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[[File:Panspermie.svg|thumb|right|300px|Illustration of a comet (center) transporting a bacterial life form (inset) through space to the Earth (left)]]
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'''Panspermia''' ({{lang-el|πανσπερμία from πᾶς/πᾶν (''pas''/pan) "all" and σπέρμα (''sperma'') "seed"}}) is the [[hypothesis]] that [[life]] exists throughout the [[Universe]], distributed by [[meteoroids]], [[asteroids]], [[comets]]<ref name="cometary panspermia">{{cite journal| last=Wickramasinghe|first=Chandra| title=Bacterial morphologies supporting cometary panspermia: a reappraisal| journal=International Journal of Astrobiology| date=10 June 2010|year=2011|volume=10| issue=1| pages=25–30 | doi=10.1017/S1473550410000157 |bibcode = 2011IJAsB..10...25W }}</ref><ref>{{cite journal|last=Napier|first=William|title=Exchange of Biomaterial Between Planetary Systems| year=2011 |month=10|volume=16| pages=6616–6642| url=http://journalofcosmology.com/JoC16pdfs/12_Napier.pdf}}</ref>  and [[Small Solar System body|planetoids]].<ref>Rampelotto, P. H. (2010). Panspermia: A promising field of research. In: Astrobiology Science Conference. Abs 5224.</ref>


Panspermia is the proposal that life forms that can survive the effects of space, such as [[extremophile]]s, become trapped in debris that is ejected into space after collisions between planets that harbor life and [[small Solar System bodies]] (SSSB). Some [[organism]]s may travel dormant for an extended amount of time before colliding randomly with other planets or intermingling with [[protoplanetary disk]]s. If met with ideal conditions on a new planet's surfaces, the organisms become active and the process of [[evolution]] begins. Panspermia is not meant to address [[abiogenesis|how life began]], just the method that may cause its distribution in the universe.<ref name= "Wesson">A variation of the panspermia hypothesis is '''necropanspermia''' which is described by astronomer Paul Wesson as follows: "The vast majority of organisms reach a new home in the Milky Way in a technically dead state ... Resurrection may, however, be possible."  {{Cite news|last=Grossman |first=Lisa |title = All Life on Earth Could Have Come From Alien Zombies |publisher=''[[Wired (magazine)|Wired]]'' |date=2010-11-10 |url=http://www.wired.com/wiredscience/2010/11/necropanspermia/|accessdate=2010-11-10}}</ref><ref name="Hoyle, F 1981. pp35-49">Hoyle, F. and Wickramasinghe, N.C., 1981. Evolution from Space (Simon & Schuster Inc., NY, 1981 and J.M. Dent and Son, Lond, 1981), ch3 pp35-49</ref><ref name="Wickramasinghe, J. 2010. pp 137-154">Wickramasinghe, J., Wickramasinghe, C. and Napier, W., 2010. Comets and the Origin of Life (World Scientific, Singapore. 1981), ch6 pp 137-154</ref>
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==History==
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The first known mention of the  term was in the writings of the 5th century BC [[Ancient Greece|Greek]] philosopher [[Anaxagoras]].<ref>Margaret O'Leary (2008) Anaxagoras and the Origin of Panspermia Theory, iUniverse publishing Group, # ISBN 978-0-595-49596-2</ref> Panspermia began to assume a more scientific form through the proposals of [[Jöns Jacob Berzelius]] (1834),<ref>{{Cite journal| last =  Berzelius (1799-1848) | first =  J. J. | title = Analysis of the Alais meteorite and implications about life in other worlds}}</ref> Hermann E. Richter (1865),<ref>{{cite book | last1 = Lynn J. Rothschild and Adrian M. Lister | title = Evolution on Planet Earth - The Impact of the Physical Environment | publisher = Academic Press | date = June 2003 | pages = 109–127 | accessdate = 2013-07-21 | isbn = 978-0-12-598655-7 }}</ref> [[William Thomson, 1st Baron Kelvin|Kelvin]] (1871),<ref>{{Cite journal| last = Thomson (Lord Kelvin) | first = W. | title =  Inaugural Address to the British Association Edinburgh. "We must regard it as probably to the highest degree that there are countless seed-bearing meteoritic stones moving through space." | journal = Nature | volume = 4 | pages = 261–278 [262] | year = 1871 |doi=10.1038/004261a0| issue = 92 |bibcode = 1871Natur...4..261. }}</ref> [[Hermann von Helmholtz]] (1879)<ref>{{cite journal | title = The word: Panspermia | journal = New Scientist | date = 7 March 2006 | issue = 2541| id = | url = http://www.newscientist.com/article/mg18925411.900-the-word-panspermia.html | accessdate = 2013-07-25}}</ref><ref>{{cite web | url = http://www.panspermia-theory.com/home-page/history-of-panspermia | title = History of Panspermia | accessdate = 2013-07-25}}</ref> and finally reaching the level of a detailed [[hypothesis]] through the efforts of the Swedish chemist [[Svante Arrhenius]] (1903).<ref>Arrhenius, S., ''Worlds in the Making: The Evolution of the Universe''. New York, Harper & Row, 1908,</ref>


[[Sir Fred Hoyle]] (1915–2001) and [[Chandra Wickramasinghe]] (born 1939) were influential proponents of panspermia.<ref name=Pollination>{{cite journal|last=Napier|first=W.M.|title=Pollination of exoplanets by nebulae|journal=Int.J.Astrobiol|year=2007|volume=6|issue=3|pages=223–228|bibcode=2007IJAsB...6..223N|doi=10.1017/S1473550407003710}}</ref><ref name=Panspermia>{{cite journal|last=Line|first=M.A.|title=Panspermia in the context of the timing of the origin of life and microbial phylogeny|journal=Int. J. Astrobiol|year=2007|volume=6|series=3|pages=249–254|bibcode=2007IJAsB...6..249L|doi=10.1017/S1473550407003813|issue=3}}</ref> In 1974 they proposed the hypothesis that some [[Cosmic dust|dust in interstellar space]] was largely [[Organic compound|organic]] (containing carbon), which Wickramasinghe later proved to be correct.<ref>{{cite journal |doi=10.1038/287518a0 |title=The 3.4-µm interstellar absorption feature |year=1980 |last1=Wickramasinghe |first1=D. T. |last2=Allen |first2=D. A. |journal=Nature |volume=287 |issue=5782 |pages=518}}</ref><ref>{{cite journal |doi=10.1038/294239a0 |title=Diffuse interstellar absorption bands between 2.9 and 4.0 µm |year=1981 |last1=Allen |first1=D. A. |last2=Wickramasinghe |first2=D. T. |journal=Nature |volume=294 |issue=5838 |pages=239}}</ref><ref>{{cite journal |doi=10.1007/BF00653492 |title=Three components of 3?4 ?m absorption bands |year=1983 |last1=Wickramasinghe |first1=D. T. |last2=Allen |first2=D. A. |journal=Astrophysics and Space Science |volume=97 |issue=2 |pages=369 |bibcode=1983Ap&SS..97..369W}}</ref> Hoyle and Wickramasinghe further contended that life forms continue to enter the Earth's atmosphere, and may be responsible for epidemic outbreaks, new diseases, and the genetic novelty necessary for [[macroevolution]].<ref>{{cite book|author=Fred Hoyle, Chandra Wickramasinghe and John Watson| title=Viruses from Space and Related Matters| publisher=University College Cardiff Press| year=1986}}</ref>
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In a presentation on April 7, 2009, physicist [[Stephen Hawking]] stated his opinion about what humans may find when venturing into space, such as the possibility of alien life through the theory of panspermia.<ref name="Stephen Hawking">{{cite news|last=Weaver|first=Rheyanne|title=Ruminations on other worlds|url=http://www.statepress.com/archive/node/5745|accessdate=25 July 2013|newspaper=statepress.com|date=April 7, 2009}}</ref> {{cquote|Life could spread from planet to planet or from stellar system to stellar system, carried on meteors.|author=Stephen Hawking|source=Origins Symposium, 2009<ref name="Stephen Hawking" />}}
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==Proposed mechanisms==
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Panspermia can be said to be either interstellar (between [[star system]]s) or interplanetary (between [[Planetary system|planets in the same star system]]); its transport mechanisms may include [[comets]],<ref name="Comets and Panspermia">{{cite book|last=Wickramasinghe|first=Chandra|title=Comets and the Origin of Life|year=2009|publisher=World Scientific Press|isbn=978-981-256-635-5|url=http://www.worldscientific.com/worldscibooks/10.1142/6008}}</ref><ref name=Comets>{{cite web|last=Wall|first=Mike|title=Comet Impacts May Have Jump-Started Life on Earth| url=http://www.space.com/20102-comets-impact-earth-life.html|publisher=space.com|accessdate=1 August 2013}}</ref> [[radiation pressure]] and lithopanspermia (microorganisms embedded in rocks).<ref>{{Cite journal| last = Weber | first = P | last2 = Greenberg| title =Can spores survive in interstellar space? | journal =  Nature | volume = 316 |  pages =  403–407 | year = 1985| doi =10.1038/316403a0|first2 =  J. M. |bibcode = 1985Natur.316..403W | issue=6027}}</ref><ref>{{Cite journal| last = Melosh | first = H. J. | title =  The rocky road to panspermia | journal = Nature | volume =  332 | pages = 687–688 | year = 1988 | doi =  10.1038/332687a0 | pmid = 11536601 | issue = 6166 |bibcode = 1988Natur.332..687M }}</ref><ref name=Mileikowsky>{{cite journal| title =  Risks threatening viable transfer of microbes between bodies in our solar system | author = C. Mileikowsky, F. A. Cucinotta, J. W. Wilson, B. Gladman, G. Horneck, L. Lindegren, J. Melosh, Hans Rickman, M. Valtonen, J. Q. Zheng | journal = Planetary and Space Science| year = 2000| volume = 48| issue = 11 | pages = 1107–1115 | doi =  10.1016/S0032-0633(00)00085-4 | bibcode=2000P&SS...48.1107M}}</ref> Interplanetary transfer of material is well documented, as evidenced by [[Mars meteorite|meteorites of Martian origin]] found on Earth.<ref name=Mileikowsky/> [[Space probe]]s may also be a viable transport mechanism for interplanetary cross-pollination in our Solar System or even beyond. However, space agencies have implemented [[planetary protection]] procedures to reduce the risk of planetary contamination.<ref>[http://www.aero.org/news/newsitems/sterilization073001.html Studies Focus On Spacecraft Sterilization]</ref><ref>[http://www.esa.int/esaMI/Aurora/SEMBJG9ATME_0.html European Space Agency: Dry heat sterilisation process to high temperatures]</ref> In 2012, mathematician [[Edward Belbruno]] and astronomers Amaya Moro-Martín and Renu Malhotra proposed that gravitational [[low energy transfer]] of rocks among the young planets of stars in their [[birth cluster]] is commonplace, and not rare in the general galactic stellar population.<ref name='Belbruno2012'>{{cite journal | title = Chaotic Exchange of Solid Material between Planetary | journal = Astrobiology | year = 2012 | first = Edward Belbruno | coauthors = Amaya Moro-Martı´n, Renu Malhotra, and Dmitry Savransky | volume = 12 | issue = 8 | doi = 10.1089/ast.2012.0825 | last1 = Belbruno | pages = 754–74 | pmid = 22897115 | pmc = 3440031|arxiv = 1205.1059 |bibcode = 2012AsBio..12..754B }}</ref><ref>[http://www.princeton.edu/main/news/archive/S34/82/42M30/ Slow-moving rocks better odds that life crashed to Earth from space] News at Princeton, Sep 24, 2012</ref>  Deliberate directed panspermia from space to seed Earth<ref name="Crick_Orgel">{{Cite journal| last = Crick | first =  F. H. | last2 = Orgel | first2 = L. E.| title = Directed Panspermia | journal = Icarus | volume = 19 | pages =  341–348 | year = 1973| doi = 10.1016/0019-1035(73)90110-3 |bibcode=1979JBIS...32..419M| issue = 3}}</ref> or sent from Earth to seed other solar systems have also been proposed.<ref>{{Cite book | last = Mautner | first = Michael N. | title = Seeding the Universe with Life: Securing Our Cosmological Future | publisher = Legacy Books (www.amazon.com) | location = Washington D. C. | year = 2000 |  isbn = 0-476-00330-X | url = http://www.astro-ecology.com/PDFSeedingtheUniverse2005Book.pdf }}</ref><ref>{{Cite journal| last =  Mautner  | first = M | last2 = Matloff | title = Directed panspermia: A technical evaluation of seeding nearby solar systems | journal =  J. British Interplanetary Soc. | volume = 32 | pages = 419 |  year = 1979|first2 =  G. | url=http://www.astro-ecology.com/PDFDirectedPanspermia1JBIS1979Paper.pdf }}</ref><ref name="autogenerated1">{{Cite journal| last =  Mautner | first = M. N. | title = Directed panspermia. 3. Strategies and motivation for seeding star-forming clouds | journal = J. British Interplanetary Soc. | volume = 50 | pages = 93–102 | year =  1997 | url=http://www.astro-ecology.com/PDFDirectedPanspermia3JBIS1997Paper.pdf| bibcode =  1997JBIS...50...93M }}</ref><ref name="BBC-2011">{{cite web |author=BBC Staff |title=Impacts 'more likely' to have spread life from Earth |url=http://www.bbc.co.uk/news/science-environment-14637109 |date=23 August 2011 |publisher=[[BBC]] |accessdate=2011-08-24}}</ref> One twist to the hypothesis by engineer Thomas Dehel (2006), proposes that [[plasmoid]] magnetic fields ejected from the [[magnetosphere]] may move the few spores lifted from the Earth's atmosphere with sufficient speed to cross interstellar space to other systems before the spores can be destroyed.<ref>{{Cite web| url=http://space.newscientist.com/article/dn9601-electromagnetic-space-travel-for-bugs.html | title=Electromagnetic space travel for bugs? - space - 21 July 2006 - New Scientist Space |publisher=Space.newscientist.com |date= |accessdate=2009-08-20}}</ref><ref>{{cite journal | title=Uplift and Outflow of Bacterial Spores via Electric Field |publisher=Adsabs.harvard.edu |date=2006-07-23| bibcode=2006cosp...36....1D |author1=Dehel |first1=T. |volume=36 |pages=1 |journal=36th COSPAR Scientific Assembly. Held 16–23 July 2006|arxiv = hep-ph/0612311 }}</ref>
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===Radiopanspermia===
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In 1903, [[Svante Arrhenius]] published in his article ''The Distribution of Life in Space'',<ref>"Die Verbreitung des Lebens im Weltenraum" (the "Distribution of Life in Space").  Published in Die Umschau. 1903.</ref> the hypothesis now called radiopanspermia, that microscopic forms of life can be propagated in space, driven by the [[radiation pressure]] from stars.<ref>''Ancient micronauts: interplanetary transport of microbes by cosmic impacts.'' Wayne L. Nicholson. Trends in Microbiology, Vol. 17, No. 6. (June 2009), pp. 243-250, {{DOI|10.1016/j.tim.2009.03.004}}</ref> Arrhenius argued that particles at a critical size below 1.5 μm would be propagated at high speed by radiation pressure of the Sun. However, because its effectiveness decreases with increasing size of the particle, this mechanism holds for very tiny particles only, such as single bacterial spores.<ref name='Gerda Horneck'/> The main criticism of radiopanspermia hypothesis came from Shklovskii and Sagan, who pointed out the proofs of the lethal action of space radiations ([[UV]] and [[X-ray]]s) in the cosmos.<ref>{{cite book | last1 = I. S. Shklovskii | last2 = Carl Sagan | title = Intelligent Life in the Universe | publisher = Emerson-Adams Press, Incorporated | year = 1966 | accessdate = 2013-08-03 | isbn = 10: 189280302X }}</ref> Regardless of the evidence, Wallis and Wickramasinghe argued in 2004 that the transport of individual bacteria or clumps of bacteria, is overwhelmingly more important than lithopanspermia in terms of numbers of microbes transferred, even accounting for the death rate of unprotected bacteria in transit.<ref>{{cite journal|last=Wickramasinghe|first=M.K.|coauthors=Wickramasinghe, C.|title=Interstellar transfer of planetary microbiota,|journal=Mon. Not.R. Astr. Soc.|year=2004|volume=348|pages=52–57|bibcode=2004MNRAS.348...52W|doi=10.1111/j.1365-2966.2004.07355.x}}</ref>
 
 
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Then, data gathered by the orbital experiments [[Exobiology Radiation Assembly|ERA]], [[BIOPAN]], [[EXOSTACK]] and [[EXPOSE]], determined that isolated spores, including those of ''[[B. subtilis]]'', were killed by several orders of magnitude if exposed to the full space environment for a mere few seconds, but if shielded against solar [[UV]], the spores were capable of surviving in space for up to 6 years while embedded in clay or meteorite powder (artificial meteorites).<ref name='Gerda Horneck'/><ref name=Protection>''Protection of Bacterial Spores in Space, a Contribution to the Discussion on Panspermia''. Gerda Horneck, Petra Rettberg, Günther Reitz, Jörg Wehner, Ute Eschweiler, Karsten Strauch, Corinna Panitz, Verena Starke, Christa Baumstark-Khan. Origins of life and evolution of the biosphere. December 2001, Volume 31, Issue 6, pp 527-547</ref> Though minimal protection is required to shelter a spore against UV radiation, exposure to solar UV and cosmic ionizing radiation of unprotected DNA, break it up into its bases.<ref>R.O. Rahn, J.L. Hosszu, Influence of relative humidity on the photochemistry of DNA films, Biochim. Biophys Acta 190 (1969) 126–131.</ref><ref>M.H. Patrick, D.M. Gray, Independence of photproduct formation on DNA conformation, Photochem. Photobiol. 24 (1976) 507–513.</ref><ref name='transport'>{{cite journal | title = The solar UV environment and bacterial spore UV resistance: considerations for Earth-to-Mars transport by natural processes and human spaceflight | journal = Mutation Research | date = 21 January 2005 | first = Wayne L. Nicholson | coauthors = Andrew C. Schuerger, Peter Setlow. | volume = 571 | pages = 249–264| id = | url = http://www.bi.ku.dk/dna/course/papers/F1b.nicholson.pdf | accessdate = 2013-08-02 | doi = 10.1016/j.mrfmmm.2004.10.012 | pmid = 15748651 | last1 = Nicholson | issue = 1–2}}</ref> Also, exposing DNA to the ultrahigh vacuum of space alone is sufficient to cause DNA damage, so the transport of unprotected DNA or RNA during interplanetary flights is extremely unlikely.<ref name='transport'/>
 
 
</ul>
Based on experimental data on radiation effects and DNA stability, it has been concluded that for such long travel times, boulder sized rocks which are greater than or equal to 1 meter in diameter are required to effectively shield resistant microorganisms, such as bacterial spores against galactic [[cosmic radiation]].<ref>Clark BC., Planetary interchange of bioactive material: probability factors and implications Origins Life Evol Biosphere 2001; 31: 185-97</ref><ref>Mileikowsky C. et al Natural Transfer of Microbes in space, part I: from Mars to Earth and Earth to Mars Icarus 2000; 145; 391-427</ref> These results clearly negate the radiopanspermia hypothesis, which requires single spores accelerated by the radiation pressure of the Sun, requiring many years to travel between the planets, and support the likelihood of interplanetary transfer of microorganisms within [[asteroid]]s or [[comet]]s, the so-called '''lithopanspermia''' hypothesis.<ref name='Gerda Horneck'/><ref name=Protection/>
 
===Lithopanspermia===
 
Lithopanspermia, the transfer of organisms in rocks from one planet to another either through interplanetary or interstellar space, remains speculative. Although there is no evidence that lithopanspermia has occurred in our own Solar System, the various stages have become amenable to experimental testing.<ref name='Experimental methods'>{{cite journal |doi=10.1016/j.mimet.2009.10.004 |title=Experimental methods for studying microbial survival in extraterrestrial environments |year=2010 |last1=Olsson-Francis |first1=Karen |last2=Cockell |first2=Charles S. |journal=Journal of Microbiological Methods |volume=80 |pages=1–13 |pmid=19854226 |issue=1}}</ref>
 
*'''Planetary ejection''' — For lithopanspermia to occur, microorganisms must survive ejection from a planetary surface which involves extreme forces of acceleration and shock with associated temperature excursions.  Hypothetical values of shock pressures experienced by ejected rocks are obtained with Martian meteorites, which suggest the shock pressures of approximately 5 to 55 GPa, acceleration of 3×10<sup>6</sup> m/s<sup>2</sup> and [[Jerk (physics)|jerk]] of 6×10<sup>9</sup> m/s<sup>3</sup> and post-shock temperature increases of about 1 K to 1000 K.<ref name='Cockell 2008'>{{cite journal |doi=10.1007/s11084-007-9112-3 |title=The Interplanetary Exchange of Photosynthesis |year=2007 |last1=Cockell |first1=Charles S. |journal=Origins of Life and Evolution of Biospheres |volume=38 |pages=87}}</ref><ref>{{cite journal |doi=10.1089/ast.2007.0134 |title=Microbial Rock Inhabitants Survive Hypervelocity Impacts on Mars-Like Host Planets: First Phase of Lithopanspermia Experimentally Tested |year=2008 |last1=Horneck |first1=Gerda |last2=Stöffler |first2=Dieter |last3=Ott |first3=Sieglinde |last4=Hornemann |first4=Ulrich |last5=Cockell |first5=Charles S. |last6=Moeller |first6=Ralf |last7=Meyer |first7=Cornelia |last8=De Vera |first8=Jean-Pierre |last9=Fritz |first9=Jörg |last10=Schade |first10=Sara |last11=Artemieva |first11=Natalia A. |journal=Astrobiology |volume=8 |pages=17–44 |pmid=18237257 |issue=1}}</ref> To determine the effect of acceleration during ejection on microorganisms, rifle and ultracentrifuge methods were successfully used under simulated outer space conditions.<ref name="Experimental methods"/>
*'''Survival in transit''' — The survival of microorganisms has been studied extensively using both simulated facilities and in low Earth orbit. A large number of microorganisms have been selected for exposure experiments. It is possible to separate these microorganisms into two groups, the human-borne, and the [[extremophiles]]. Studying the human-borne microorganisms is significant for human welfare and future manned missions; whilst the extremophiles are vital for studying the physiological requirements of survival in space.<ref name="Experimental methods"/>
*'''Atmospheric entry''' — An important aspect of the lithopanspermia hypothesis to test is that microbes situated on or within rocks could survive hypervelocity entry from space through Earth's atmosphere (Cockell, 2008). As with planetary ejection, this is experimentally tractable, with sounding rockets and orbital vehicles being used for microbiological experiments.<ref name="Experimental methods"/><ref name='Cockell 2008'/> ''[[B. subtilis]]'' spores inoculated onto [[granite]] domes were subjected to hypervelocity atmospheric transit (twice) by launch to a ∼120&nbsp;km altitude on an Orion two-stage rocket. The spores were shown to have survived on the sides of the rock, but they did not survive on the forward-facing surface that was subjected to a maximum temperature of 145 °C.<ref>{{cite journal |doi=10.1089/ast.2005.5.726 |title=Bacillus subtilisSpores on Artificial Meteorites Survive Hypervelocity Atmospheric Entry: Implications for Lithopanspermia |year=2005 |last1=Fajardo-Cavazos |first1=Patricia |last2=Link |first2=Lindsey |last3=Melosh |first3=H. Jay |last4=Nicholson |first4=Wayne L. |journal=Astrobiology |volume=5 |issue=6 |pages=726–36 |pmid=16379527}}</ref>  In separate experiments, as part of the ESA STONE experiment, numerous organisms were embedded in different types or rocks and were mounted in the heat shield of six [[Foton (space programs)|Foton re-entry capsules]]. On reentry, the rock samples were subjected to temperatures and pressure loads comparable to those experienced in meteorites.<ref>{{cite journal |doi=10.1016/S0032-0633(02)00018-1 |title=Do meteoroids of sedimentary origin survive terrestrial atmospheric entry? The ESA artificial meteorite experiment STONE |year=2002 |last1=Brack |first1=A. |last2=Baglioni |first2=P. |last3=Borruat |first3=G. |last4=Brandstätter |first4=F. |last5=Demets |first5=R. |last6=Edwards |first6=H.G.M. |last7=Genge |first7=M. |last8=Kurat |first8=G. |last9=Miller |first9=M.F. |last10=Newton |first10=E.M. |last11=Pillinger |first11=C.T. |last12=Roten |first12=C.-A. |last13=Wäsch |first13=E. |journal=Planetary and Space Science |volume=50 |issue=7–8 |pages=763 |bibcode=2002P&SS...50..763B}}</ref> The exogenous arrival of [[photosynthesis|photosynthetic]] microorganisms could have quite profound consequences for the course of biological evolution on the inoculated planet. As photosynthetic organisms must be close to the surface of a rock to obtain sufficient light energy, atmospheric transit might act as a filter against them by ablating the surface layers of the rock. Although [[cyanobacteria]] have been shown to survive the desiccating, freezing conditions of space in orbital experiments, this would be of no benefit as the STONE experiment showed that they cannot survive atmospheric entry.<ref name='filter'>{{cite journal |doi=10.1089/ast.2006.0038 |title=Interplanetary Transfer of Photosynthesis: An Experimental Demonstration of a Selective Dispersal Filter in Planetary Island Biogeography |year=2007 |last1=Cockell |first1=Charles S. |last2=Brack |first2=André |last3=Wynn-Williams |first3=David D. |last4=Baglioni |first4=Pietro |last5=Brandstätter |first5=Franz |last6=Demets |first6=René |last7=Edwards |first7=Howell G.M. |last8=Gronstal |first8=Aaron L. |last9=Kurat |first9=Gero |last10=Lee |first10=Pascal |last11=Osinski |first11=Gordon R. |last12=Pearce |first12=David A. |last13=Pillinger |first13=Judith M. |last14=Roten |first14=Claude-Alain |last15=Sancisi-Frey |first15=Suzy |journal=Astrobiology |volume=7 |pages=1–9 |pmid=17407400 |issue=1}}</ref> Thus, non-photosynthetic organisms deep within rocks have a chance to survive the exit and entry process. (See also: [[Impact survival]].)
 
===Accidental panspermia===
[[Thomas Gold]], a professor of [[astronomy]], suggested in 1960 the hypothesis of "Cosmic Garbage", where that life on Earth might have originated from a pile of [[waste]] products accidentally dumped on Earth long ago by extraterrestrial beings.<ref>Gold, T. "Cosmic Garbage," Air Force and Space Digest, 65 (May 1960).</ref>
 
===Directed panspermia===
{{Main|Directed panspermia}}
Directed panspermia concerns the deliberate transport of microorganisms in space, sent to Earth to start life here, or sent from Earth to seed new [[solar systems]] with life by [[introduced species]] of microorganisms on lifeless planets. The Nobel prize winner [[Francis Crick]], along with [[Leslie Orgel]] proposed that life may have been purposely spread by an advanced extraterrestrial civilization,<ref name="Crick_Orgel"/> but considering an early "[[RNA world]]" Crick noted later that life may have originated on Earth.<ref>"[http://www.fasebj.org/cgi/reprint/7/1/238.pdf Anticipating an RNA world. Some past speculations on the origin of life: where are they today?]" by L. E. Orgel and F. H. C. Crick in ''FASEB J.'' (1993) Volume 7 pages 238-239.</ref> It has been suggested that 'directed' panspermia was proposed in order to counteract various objections, including the argument that microbes would be inactivated by the space environment and [[cosmic radiation]] before they could make a chance encounter with Earth.<ref name='C. Clark'>{{cite journal | title = Planetary Interchange of Bioactive Material: Probability Factors and Implications | journal = Origins of life and evolution of the biosphere | date = February 2001 | first = Benton C. Clark | volume = 31 | issue = 1–2 | pages = 185–197| id = |bibcode = 2001OLEB...31..185C| doi = 10.1023/A:1006757011007 | last1 = Clark | pmid = 11296521 }}</ref>
 
Conversely, active directed panspermia has been proposed to secure and expand life in space.<ref name="autogenerated1" /> This may be motivated by biotic ethics that values, and seeks to propagate, the basic patterns of our organic gene/protein life-form.<ref>{{Cite journal | last = Mautner | first = Michael N. | title =  Life-centered ethics, and the human future in space | journal = Bioethics | volume = 23 | pages = 433–440 | year = 2009 | doi = 10.1111/j.1467-8519.2008.00688.x | pmid=19077128 | url = http://www.astro-ecology.com/PDFLifeCenteredBioethics2009Paper.pdf | issue = 8 }}</ref>  The panbiotic program would seed new solar systems nearby, and clusters of new stars in interstellar clouds. These young targets, where local life would not have formed yet, avoid any interference with local life.
 
For example, microbial payloads launched by solar sails at speeds up to 0.0001 ''c'' (30,000&nbsp;m/s) would reach targets at 10 to 100 light-years in 0.1 million to 1  million years. Fleets of microbial capsules can be aimed at clusters of new stars in star-forming clouds, where they may land on planets or captured by asteroids and comets and later delivered to planets. Payloads may contain [[extremophile]]s for diverse environments and [[cyanobacteria]] similar to early microorganisms. Hardy multicellular organisms (rotifer cysts) may be included to induce higher evolution.<ref>{{Cite book| last =  Mautner | first = Michael Noah Ph.D.| author-link =  Michael Noah  Mautner | title =  Seeding the Universe with Life: Securing our Cosmological Future | publisher =  Legacy Books  (www.amazon.com) | year = 2000 | isbn = 0-476-00330-X | url = http://www.astro-ecology.com/PDFSeedingtheUniverse2005Book.pdf }}</ref>
 
The probability of hitting the target zone can be calculated from <math>P(target) = \frac{A(target)}{\pi (dy)^2} = \frac{a r(target)^2 v^2}{(tp)^2 d^4}</math> where ''A''(target) is the cross-section of the target area, ''dy'' is the positional uncertainty at arrival; ''a'' - constant (depending on units),  ''r''(target) is the radius of the target area; ''v'' the velocity of the probe; (tp) the targeting precision (arcsec/yr); and ''d'' the distance to the target, guided by high-resolution astrometry of 1×10<sup>−5</sup> arcsec/yr (all units in SIU). These calculations show that relatively near target stars(Alpha PsA, Beta Pictoris) can be seeded by milligrams of launched microbes; while seeding the Rho Ophiochus star-forming cloud requires hundreds of kilograms of dispersed capsules.<ref name="autogenerated1" />
 
Theoretically, unintended panspermia may occur by spacecraft travelling to other celestial bodies. This may concern space researchers who try to prevent [[Interplanetary contamination|contamination]]. However, directed panspermia may reach a few dozen target systems, leaving billions in the galaxy untouched. In any case, matter is exchanged by [[Impact event|meteor impacts]] in the solar system even without human intervention.
 
Directed panspermia to secure and expand life in space is becoming possible due to developments in [[solar sails]], precise [[astrometry]], [[extrasolar planets]], [[extremophiles]] and microbial [[genetic engineering]]. After determining the composition of chosen meteorites, [[Astroecology|astroecologists]] performed laboratory experiments that suggest that many colonizing microorganisms and some plants could obtain many of their chemical nutrients from asteroid and cometary materials.<ref name=bioresources>{{Cite journal | last = Mautner | first = Michael N. | title = Planetary bioresources and astroecology. 1. Planetary microcosm bioessays of Martian and meteorite materials: soluble electrolytes, nutrients, and algal and plant responses|journal = Icarus |volume = 158 | pages = 72–86 |  year = 2002 | doi=10.1006/icar.2002.6841 | bibcode=2002Icar..158...72M | url = http://www.astro-ecology.com/PDFBioresourcesIcarus2002Paper.pdf | pmid=12449855}}</ref> However, the scientists noted that phosphate (PO<sub>4</sub>) and [[nitrate]] (NO<sub>3</sub>–N) critically limit nutrition to many terrestrial lifeforms.<ref name=bioresources/>  With such materials, and energy from long-lived stars, microscopic life planted by directed panspermia could find an immense future in the galaxy.<ref>{{Cite journal|last = Mautner |first = Michael N. | title = Life in the cosmological future: Resources, biomass and populations  | journal = Journal of the British Interplanetary Society | year = 2005 | volume = 58 | pages = 167–180 | url=http://www.astro-ecology.com/PDFCosmologyJBIS2005Paper.pdf |bibcode = 2005JBIS...58..167M }}</ref>
 
===Pseudo-panspermia===
{{further| List of interstellar and circumstellar molecules|Abiogenesis#Extraterrestrial organic molecules}}
 
Pseudo-panspermia (sometimes called ''"soft panspermia"'' or ''"molecular panspermia"'') argues that the pre-biotic organic building blocks of life originated in space and were incorporated in the solar nebula from which the planets condensed and were further —and continuously— distributed to planetary surfaces where life then emerged.<ref>{{cite web | url = http://www.panspermia.org/oseti.htm | title = Panspermia Asks New Questions | accessdate = 2013-07-25 | last = Klyce | first = Brig | year = 2001}}</ref><ref>{{cite journal |doi=10.1117/12.435366 |chapter=<title>Panspermia asks new questions</title> |title=The Search for Extraterrestrial Intelligence (SETI) in the Optical Spectrum III |series=The Search for Extraterrestrial Intelligence (SETI) in the Optical Spectrum III |year=2001 |editor1-last=Kingsley |editor1-first=Stuart A |last1=Klyce |first1=Brig |editor2-last=Bhathal |editor2-first=Ragbir |volume=4273 |pages=11}}</ref> From the early 1970s it was becoming evident that interstellar dust consisted of a large component of organic molecules.  The first suggestion came from [[Chandra Wickramasinghe]], who proposed a polymeric composition based on the molecule [[formaldehyde]] (CH<sub>2</sub>O).<ref>N.C. Wickramasinghe, Formaldehyde Polymers in Interstellar Space, Nature, 252, 462, 1974</ref>  Interstellar molecules are formed by chemical reactions within very sparse interstellar or circumstellar clouds of dust and gas. Usually this occurs when a molecule becomes ionized, often as the result of an interaction with [[cosmic ray]]s. This positively charged molecule then draws in a nearby reactant by electrostatic attraction of the neutral molecule's electrons. Molecules can also be generated by reactions between neutral atoms and molecules, although this process is generally slower.<ref name=pnas103>{{cite journal |doi=10.1073/pnas.0602117103 |title=The galactic cosmic ray ionization rate |year=2006 |last1=Dalgarno |first1=A. |journal=Proceedings of the National Academy of Sciences |volume=103 |issue=33 |pages=12269–73 |pmid=16894166 |pmc=1567869}}</ref> The dust plays a critical role of shielding the molecules from the ionizing effect of ultraviolet radiation emitted by stars.<ref name=brown_pais95>{{Cite book | last1=Brown | first1=Laurie M. | last2=Pais | first2=Abraham | last3=Pippard | first3=A. B. | title=Twentieth Century Physics | page=1765 | chapter=The physics of the interstellar medium | edition=2nd | publisher=CRC Press | year=1995 | isbn=0-7503-0310-7}}</ref>
 
A 2008 analysis of <sup>12</sup>C/<sup>13</sup>C isotopic ratios of organic compounds found in the [[Murchison meteorite]] indicates a non-terrestrial origin for these molecules rather than terrestrial contamination. Biologically relevant molecules identified so far include [[uracil]], an RNA [[nucleobase]], and [[xanthine]].<ref name='Murch_base'>{{Cite journal |doi=10.1016/j.epsl.2008.03.026 |title=Extraterrestrial nucleobases in the Murchison meteorite |year=2008 |last1=Martins |first1=Zita |last2=Botta |first2=Oliver |last3=Fogel |first3=Marilyn L. |last4=Sephton |first4=Mark A. |last5=Glavin |first5=Daniel P. |last6=Watson |first6=Jonathan S. |last7=Dworkin |first7=Jason P. |last8=Schwartz |first8=Alan W. |last9=Ehrenfreund |first9=Pascale |journal=Earth and Planetary Science Letters |volume=270 |pages=130 |bibcode=2008E&PSL.270..130M}}</ref><ref>{{Cite web |author=[[Agence France-Presse|AFP]] Staff |title=We may all be space aliens: study |date=20 August 2009 |url=http://afp.google.com/article/ALeqM5j_QHxWNRNdiW35Qr00L8CkwcXyvw |publisher=[[Agence France-Presse|AFP]] |accessdate=2011-08-14 }}</ref> These results demonstrate that many organic compounds which are components of life on Earth were already present in the early Solar System and may have played a key role in life's origin.<ref>{{cite journal |doi=10.1016/j.epsl.2008.03.026 |title=Extraterrestrial nucleobases in the Murchison meteorite |year=2008 |last1=Martins |first1=Zita |last2=Botta |first2=Oliver |last3=Fogel |first3=Marilyn L. |last4=Sephton |first4=Mark A. |last5=Glavin |first5=Daniel P. |last6=Watson |first6=Jonathan S. |last7=Dworkin |first7=Jason P. |last8=Schwartz |first8=Alan W. |last9=Ehrenfreund |first9=Pascale |journal=Earth and Planetary Science Letters |volume=270 |pages=130 |bibcode=2008E&PSL.270..130M}}</ref>
 
In August 2009, NASA scientists identified one of the fundamental chemical building-blocks of life (the amino acid [[glycine]]) in a comet for the first time.<ref>{{Cite news| first= | last= | coauthors= |authorlink= | title='Life chemical' detected in comet | date=18 August 2009 | publisher=BBC News | url =http://news.bbc.co.uk/2/hi/science/nature/8208307.stm | work =[[NASA]] | pages = | accessdate = 2010-03-06 | language = }}</ref>
 
On August 2011, a report, based on [[NASA]] studies with [[meteorites]] found on [[Earth]], was published suggesting building blocks of [[DNA]] ([[adenine]], [[guanine]] and related [[organic molecules]]) may have been formed extraterrestrially in [[outer space]].<ref name="Callahan">{{cite journal |doi=10.1073/pnas.1106493108 |title=Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases |year=2011 |last1=Callahan |first1=M. P. |last2=Smith |first2=K. E. |last3=Cleaves |first3=H. J. |last4=Ruzicka |first4=J. |last5=Stern |first5=J. C. |last6=Glavin |first6=D. P. |last7=House |first7=C. H. |last8=Dworkin |first8=J. P. |journal=Proceedings of the National Academy of Sciences |volume=108 |issue=34 |pages=13995}}</ref><ref name="Steigerwald">{{cite web |last=Steigerwald |first=John |title=NASA Researchers: DNA Building Blocks Can Be Made in Space|url=http://www.nasa.gov/topics/solarsystem/features/dna-meteorites.html|publisher=[[NASA]] |date=8 August 2011 |accessdate=2011-08-10}}</ref><ref name="DNA">{{cite web |author=ScienceDaily Staff |title=DNA Building Blocks Can Be Made in Space, NASA Evidence Suggests|url=http://www.sciencedaily.com/releases/2011/08/110808220659.htm |date=9 August 2011 |publisher=[[ScienceDaily]] |accessdate=2011-08-09}}</ref>  In October 2011, scientists reported that [[cosmic dust]] contains complex [[organic compound|organic]] matter ("amorphous organic solids with a mixed [[aromatic]]-[[aliphatic]] structure") that could be created naturally, and rapidly, by [[stars]].<ref name="Space-20111026">{{cite web |last=Chow |first=Denise |title=Discovery: Cosmic Dust Contains Organic Matter from Stars |url=http://www.space.com/13401-cosmic-star-dust-complex-organic-compounds.html |date=26 October 2011 |publisher=[[Space.com]] |accessdate=2011-10-26 }}</ref><ref name="ScienceDaily-20111026">{{cite web |author=[[ScienceDaily]] Staff |title=Astronomers Discover Complex Organic Matter Exists Throughout the Universe |url=http://www.sciencedaily.com/releases/2011/10/111026143721.htm |date=26 October 2011 |publisher=[[ScienceDaily]] |accessdate=2011-10-27 }}</ref><ref name="Nature-20111026">{{cite journal |doi=10.1038/nature10542 |title=Mixed aromatic–aliphatic organic nanoparticles as carriers of unidentified infrared emission features |year=2011 |last1=Kwok |first1=Sun |last2=Zhang |first2=Yong |journal=Nature |volume=479 |issue=7371 |pages=80–3 |pmid=22031328}}</ref>  One of the scientists suggested that these complex organic compounds may have been related to the development of life on Earth and said that, "If this is the case, life on Earth may have had an easier time getting started as these organics can serve as basic ingredients for life."<ref name="Space-20111026"/>
 
On August 2012, and in a world first, astronomers at [[Copenhagen University]] reported the detection of a specific sugar molecule, [[glycolaldehyde]], in a distant star system. The molecule was found around the [[protostar|protostellar]] binary ''IRAS 16293-2422'', which is located 400 light years from Earth.<ref name="NG-20120829">{{cite journal|title=Sugar Found In Space|journal=National Geographic |last=Than |first=Ker |date=August 29, 2012 |url=http://news.nationalgeographic.com/news/2012/08/120829-sugar-space-planets-science-life/ |accessdate=August 31, 2012 }}</ref><ref name="AP-20120829">{{cite web |author=Staff |title=Sweet! Astronomers spot sugar molecule near star |url=http://apnews.excite.com/article/20120829/DA0V31D80.html |date=August 29, 2012 |publisher=[[AP News]] |accessdate=August 31, 2012 }}</ref> Glycolaldehyde is needed to form [[ribonucleic acid]], or [[RNA]], which is similar in function to [[DNA]].  This finding suggests that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation.<ref>{{cite journal |doi=10.1088/2041-8205/757/1/L4 |title=Detection of the Simplest Sugar, Glycolaldehyde, in a Solar-Type Protostar with Alma |year=2012 |last1=Jørgensen |first1=Jes K. |last2=Favre |first2=Cécile |last3=Bisschop |first3=Suzanne E. |last4=Bourke |first4=Tyler L. |last5=Van Dishoeck |first5=Ewine F. |last6=Schmalzl |first6=Markus |journal=The Astrophysical Journal |volume=757 |pages=L4 |bibcode=2012ApJ...757L...4J}}</ref>
 
In September 2012, [[NASA|NASA scientists]] reported that [[polycyclic aromatic hydrocarbons|polycyclic aromatic hydrocarbons (PAHs)]], subjected to [[Interstellar medium|interstellar medium (ISM)]] conditions, are transformed, through [[hydrogenation]], [[Oxygenate|oxygenation]] and [[hydroxylation]], to more complex [[Organic compounds|organics]] - "a step along the path toward [[amino acids]] and [[nucleotide]]s, the raw materials of [[protein]]s and [[DNA]], respectively".<ref name="Space-20120920">{{cite web |author=Staff |title=NASA Cooks Up Icy Organics to Mimic Life's Origins|url=http://www.space.com/17681-life-building-blocks-nasa-organic-molecules.html|date=September 20, 2012 |publisher=[[Space.com]] |accessdate=September 22, 2012 }}</ref><ref name="AJL-20120901">{{cite journal |doi=10.1088/2041-8205/756/1/L24 |title=In-Situ Probing of Radiation-Induced Processing of Organics in Astrophysical Ice Analogs—Novel Laser Desorption Laser Ionization Time-Of-Flight Mass Spectroscopic Studies |year=2012 |last1=Gudipati |first1=Murthy S. |last2=Yang |first2=Rui |journal=The Astrophysical Journal |volume=756 |pages=L24 |bibcode=2012ApJ...756L..24G}}</ref> Further, as a result of these transformations, the PAHs lose their [[Spectroscopy|spectroscopic signature]] which could be one of the reasons "for the lack of PAH detection in [[interstellar ice]] [[Cosmic dust#Dust grain formation|grains]], particularly the outer regions of cold, dense clouds or the upper molecular layers of [[protoplanetary disks]]."<ref name="Space-20120920" /><ref name="AJL-20120901" />
 
In 2013, the [[Atacama Large Millimeter Array]] (ALMA Project) confirmed that researchers have discovered an important pair of prebiotic molecules in the icy particles in [[interstellar space]] (ISM). The chemicals, found in a giant cloud of gas about 25,000 light-years from Earth in ISM, may be a precursor to a key component of DNA and the other may have a role in the formation of an important [[amino acid]]. Researchers found a molecule called cyanomethanimine, which produces [[adenine]], one of the four [[nucleobases]] that form the “rungs” in the ladder-like structure of DNA. The other molecule, called [[ethanamine]], is thought to play a role in forming [[alanine]], one of the twenty amino acids in the genetic code. Previously, scientists thought such processes took place in the very tenuous gas between the stars. The new discoveries, however, suggest that the chemical formation sequences for these molecules occurred not in gas, but on the surfaces of ice grains in interstellar space.<ref>{{cite journal |doi=10.1088/2041-8205/765/1/L9 |title=The Detection of Interstellar Ethanimine (Ch3Chnh) from Observations Taken During the Gbt Primos Survey |year=2013 |last1=Loomis |first1=Ryan A. |last2=Zaleski|first2=Daniel P. |last3=Steber |first3=Amanda L. |last4=Neill |first4=Justin L. |last5=Muckle|first5=Matthew T. |last6=Harris |first6=Brent J. |last7=Hollis |first7=Jan M. |last8=Jewell |first8=Philip R.|last9=Lattanzi|first9=Valerio |last10=Lovas |first10=Frank J. |last11=Martinez |first11=Oscar|last12=McCarthy|first12=Michael C. |last13=Remijan |first13=Anthony J. |last14=Pate |first14=Brooks H.|last15=Corby|first15=Joanna F. |journal=The Astrophysical Journal |volume=765 |pages=L9|bibcode=2013ApJ...765L...9L}}</ref> NASA ALMA scientist Anthony Remijan stated that finding these molecules in an interstellar gas cloud means that important building blocks for DNA and amino acids can 'seed' newly formed planets with the chemical precursors for life.<ref>[https://www.nrao.edu/pr/2013/newchem/ Finley, Dave,''Discoveries Suggest Icy Cosmic Start for Amino Acids and DNA Ingredients,'' The National Radio Astronomy Observatory, Feb. 28, 2013]</ref>
 
In March 2013, a simulation experiment indicate that dipeptides (pairs of amino acids) that can be building blocks of [[proteins]], can be created in interstellar dust.<ref name="Sanders">{{cite journal |doi=10.1088/0004-637X/765/2/111 |laysummary=http://phys.org/news/2013-03-evidence-comets-seeded-life-earth.html |laysource=Phys.org |laydate=Mar 05, 2013 |title=On the Formation of Dipeptides in Interstellar Model Ices |year=2013 |last1=Kaiser |first1=R. I. |last2=Stockton |first2=A. M. |last3=Kim |first3=Y. S. |last4=Jensen |first4=E. C. |last5=Mathies |first5=R. A. |journal=The Astrophysical Journal |volume=765 |issue=2 |pages=111 |bibcode=2013ApJ...765..111K}}</ref>
 
==Extraterrestrial life==
{{Main|Extraterrestrial life}}
[[Earth]] is the only planet known to harbor life in the observed universe, while the sheer number of planets in the [[Milky Way]] galaxy make it seem probable that life has arisen somewhere else in the galaxy and the [[universe]]. It is generally agreed that the conditions required for the [[evolution]] of intelligent life as we know it are probably exceedingly rare in the universe, while simultaneously noting that simple single-celled [[microorganisms]] may be more likely.<ref name="Webb, Stephen 2002">Webb, Stephen, 2002. ''If the universe is teeming with aliens, where is everybody? Fifty solutions to the Fermi paradox and the problem of extraterrestrial life''. Copernicus Books (Springer Verlag)</ref>
 
The [[extrasolar planet]] results from the [[Kepler (spacecraft)|Kepler mission]] estimate 100–400 billion exoplanets, with over 3,500 as candidates or confirmed exoplanets.<ref>{{cite journal |last1=Steffen |first1=Jason H. |coauthors=''et al.'' |title=Five Kepler target stars that show multiple transiting exoplanet candidates |doi=10.1088/0004-637X/725/1/1226 |pages=1226–1241 |volume=725 |journal=[[Astrophysical Journal]] |arxiv=1006.2763 |date=9 November 2010 |bibcode = 2010ApJ...725.1226S}}</ref> On 4 November 2013, astronomers reported, based on [[Kepler (spacecraft)|Kepler space mission]] data, that there could be as many as 40 billion [[Terrestrial planet|Earth-sized]] [[extrasolar planets|planets]] orbiting in the [[habitable zone]]s of [[sun-like|sun-like stars]] and [[red dwarf stars]] within the [[Milky Way Galaxy]].<ref name="NYT-20131104">{{cite news |last=Overbye |first=Dennis |title=Far-Off Planets Like the Earth Dot the Galaxy |url=http://www.nytimes.com/2013/11/05/science/cosmic-census-finds-billions-of-planets-that-could-be-like-earth.html |date=November 4, 2013 |work=[[New York Times]]|accessdate=November 5, 2013 }}</ref><ref name="PNAS-20131031">{{cite journal |last1=Petigura|first1=Eric A. |last2=Howard |first2=Andrew W. |last3=Marcy |first3=Geoffrey W.|title=Prevalence of Earth-size planets orbiting Sun-like stars|url=http://www.pnas.org/content/early/2013/10/31/1319909110 |date=October 31, 2013 |journal=[[Proceedings of the National Academy of Sciences of the United States of America]]|doi=10.1073/pnas.1319909110 |accessdate=November 5, 2013 }}</ref> 11 billion of these estimated planets may be orbiting sun-like stars.<ref name="LATimes-20131104">{{cite news |last=Khan |first=Amina |title=Milky Way may host billions of Earth-size planets |url=http://www.latimes.com/science/la-sci-earth-like-planets-20131105,0,2673237.story |date=November 4, 2013 |work=[[Los Angeles Times]] |accessdate=November 5, 2013 }}</ref> The nearest such planet may be 12 light-years away, according to the scientists.<ref name="NYT-20131104" /><ref name="PNAS-20131031" />
 
It is estimated that space travel over cosmic distances would take an incredibly long time to an outside observer, and with vast amounts of energy required. However, there are reasons to hypothesize that faster-than-light interstellar space travel might be feasible. This has been explored by NASA scientists since at least 1995.<ref>{{cite journal |last=Crawford|first=I.A. |title=Some Thoughts on the Implications of Faster-Than-Light Interstellar Space Travel |journal=Quarterly Journal of the Royal Astronomical Society |date=Sep 1995|volume=36|issue=3|page=205|bibcode=1995QJRAS..36..205C}}</ref>
 
=== Hypotheses on extraterrestrial sources of illnesses ===
 
Hoyle and Wickramasinghe have speculated that several outbreaks of illnesses on Earth are of extraterrestrial origins, including the [[1918 flu pandemic]], and certain outbreaks of [[polio]] and [[mad cow disease]]. For the 1918 flu pandemic they hypothesized that [[comet]]ary dust brought the virus to Earth simultaneously at multiple locations—a view almost universally dismissed by experts on this pandemic. Hoyle also hypothesized that [[AIDS]] came from outer space.<ref name="Byrne2008">{{cite book|author=Joseph Patrick Byrne|title=Encyclopedia of Pestilence, Pandemics, and Plagues (entry on Panspermia)|url=http://books.google.com/books?id=5Pvi-ksuKFIC&pg=PA454|year=2008|publisher=ABC-CLIO|isbn=978-0-313-34102-1|pages=454–455}}</ref> After Hoyle's death, ''[[The Lancet]]'' published a [[letter to the editor]] from Wickramasinghe and two of his colleagues,<ref>{{cite journal|last=Wickramasinghe|first=C|coauthors=Wainwright, M; Narlikar, J|title=SARS—a clue to its origins?|journal=Lancet|date=May 24, 2003|volume=361|issue=9371|pages=1832|pmid=12781581|doi=10.1016/S0140-6736(03)13440-X}}</ref> in which they hypothesized that the [[virus]] that causes [[severe acute respiratory syndrome]] (SARS) could be extraterrestrial in origin and not originated from chickens. ''The Lancet'' subsequently published three responses to this letter, showing that the hypothesis was not evidence-based, and casting doubts on the quality of the experiments referenced by Wickramasinghe in his letter.<ref>{{cite journal|last=Willerslev|first=E|coauthors=Hansen, AJ; Rønn, R; Nielsen, OJ|title=Panspermia--true or false?|journal=Lancet|date=Aug 2, 2003|volume=362|issue=9381|pages=406; author reply 407–8|pmid=12907025|doi=10.1016/S0140-6736(03)14039-1}}</ref><ref>{{cite journal|last=Bhargava|first=PM|title=Panspermia--true or false?|journal=Lancet|date=Aug 2, 2003|volume=362|issue=9381|pages=407; author reply 407–8|pmid=12907028|doi=10.1016/S0140-6736(03)14041-X}}</ref><ref>{{cite journal|last=Ponce de Leon|first=S|coauthors=Lazcano, A|title=Panspermia--true or false?|journal=Lancet|date=Aug 2, 2003|volume=362|issue=9381|pages=406–7; author reply 407–8|pmid=12907026}}</ref> A 2008 encyclopedia notes that "Like other claims linking terrestrial disease to extraterrestrial pathogens, this proposal was rejected by the greater research community."<ref name="Byrne2008"/>
 
===Case studies===
* A [[meteorite]] originating from [[Mars]] known as [[ALH84001]] was shown in 1996 to contain [[microscopic]] structures resembling small terrestrial [[nanobacteria]]. When the discovery was announced, many immediately conjectured that these were [[fossil]]s and were the first evidence of [[extraterrestrial life]] — making headlines around the world. Public interest soon started to dwindle as most experts started to agree that these structures were not indicative of life, but could instead be formed abiotically from [[organic molecules]]. However, in November 2009, a team of scientists at [[Johnson Space Center]], including David McKay, reasserted that there was "strong evidence that life may have existed on ancient Mars", after having reexamined the meteorite and finding [[magnetite#Biological occurrences|magnetite]] crystals.<ref>{{Cite web| title=New Study Adds to Finding of Ancient Life Signs in Mars Meteorite | url=http://www.nasa.gov/centers/johnson/news/releases/2009/J09-030.html | publisher=[[NASA]] | date=2009-11-30 | accessdate=2009-12-01}}</ref><ref>{{cite journal|author=Thomas-Keprta, K., S. Clemett, D. McKay, E. Gibson and S. Wentworth|year=2009|title=Origin of Magnetite Nanocrystals in Martian Meteorite ALH84001|journal=Geochimica et Cosmochimica Acta|issue=73|pages=6631–6677|doi=10.1016/j.gca.2009.05.064|volume=73|bibcode = 2009GeCoA..73.6631T }}</ref>
* On May 11, 2001, two researchers from the [[University of Naples Federico II|University of Naples]] claimed to have found live extraterrestrial bacteria inside a meteorite. Geologist Bruno D'Argenio and molecular biologist Giuseppe Geraci claim the bacteria were wedged inside the crystal structure of minerals, but were resurrected when a sample of the rock was placed in a culture medium. They believe that the bacteria were not terrestrial because they survived when the sample was sterilized at very high temperature and washed with alcohol. They also claim that the bacteria's DNA is unlike any on Earth.<ref>{{Cite web|url=http://space.newscientist.com/article/dn725 |title=Alien visitors - 11 May 2001 - New Scientist Space |publisher=Space.newscientist.com |date= |accessdate=2009-08-20}}</ref><ref>{{Cite journal|title=Microbes in rocks and meteorites: a new form of life unaffected by time, temperature, pressure |journal=Rendiconti Lincei|date=March 2001|first=Bruno|last=D’Argenio|coauthors=Giuseppe Geraci and Rosanna del Gaudio|volume=12|issue=1|pages=51–68|doi= 10.1007/BF02904521|url=http://www.springerlink.com/content/q3215249n6853188/|format=|accessdate=2009-10-13 }}</ref>  They presented a report on May 11, 2001, concluding that this is the first evidence of extraterrestrial life, documented in its genetic and morphological properties. Some of the bacteria they discovered were found inside meteorites that have been estimated to be over 4.5 billion years old, and were determined to be related to modern day [[Bacillus subtilis]] and Bacillus pumilis bacteria on Earth but appears to be a different strain.<ref>http://www.lincei.it/pubblicazioni/rendicontiFMN/rol/pdf/S2001-01-04.pdf</ref>
* An Indian and British team of researchers led by Chandra Wickramasinghe reported on 2001 that air samples over [[Hyderabad, India]], gathered from the stratosphere by the [[Indian Space Research Organization]], contained clumps of living cells. Wickramasinghe calls this "unambiguous evidence for the presence of clumps of living cells in air samples from as high as 41 km, above which no air from lower down would normally be transported".<ref name="sciam_Wick">{{Cite web|url=http://www.sciam.com/article.cfm?articleID=000D499B-4662-1C60-B882809EC588ED9F |title=Scientists Say They Have Found Extraterrestrial Life in the Stratosphere But Peers Are Skeptical: Scientific American |publisher=Sciam.com |date=2001-07-31 |accessdate=2009-08-20}}</ref><ref name="Narlikar 2003">{{Cite journal|author=Narlikar JV, Lloyd D, Wickramasinghe NC, et al. |title=Balloon experiment to detect micro-organisms in the outer space |journal=Astrophys Space Science |volume=285 |issue=2 |pages=555–62 |year=2003 |doi=10.1023/A:1025442021619 |url=http://www.springerlink.com/content/jp0232r03v023701/?p=3a22f0bdfe244d668927fa1d782a2b8e&pi=27|bibcode = 2003Ap&SS.285..555N }}</ref>  Two bacterial and one fungal species were later independently isolated from these filters which were identified as ''Bacillus simplex'', ''Staphylococcus pasteuri'' and ''Engyodontium album'' respectively.<ref name="Wainwright, 2003">{{Cite web|url=http://meghnad.iucaa.ernet.in/~jvn/FEMS.html |title=Microorganisms cultured from stratospheric air samples obtained at 41km |author=M. Wainwright, N.C. Wickramasinghe, J.V. Narlikar, P. Rajaratnam |accessdate=2007-05-11}}{{Cite journal|author=Wainwright M |title= A microbiologist looks at panspermia |journal=Astrophys Space Science |volume=285 |issue=2 |pages=563–70 |year=2003 |doi=10.1023/A:1025494005689|bibcode = 2003Ap&SS.285..563W }}</ref> The experimental procedure suggested that these were not the result of laboratory contamination, although similar isolation experiments at separate laboratories were unsuccessful.
:A reaction report at [[NASA Ames Research Center|NASA Ames]] indicated skepticism towards the premise that Earth life cannot travel to and reside at such altitudes.<ref>{{Cite news|author=By Richard StengerCNN.com Writer |url=http://archives.cnn.com/2000/TECH/space/11/24/alien.microbe.claim/index.html |title= Space - Scientists discover possible microbe from space  |date= 2000-11-24|accessdate=2009-08-20}}</ref> Max Bernstein, a space scientist associated with [[SETI]] and Ames, argues the results should be interpreted with caution, noting that "it would strain one's credulity less to believe that terrestrial organisms had somehow been transported upwards than to assume that extraterrestrial organisms are falling inward".<ref name="sciam_Wick" />  Pushkar Ganesh Vaidya from the [http://www.astrobiology.co.in/ Indian Astrobiology Research Centre] reported in his 2009 paper that "the three microorganisms captured during the balloon experiment do not exhibit any distinct adaptations expected to be seen in microorganisms occupying a cometary niche".<ref>{{Cite journal|title=Critique on Vindication of Panspermia|journal=Apeiron|date=July 2009|first=Pushkar Ganesh Vaidya|last=|coauthors=|volume=16|issue=3|pages=|id= |url=http://redshift.vif.com/JournalFiles/V16NO3PDF/V16N3VAI.pdf|format=PDF|accessdate=2009-11-28 }}</ref><ref>[http://www.aol.in/news-story/mumbai-scientist-challenges-theory-that-bacteria-came-from-space/569555 Mumbai scientist challenges theory that bacteria came from space]</ref>
* In 2005 an improved experiment was conducted by [[ISRO]]. On April 10, 2005 air samples were collected from six places at different altitudes from the Earth ranging from 20&nbsp;km to more than 40&nbsp;km. The samples were tested at two labs in India. The labs found 12 bacterial and 6 different fungal species in these samples. The fungi were ''[[Penicillium]] decumbens'', ''[[Cladosporium cladosporioides]]'', ''[[Alternaria]] sp.'' and ''Tilletiopsis albescens''. Out of the 12 bacterial samples, three were identified as new species and named ''Janibacter hoyeli.sp.nov'' (after [[Fred Hoyle]]), ''Bacillus isronensis.sp.nov'' (named after ISRO) and ''Bacillus aryabhati'' (named after the ancient Indian mathematician, [[Aryabhata]]). These three new species showed that they were more resistant to [[UV radiation]] than similar bacteria.<ref>''Janibacter hoylei sp. nov.'', ''Bacillus isronensis sp. nov.'' and ''Bacillus aryabhattai sp. nov.'', isolated from cryotubes used for collecting air from upper atmosphere. ''International Journal of Systematic and Evolutionary Microbiology'' 2009. http://ijs.sgmjournals.org/cgi/content/abstract/ijs.0.002527-0v1</ref><ref>[http://www.physorg.com/news156626262.html Discovery of New Microorganisms in the Stratosphere.]</ref>  Atmospheric sampling by NASA in 2010 before and after hurricanes, collected 314 different types of bacteria; the study suggests that large-scale convection during tropical storms and hurricanes can then carry this material from the surface higher up into the atmosphere.<ref name='hurricanes'>{{cite news | first = Timothy Oleson | title = Lofted by hurricanes, bacteria live the high life | date = May 5, 2013 | publisher = Earth Magazine | url = http://www.earthmagazine.org/article/lofted-hurricanes-bacteria-live-high-life | work = NASA | accessdate = 2013-09-21}}</ref><ref>{{cite journal | title = High-flying bacteria spark interest in possible climate effects | journal = Nature News | date = 28 January 2013 | first = Helen Shen| id = | url = http://www.nature.com/news/high-flying-bacteria-spark-interest-in-possible-climate-effects-1.12310 | accessdate = 2013-09-21}}</ref>
{{anchor|NIH-2006}}
[[File:Regression of genome increase.jpg|thumb|right|[[Regression analysis|Regression]] of complexity increase.]]
{{anchor|Complexity}}
* Recent studies, applying the equivalent of [[Moore's Law]] to biological evolution and extrapolating backwards, propose that [[life]] began "{{val|9.7|2.5}} billion years ago", billions of years before the [[Earth]] was formed.<ref name="arXiv-20130328">{{cite journal |last1=Sharov |first1=Alexei A. |last2=Gordon |first2=Richard |title=Life Before Earth |url=http://arxiv.org/ftp/arxiv/papers/1304/1304.3381.pdf |date=28 March 2013 |journal=[[arXiv]] |arxiv=1304.3381v1 |accessdate=16 April 2013 }}</ref><ref name="NIH-20060612">{{cite journal |last=Sharov |first=Alexei A. |title=Genome increase as a clock for the origin and evolution of life |journal=[[Biology Direct]] |volume=1 |pages=1–17 |date=12 June 2006 |issue=  |doi=10.1186/1745-6150-1-17 |pmc=1526419 }}</ref> In the case of [[evolution]], empirical evidence suggested a doubling of complexity every 376 million years. As the age of trees can be measured by the number of rings, the [[hypothesis]] that the age of life could be measured by biological [[complexity]] (i.e., the length of functional non-redundant [[DNA]] in the [[genome]]) was studied.<ref name="arXiv-20130328" /><ref name="NIH-20060612" /> If log-transformed complexity is plotted against the time of origin of large evolutionary lineages, then the points fit to a straight line (see figure). The exponential increase in complexity can be explained by a positive self-activating [[Feedback loop#Biology|feed back loop]].<ref name="NIH-20060612" /> The regression line hits zero (i.e., one [[nucleotide]]) at "{{val|9.7|2.5}} billion years ago".<ref name="arXiv-20130328" /> If this model is correct, and since our [[Solar System]] is 4.6 billion years ago,<ref name="NG-2010">The date is based on the oldest [[inclusion (mineral)|inclusions]] found to date in [[meteorite]]s, and is thought to be the date of the formation of the first solid material in the collapsing [[nebula]].<br>{{cite journal |last1=Bouvier |first1=A. |last2=Wadhwa |first2=M. |title=The age of the solar system redefined by the oldest Pb-Pb age of a meteoritic inclusion |url=http://www.nature.com/ngeo/journal/v3/n9/abs/ngeo941.html |journal=[[Nature Geoscience]] |volume=3 |pages=637–641 |date=22 August 2010 |doi=10.1038/NGEO941 |accessdate=17 April 2013 |issue=9 |bibcode = 2010NatGe...3..637B }}</ref> then life somehow arrived to Earth from older stellar systems. This hypothesis was criticized by [[Eugene Koonin]] who suggested that the rates of early biological evolution might have been much faster due to the absence of competition on early Earth.<ref name="Koonin-2002">{{cite book |author=Koonin, E. V. |coauthors=Galperin, M. Y. |title=Sequence - Evolution - Function: Computational Approaches in Comparative Genomics |url=http://www.springer.com/life+sciences/animal+sciences/book/978-1-4020-7274-1 |publisher=[[Springer Verlag]] |year=2002 }}</ref> [[Chris Adami]] argued that "it is inconceivable that life began with just a few nucleotides" (see discussion<ref name="NIH-20060612" />). To answer this criticism, Sharov proposed a hypothetical [[abiogenesis]] scenario that starts from [[Coenzyme|coenzyme-like molecules]] that are functionally equivalent to single [[nucleotides]].<ref name="Sharov-2009">{{cite journal |author=Sharov, A.A. |title=Coenzyme autocatalytic network on the surface of oil microspheres as a model for the origin of life |url=http://www.mdpi.com/1422-0067/10/4/1838 |journal=[[International Journal of Molecular Sciences]] |volume=10 |issue= 4|pages =1838–1852 |date=22 April 2009 |doi=10.3390/ijms10041838  }}</ref><ref name="Coenzyme-2011">{{cite book |last=Raffaelli |first=Nadia |title=Origins of Life: The Primal Self-Organization - Nicotinamide Coenzyme Synthesis: A Case of Ribonucleotide Emergence or a Byproduct of the RNA World? |url=http://link.springer.com/content/pdf/10.1007/978-3-642-21625-1_9 |year=2011 |pages=185–208 |publisher=[[Springer Berlin Heidelberg]] |isbn=978-3-642-21624-4  |doi=10.1007/978-3-642-21625-1_9 |accessdate=17 April 2013 }}</ref> (also see [[Abiogenesis#Coenzyme world]] and [[Rare Earth hypothesis#Enough time elapsed since the big bang for evolution to occur]])
*A NASA research group found a small number of ''[[Streptococcus mitis]]'' bacteria living inside the camera of the [[Surveyor 3]] spacecraft when it was brought back to Earth by [[Apollo 12]]. They believed that the bacteria survived since the time of the craft's launch to the Moon.<ref>{{Cite web| url=http://www.lpi.usra.edu/lunar/missions/apollo/apollo_12/experiments/surveyor/| title=Apollo 12 Mission|publisher=Lunar and Planetary Institute|accessdate=2008-02-15}}</ref> However, these reports are disputed by Leonard D. Jaffe, who was Surveyor program scientist and custodian of the Surveyor 3 parts brought back from the Moon, stated in a letter to the Planetary Society that an unnamed member of his staff reported that a "breach of sterile procedure" took place at just the right time to produce a false positive result.<ref>{{Cite web| url=http://www.astrobio.net/exclusive/1311/apollo-12-remembered| title=Apollo 12 Remembered|publisher=Astrobiology Magazine (online 21 Nov 2004)|accessdate=2011-02-05}}</ref> NASA was funding an archival study in 2007 that was trying to locate the film of the camera-body microbial sampling to confirm the report of a breach in sterile technique. NASA currently stands by its original assessment: see [[Reports of Streptococcus mitis on the moon]].{{Citation needed|date=January 2011}}
 
*On January 10, 2013, [[Chandra Wickramasinghe]] reported in the [[fringe science]] [[Journal of Cosmology]], of shapes resembling fossil [[diatom]] [[frustule]]s in a new carbonaceous meteorite called [[Polonnaruwa (meteorite)|Polonnaruwa]] that landed in the North Central Province of Sri Lanka on 29 December 2012.<ref name="JCosmology-20130110">{{Cite journal |authors=Samaranayake, N.C. et al. |title=Fossil Diatoms in a New Carbonaceous Meteorite |date=January 10, 2013 |url=http://www.buckingham.ac.uk/wp-content/uploads/2011/09/Polonnaruwa-meteorite.pdf |publisher=[[Journal of Cosmology]] |volume=21 |pages=1–14 |accessdate=January 14, 2013 |issue=37 |bibcode=2013arXiv1303.2398W |arxiv=1303.2398 |journal=Journal of Cosmology, Vol (), January }}</ref> Early on, there was criticism that that Wickramasinghe's article was not an examination of the Polonnaruwa meteorite but of some terrestrial rock passed off as the meteorite.<ref name='Slate'>{{cite news | first = [[Phil Plait]] | title = No, Diatoms Have Not Been Found in a Meteorite | date = 15 January 2013 | url = http://www.slate.com/blogs/bad_astronomy/2013/01/15/life_in_a_meteorite_claims_by_n_c_wickramasinghe_of_diatoms_in_a_meteorite.html | work = Slate.com - Astronomy | accessdate = 2013-01-16}}</ref>
 
:Wickramasinghe's team remark that they are aware that a large number of unrelated stones have been submitted for analysis, and have no knowledge regarding the nature, source or origin of the stones their critics have examined, so Wickramasinghe clarifies that he is using the stones submitted by the Medical Research Institute in Sri Lanka.<ref name='isotopes'>{{cite journal | title = The Polonnaruwa meteorite: oxygen isotope, crystalline and biological composition | journal = [[Journal of Cosmology]] | date = 5 March 2013 | authors=Wallis, Jamie et al. | volume = 22 | issue = 2 | url = http://journalofcosmology.com/JOC22/Paper22%282%29.pdf | accessdate = 2013-03-07 | bibcode = 2013arXiv1303.1845W | pages = 1845 | arxiv = 1303.1845 }}</ref> In response to the criticism from other scientists, Wickramasinghe performed [[X-ray diffraction]] <ref name='authenticity'>{{cite journal | title = Authenticity of the life-bearing Polonnaruwa meteorite | journal = [[Journal of Cosmology]] | date = 4 February 2013 | first = N.C. | last = N.C. Wickramasinghe | coauthors = J. Wallis, N. Miyake, Anthony Oldroyd, D.H. Wallis, Anil Samaranayake, K. Wickramarathne , Richard B. Hoover and M.K. Wallis | url = http://journalofcosmology.com/JOC21/Polonnaruwa5R.pdf | accessdate = 2013-02-04}}</ref> and [[isotope]]<ref name='isotopes'/> analyses to verify its meteoritic origin. His analysis revealed a 95% [[silica]] and 3% [[quartz]] content,<ref name='authenticity'/> and interpreted this result as a "[[Carbonaceous chondrite|carbonaceous meteorite]] of unknown type".<ref name='authenticity'/>  In addition, Wickramasinghe's team remarked that the temperature at which sand must be heated by lightning to melt and form a fulgurite (1770 °C) would have vaporized and burned all carbon-rich organisms and melted and thus destroyed the delicately marked silica frustules of the diatoms,<ref name='isotopes'/> and that the oxygen isotope data confirms its meteoric origin.<ref name='isotopes'/> Wickramasinghe's team also argues that since living diatoms require [[nitrogen fixation]] to synthetize amino acids, proteins, DNA, RNA and other life-critical biomolecules, a population of extraterrestrial [[cyanobacteria]] must have been a required component of the comet (Polonnaruwa meteorite) "ecosystem".<ref name='isotopes'/>
 
* In 2013, Dale Warren Griffin, a microbiologist working at the United States Geological Survey noted that viruses are the most numerous entities on Earth. Griffin speculates that viruses evolved in comets and on other planets and moons may be pathogenic to humans, so he proposed to also look for viruses on moons and planets of the Solar System.<ref name="Astro-20130814">{{cite journal |last=Griffin| first=Dale Warren |title=The Quest for Extraterrestrial Life: What About the Viruses? |url=http://online.liebertpub.com/doi/abs/10.1089/ast.2012.0959  |journal=[[Astrobiology (journal)|Astrobiology]] |date=14 August 2013 |volume=13 |issue=8 |pages=774–783 |doi=10.1089/ast.2012.0959 |accessdate=12 September 2013|bibcode = 2013AsBio..13..774G }}</ref>
 
===Hoaxes===
A separate fragment of the [[Orgueil (meteorite)|Orgueil]] meteorite (kept in a sealed glass jar since its discovery) was found in 1965 to have a seed capsule embedded in it, whilst the original glassy layer on the outside remained undisturbed. Despite great initial excitement, the seed was found to be that of a European [[Juncaceae]] or Rush plant that had been glued into the fragment and camouflaged using [[coal dust]]. The outer "fusion layer" was in fact glue. Whilst the perpetrator of this hoax is unknown, it is thought he sought to influence the 19th century debate on [[spontaneous generation]] — rather than panspermia — by demonstrating the transformation of inorganic to biological matter.<ref>Edward Anders, Eugene R. DuFresne,Ryoichi Hayatsu, Albert Cavaille, Ann DuFresne, and Frank W. Fitch. "Contaminated Meteorite," ''Science, New Series'', Volume 146, Issue 3648 (Nov.27, 1964), 1157-1161.</ref>
 
==Extremophiles==
{{see also|Extremophile}}
[[Image:Blacksmoker in Atlantic Ocean.jpg|thumb|left|[[Hydrothermal vent]]s are able to support [[extremophile|extremophile bacteria]] on [[Earth]] and may also support life in other parts of the cosmos.]]
Until the 1970s, [[life]] was believed to depend on its access to [[sunlight]]. Even life in the ocean depths, where sunlight cannot reach, was believed to obtain its nourishment either from consuming organic detritus rained down from the surface waters or from eating animals that did.<ref name="smoker">{{cite web |title= Black Smokers and Giant Worms |author=Chamberlin, Sean |year=1999 |work=Fullerton College |url=http://www.courseworld.com/ocean/smokers.html |accessdate=11 February 2011 }}</ref> However, in 1977, during an exploratory dive to the [[Galapagos Rift]] in the deep-sea exploration submersible ''[[DSV Alvin|Alvin]]'', scientists discovered colonies of assorted creatures clustered around undersea volcanic features known as [[black smoker]]s.<ref name="smoker"/>  It was soon determined that the basis for this food chain is a form of [[bacterium]] <!--Find name/species.-->that derives its energy from [[Redox|oxidation]] of reactive chemicals, such as [[hydrogen]] or [[hydrogen sulfide]], that bubble up from the Earth's interior. This [[chemosynthesis]] revolutionized the study of biology by revealing that terrestrial life need not be Sun-dependent; it only requires water and an energy gradient in order to exist.
 
It is now known that [[extremophiles]], microorganisms with extraordinary capability to thrive in the harshest environments on Earth, can specialize to thrive in the deep-sea,<ref name="LS-20130317">{{cite web |last=Choi |first=Charles Q. |title=Microbes Thrive in Deepest Spot on Earth |url=http://www.livescience.com/27954-microbes-mariana-trench.html |date=17 March 2013| publisher=[[LiveScience]] |accessdate=17 March 2013 }}</ref><ref name="LS-20130314">{{cite web |last=Oskin|first=Becky |title=Intraterrestrials: Life Thrives in Ocean Floor| url=http://www.livescience.com/27899-ocean-subsurface-ecosystem-found.html |date=14 March 2013|publisher=[[LiveScience]] |accessdate=17 March 2013 }}</ref><ref name="NG-20130317">{{cite journal |last1=Glud|first1=Ronnie |last2=Wenzhöfer |first2=Frank |last3=Middelboe |first3=Mathias |last4=Oguri |first4=Kazumasa|last5=Turnewitsch |first5=Robert |last6=Canfield |first6=Donald E. |last7=Kitazato |first7=Hiroshi |title=High rates of microbial carbon turnover in sediments in the deepest oceanic trench on Earth|url=http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo1773.html |doi=10.1038/ngeo1773 |date=17 March 2013 |journal=[[Nature Geoscience]] |accessdate=17 March 2013 |volume=6 |issue=4 |pages=284 |bibcode = 2013NatGe...6..284G }}</ref> ice, boiling water, acid, the water core of nuclear reactors, salt crystals, toxic waste and in a range of other extreme habitats that were previously thought to be inhospitable for life.<ref>{{cite web| url= http://www.livescience.com/animalworld/050207_extremophiles.html |title=Wild Things: The Most Extreme Creatures |accessdate=2008-10-20 | last=Carey |first=Bjorn |date=7 February 2005 |work=Live Science }}</ref><ref name=Cavicchioli >{{cite journal |title=Extremophiles and the search for extraterrestrial life |journal=Astrobiology|date=Fall 2002 |first=R. | last=Cavicchioli |coauthors= |volume=2 |issue=3|pages=281–92 |pmid=12530238 |format= |doi=10.1089/153110702762027862 |bibcode = 2002AsBio...2..281C }}</ref><ref name="adsabs.harvard.edu">[http://adsabs.harvard.edu/abs/2008cosp...37.2602R The BIOPAN experiment MARSTOX II of the FOTON M-3 mission] July 2008.</ref><ref name="Surviving the Final Frontier">[http://www.astrobio.net/exclusive/318/surviving-the-final-frontier Surviving the Final Frontier]. 25 November 2002.</ref>  Living bacteria found in ice core samples retrieved from {{convert|3700|m|ft}} deep at [[Lake Vostok]] in [[Antarctica]], have provided data for extrapolations to the likelihood of microorganisms surviving frozen in extraterrestrial habitats or during interplanetary transport.<ref>{{cite web | url = http://etd.ohiolink.edu/view.cgi?acc_num=osu1015965965 | title = Detection, recovery, isolation, and characterization of bacteria in glacial ice and Lake Vostok accretion ice | accessdate = 2011-02-04 | first = Christner, Brent C. | year = 2002 | work = Ohio State University}}</ref> Also, bacteria have been discovered living within warm rock deep in the Earth's crust.<ref name=Nanjundiah2000>{{Cite journal| last = Nanjundiah | first =  V. | year = 2000 | title = The smallest form of life yet? | journal = Journal of Biosciences | volume = 25 | issue = 1 | pages = 9–10 | url = http://eprints.iisc.ernet.in/archive/00001799/01/25smallest25(1)-9to10mar2000.pdf | doi = 10.1007/BF02985175 | pmid = 10824192 | postscript = <!--None-->}}</ref>
 
In order to test some these organism's potential resilience in outer space, [[Spermatophyte|plant seeds]] and [[spores]] of [[Endospore|bacteria]], [[fungus|fungi]] and [[fern]]s have been exposed to the harsh space environment.<ref name="adsabs.harvard.edu"/><ref name="Surviving the Final Frontier"/><ref name='Rabbow'>{{cite journal | title = EXPOSE, an Astrobiological Exposure Facility on the International Space Station - from Proposal to Flight | journal = Orig Life Evol Biosph | date = 9 July 2009 | first = Elke Rabbow | coauthors = Gerda Horneck, Petra Rettberg, Jobst-Ulrich Schott, Corinna Panitz, Andrea L’Afflitto, Ralf von Heise-Rotenburg, Reiner Willnecker, Pietro Baglioni, Jason Hatton, Jan Dettmann, René Demets and Günther Reitz. | url = http://www.prism.gatech.edu/~alafflitto3/Documents/Rabbow_Horneck_LAfflitto_Origin_of_Life_and_Evolution_of_Biosphere.pdf | format = PDF | accessdate = 2013-07-08 | doi = 10.1007/s11084-009-9173-6 | last1 = Rabbow | volume = 39 | issue = 6 | pages = 581–98 | pmid = 19629743}}</ref>  Spores are produced as part of the normal life cycle of many [[plant]]s, [[algae]], [[fungus|fungi]] and some [[protozoan]]s, and some bacteria produce [[endospores]] or [[Microbial cyst|cysts]] during times of stress. These structures may be highly resilient to [[ultraviolet radiation|ultraviolet]] and [[gamma radiation]], [[desiccation]], [[lysozyme]], [[temperature]], [[starvation]] and chemical [[disinfectants]], while [[metabolism|metabolically]] inactive. Spores [[Germination|germinate]] when favourable conditions are restored after exposure to conditions fatal to the parent organism.
 
Although computer models suggest that a captured meteoroid would typically take some tens of millions of years before collision with a neighboring  solar system planet,<ref name='Belbruno2012'/> there are documented viable Earthly bacterial spores that are 40 million years old that are very resistant to radiation,<ref name='Belbruno2012'/><ref name="BBC-2011"/> and others able to resume life after being dormant for 25 million years,<ref>[http://commtechlab.msu.edu/sites/dlc-me/news/ns595ap1.html Bacterium revived from 25 million year sleep] Digital Center for Microbial Ecology</ref> suggesting that lithopanspermia life-transfers are possible via meteorites exceeding 1m in size.<ref name='Belbruno2012'/>
 
The discovery of deep-sea [[ecosystem]]s, along with advancements in the fields of [[astrobiology]], observational [[astronomy]] and discovery of large varieties of extremophiles, opened up a new avenue in astrobiology by massively expanding the number of possible [[Planetary habitability|extraterrestrial habitats]] and possible transport of hardy microbial life through vast distances.<ref name="Experimental methods" />
 
===Research in outer space===
General panspermia requires that life survive transfer through space, however, biology and chemistry, as opposed to physics, do not admit ideological contexts: either the biological phenomena are real, or they are abstract.<ref name=BC>{{cite web | url = http://biocab.org/Astrobiology.html | title = Astrobiology | accessdate = 2011-01-17 | date = 26 September 2006 | publisher = Biology Cabinet| archiveurl= http://web.archive.org/web/20101212184044/http://biocab.org/Astrobiology.html| archivedate= 12 December 2010 }}</ref> Biologists cannot say that a process or phenomenon, by being mathematically possible, have to exist forcibly in the real nature. For biologists, the ground of speculations is well noticeable, and biologists specify what is speculative and what is not.<ref name=BC/> The question of whether certain [[microorganism]]s can survive in the harsh environment of outer space has intrigued biologists since the beginning of spaceflight, and opportunities were provided to expose samples to space.
 
The first tests were made in 1966, during the [[Gemini IX]] and [[Gemini XII|XII]] missions, when samples of [[bacteriophage]] T1 and spores of ''[[Penicillium roqueforti]]'' were exposed to outer space for 16.8 h and 6.5 h, respectively.<ref name='Gerda Horneck'/><ref name="Experimental methods"/> Other basic life sciences research in [[low Earth orbit]] started in 1966 with the Soviet biosatellite program [[Bion (satellite)|Bion]] and the U.S. [[Biosatellite program]]. Thus, the plausibility of panspermia can be evaluated by examining life forms on Earth for their capacity to survive in space.<ref>{{cite journal | title = The origin of life, panspermia and a proposal to seed the Universe | journal = Plant Science | date = December 2008 | first = David Tepfer | volume = 175 | issue = 6 | pages = 756–760 | doi = 10.1016/j.plantsci.2008.08.007 | last1 = Tepfer}}</ref> The following experiments carried on [[low Earth orbit]] specifically tested some aspects of panspermia or lithopanspermia:
 
====ERA====
[[File:STS-46 EURECA deployment.jpg|thumb|[[European Retrievable Carrier|EURECA]] facility deployment in 1992]]
The [[Exobiology Radiation Assembly]] (ERA) was a 1992 experiment on board the [[European Retrievable Carrier]] (EURECA) on the biological effects of [[space radiation]]. EURECA was an unmanned 4.5 tonne satellite with a payload of 15 experiments.<ref>{{cite web | url = http://nssdc.gsfc.nasa.gov/nmc/experimentDisplay.do?id=1992-049B-03 | title = Exobiology and Radiation Assembly (ERA) | accessdate = 2013-07-22 | year = 1992 | work = [[ESA]] | publisher = NASA}}</ref>  It was an [[astrobiology]] mission developed by the [[European Space Agency]] (ESA). [[Spore]]s of different strains of ''[[Bacillus subtilis]]'' and the ''[[Escherichia coli]]'' [[plasmid]] [[pUC19]] were exposed to selected conditions of space (space vacuum and/or defined wavebands and intensities of solar ultraviolet radiation). After the approximately 11 month mission, their responses were studied in terms of survival, [[mutagenesis]] in the ''his'' (''B. subtilis'') or ''[[lac operon|lac]]'' [[Locus (genetics)|locus]] (pUC19), induction of [[DNA]] strand breaks, efficiency of [[DNA repair]] systems, and the role of external protective agents. The data were compared with those of a simultaneously running ground control experiment:<ref name=Dose>{{cite journal | title = ERA-experiment "space biochemistry" | journal = Advances in Space Research | first = K. Dose | coauthors = A. Bieger-Dose, R. Dillmann, M. Gill, O. Kerz, A. Klein, H. Meinert, T. Nawroth, S. Risi , C. Stride., | volume = 16 | issue = 8 | year= 1995 |pages = 119–129 |doi = 10.1016/0273-1177(95)00280-R | last1 = Zhang | pmid = 11542696 }}</ref><ref name='Horneck'>{{cite journal | title = Biological responses to space: results of the experiment "Exobiological Unit" of ERA on EURECA I | journal = Adv Space Res. | year = 1995 | first = Horneck G | coauthors = Eschweiler U, Reitz G, Wehner J, Willimek R, Strauch K. | volume = 16 | issue = 8 | pages = 105–18| pmid = 11542695 | bibcode = 1995AdSpR..16..105V |doi = 10.1016/0273-1177(95)00279-N | last1 = Vaisberg }}</ref>
 
*The survival of spores treated with the vacuum of space, however shielded against solar radiation, is substantially increased, if they are exposed in multilayers and/or in the presence of [[glucose]] as protective.
*All spores in "artificial meteorites", i.e. embedded in [[clay]]s or simulated [[Martian soil]], are killed.
*Vacuum treatment leads to an increase of [[mutation frequency]] in spores, but not in [[plasmid]] DNA.
*Extraterrestrial solar [[ultraviolet]] radiation is [[mutagen]]ic, induces strand breaks in the DNA and reduces survival substantially.
*Action [[spectroscopy]] confirms results of previous space experiments of a synergistic action of  [[Vacuum#Outer space|space vacuum]] and [[Ultraviolet|solar UV radiation]] with [[DNA]] being the critical target.
*The decrease in viability of the microorganisms could be correlated with the increase in [[DNA repair#DNA damage|DNA damage]].
*The purple membranes, amino acids and urea were not measurably affected by the dehydrating condition of open space, if sheltered from solar radiation. Plasmid DNA, however, suffered a significant amount of strand breaks under these conditions.<ref name=Dose/>
 
====BIOPAN====
[[BIOPAN]] is a multi-user experimental facility installed on the external surface of the Russian [[Foton (space programs)|Foton descent capsule]]. Experiments developed for BIOPAN are designed to investigate the effect of the space environment on biological material after exposure between 13 to 17 days.<ref name='Kayser'>{{cite web | url = http://www.kayser.it/index.php/life-science/facilities/biopan | title = BIOPAN Pan for exposure to space environment | accessdate = 2013-07-17 | year = 2013 | work = Kayser Italia}}</ref> The experiments in BIOPAN are exposed to [[Insolation|solar]] and [[Cosmic ray|cosmic radiation]], the space vacuum and weightlessness, or a selection thereof. Of the 6 missions flown so far on BIOPAN between 1992 and 2007, dozens of experiments were conducted, and some analyzed the likelihood of panspermia. Some bacteria, [[lichen]]s (''[[Xanthoria elegans]]'', ''[[Rhizocarpon geographicum]]'' and their mycobiont cultures, the black Antarctic microfungi ''Cryomyces minteri'' and ''Cryomyces antarcticus''), spores, and even one animal ([[tardigrades]]) were found to have survived the harsh outer space environment and [[cosmic radiation]].<ref name=Foton >{{cite journal |bibcode=2008cosp...37..660D |title=Experiment lithopanspermia: Test of interplanetary transfer and re-entry process of epi- and endolithic microbial communities in the FOTON-M3 Mission |author1=De La Torre Noetzel |first1=Rosa |volume=37 |year=2008 |pages=660 |journal=37th COSPAR Scientific Assembly. Held 13–20 July 2008}}</ref><ref name=Foton-M3>{{Cite web|url=http://www.congrex.nl/08a09/Sessions/26-06%20Session%202a.htm |title=Life in Space for Life ion Earth - Biosatelite Foton M3 |accessdate=2009-10-13 |date=June 26, 2008 }}</ref><ref name=Tardigrades >{{Cite journal|title=Tardigrades survive exposure to space in low Earth orbit |journal=Current Biology| date=9 September 2008| first=K. Ingemar Jönsson|last=| coauthors=Elke Rabbow, Ralph O. Schill, Mats Harms-Ringdahl and Petra Rettberg|volume=18 |issue=17| pages=R729–R731|doi= 10.1016/j.cub.2008.06.048 | pmid=18786368| last1=Jönsson }}</ref><ref>{{Cite journal | url = http://www.researchgate.net/publication/224992640_ESA-space_experiments_from_BIOPAN_6_experiment_Lithopanspermia_to_EXPOSE | title = COSPAR 2010 Conferene | author = de Vera, J.P.P. et al. |publisher = Research Gate | year = 2010| id = | accessdate = 2013-07-17 | postscript = <!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}}}}</ref>
 
====EXOSTACK====
[[File:1990 s32 LDEF stow.jpg|thumb|left|EXOSTACK on the [[Long Duration Exposure Facility]] satellite.]]
 
The German [[EXOSTACK]] experiment was deployed in 7 April 1984 on board the [[Long Duration Exposure Facility]] statellite.  30% of ''[[Bacillus subtilis]]'' [[spores]] survived the nearly 6 years exposure when embedded in salt crystals, whereas 80% survived in the presence of glucose, which stabilize the structure of the cellular macromolecules, especially during vacuum-induced dehydration.<ref name='Gerda Horneck'>{{cite journal |doi=10.1128/MMBR.00016-09 |title=Space Microbiology |year=2010 |last1=Horneck |first1=G. |last2=Klaus |first2=D. M. |last3=Mancinelli |first3=R. L. |journal=Microbiology and Molecular Biology Reviews |volume=74 |pages=121–56 |pmid=20197502 |issue=1 |pmc=2832349}}</ref><ref name='Clancy'>{{cite book | last1 = Paul Clancy | title = Looking for Life, Searching the Solar System | publisher = Cambridge University Press | date = Jun 23, 2005 | accessdate = 2013-07-29}}{{page needed|date=November 2013}}</ref>
 
If shielded against solar [[UV]], spores of ''B. subtilis'' were capable of surviving in space for up to 6 years, especially if embedded in clay or meteorite powder (artificial meteorites). The data support the likelihood of interplanetary transfer of microorganisms within meteorites, the so-called [[lithopanspermia]] hypothesis.<ref name='Gerda Horneck'/>
 
====EXPOSE====
[[File:EXPOSE location on the ISS.jpg|thumb|Location of the astrobiology EXPOSE-E and EXPOSE-R facilities on the [[International Space Station]]]]
[[EXPOSE]] is a multi-user facility mounted outside the [[International Space Station]] dedicated to [[astrobiology]] experiments.<ref name='Rabbow'/> Results from the orbital mission, especially the experiments ''SEEDS''<ref>{{cite journal | title = Survival of Plant Seeds, Their UV Screens, and nptII DNA for 18 Months Outside the International Space Station | journal = Astrobiology | date = May 2012 | first = David Tepfer | coauthors = Andreja Zalar, and Sydney Leach. | volume = 12 | issue = 5 | pages = 517–528 |bibcode = 2012AsBio..12..517T |doi = 10.1089/ast.2011.0744 | last1 = Tepfer | pmid = 22680697 }}</ref> and ''LiFE'',<ref name='LiFE results'>{{cite journal | title = LIFE Experiment: Isolation of Cryptoendolithic Organisms from Antarctic Colonized Sandstone Exposed to Space and Simulated Mars Conditions on the International Space Station | journal = Origins of Life and Evolution of Biospheres | date = 1 June 2012 |first =  Giuliano Scalzi | coauthors = Laura Selbmann, Laura Zucconi, Elke Rabbow, Gerda Horneck, Patrizia Albertano, Silvano Onofri. | volume = 42 | issue = 2 – 3 | pages = 253–262| doi = 10.1007/s11084-012-9282-5 | last1 = Scalzi}}</ref> concluded that after an 18-month exposure, some seeds and lichens (''[[Stichococcus]] sp.''  and ''[[Acarospora]] sp''., a lichenized fungal genus) may be capable to survive interplanetary travel if sheltered inside comets or rocks from [[cosmic radiation]] and [[UV]] radiation.<ref name='Rabbow'>{{cite journal | title = EXPOSE, an Astrobiological Exposure Facility on the International Space Station - from Proposal to Flight | journal = Orig Life Evol Biosph | date = 9 July 2009 | first = Elke Rabbow | coauthors = Gerda Horneck, Petra Rettberg, Jobst-Ulrich Schott, Corinna Panitz, Andrea L’Afflitto, Ralf von Heise-Rotenburg, Reiner Willnecker, Pietro Baglioni, Jason Hatton, Jan Dettmann, René Demets and Günther Reitz | doi = 10.1007/s11084-009-9173-6 | last1 = Rabbow | volume = 39 | issue = 6 | pages = 581–98 | pmid = 19629743|bibcode = 2009OLEB...39..581R }}</ref><ref>{{cite journal | title = Survival of Rock-Colonizing Organisms After 1.5 Years in Outer Space | journal = Astrobiology | date = May 2012 | first = Silvano Onofri | coauthors = Rosa de la Torre, Jean-Pierre de Vera, Sieglinde Ott, Laura Zucconi, Laura Selbmann, Giuliano Scalzi,1, Kasthuri J. Venkateswaran, Elke Rabbow, Francisco J. Sánchez Iñigo, and Gerda Horneck. | volume = 12 | issue = 5 | pages = 508–516 | doi = 10.1089/ast.2011.0736 | last1 = Onofri | pmid = 22680696|bibcode = 2012AsBio..12..508O }}</ref>  The survival of some lichen species in space has also been characterized in simulated laboratory experiments.<ref name="Skymania-20120426">{{cite web |last=Baldwin |first=Emily |title=Lichen survives harsh Mars environment |url=http://www.skymania.com/wp/2012/04/lichen-survives-harsh-martian-setting.html |date=26 April 2012 |publisher=Skymania News |accessdate=27 April 2012 }}</ref><ref name="EGU-20120426">{{cite web |last1=de Vera |first1=J.-P. |last2=Kohler |first2=Ulrich |title=The adaptation potential of extremophiles to Martian surface conditions and its implication for the habitability of Mars |url=http://media.egu2012.eu/media/filer_public/2012/04/05/10_solarsystem_devera.pdf |date=26 April 2012 |publisher=[[European Geosciences Union]] |accessdate=27 April 2012 }}</ref>
 
A separate experiment on EXPOSE called Beer was designed to find microbes that could be used in life-support recycling equipment and future "bio-mining" projects on Mars. A group of microbes called OU-20 resembling [[cyanobacteria]] genus ''[[Gloeocapsa]]'' survived 553 days exposure outside the ISS.<ref name=Beer>{{cite news|last=Amos|first=Jonathan|title=Beer microbes live 553 days outside ISS|url=http://www.bbc.co.uk/news/science-environment-11039206|accessdate=31 July 2013|newspaper=BBC News - Science and Technology|date=23 August 2010}}</ref>
<!--
====O/OREOS====
Still searching for the published results of the SESLO experiment on O/OREOS.-->
 
====Rosetta====
In 2014, the ''[[Rosetta (spacecraft)|Rosetta]]'' spacecraft will arrive at COMET [[67P/Churyumov–Gerasimenko]]. A few months after arriving at the comet, ''Rosetta'' will release a small lander onto its surface. Then, for almost two years it will investigate Churyumov-Gerasimenko from close up. Rosetta's Project Scientist, Gerhard Schwehm, stated that sterilization is generally not crucial since comets are usually regarded as objects where prebiotic molecules can be found, but not living microorganisms.<ref>{{cite web | url = http://sci.esa.int/home/30313-no-bugs-please-this-is-a-clean-planet/ | title = Nol bugs please, this is a clean planet! | accessdate = 2013-07-16 | date = 30 July 2002 | publisher = European Space Agency (ESA)}}</ref>  Notwithstanding, other scientists think it will be an opportunity to gather evidence for one of panspermia's hypotheses: the possibility of both active and dormant microbes inside comets.<ref name="Hoyle, F 1981. pp35-49"/><ref name="Wickramasinghe, J. 2010. pp 137-154"/>
 
====Phobos LIFE====
The ''Phobos LIFE'' or ''[[Living Interplanetary Flight Experiment]]'', was developed by the [[Planetary Society]] and intended to send selected microorganisms on a three-year interplanetary round-trip in a small capsule aboard the Russian [[Fobos-Grunt]] [[spacecraft]] in 2011. Unfortunately, the spacecraft suffered technical difficulties soon after launch and fell back to Earth, so the experiment was never carried out.  The experiment would have tested one aspect of panspermia: lithopanspermia, the hypothesis that life could survive space travel, if protected inside rocks blasted by impact off one planet to land on another.<ref>{{Cite web|url=http://www.planetary.org/programs/projects/life/ |title=LIFE Experiment |publisher=Planetary.org |date= |accessdate=2009-08-20}}</ref><ref>{{Cite web|url=http://www.lpi.usra.edu/meetings/phobosdeimos2007/pdf/7043.pdf |title=Living interplanetary flight experiment: an experiment on survivability of microorganisms during interplanetary transfer |format=PDF |date= |accessdate=2009-08-20}}</ref><ref name=planetary-life>{{cite web |url=http://www.planetary.org/programs/projects/innovative_technologies/life/ |title=Projects: LIFE Experiment: Phobos |publisher=[[The Planetary Society]] |accessdate=2 April 2011}}</ref><ref name=Zak1>{{cite web |url=http://www.airspacemag.com/space-exploration/Mission_Possible.html?c=y&page=4 |first=Anatoly |last=Zak |title=Mission Possible |date=1 September 2008 |work=[[Air & Space Magazine]] |publisher=[[Smithsonian Institution]] |accessdate=26 May 2009}}</ref>
 
<!--Reviewing and researching this section:
==Criticism==
Panspermia is heavily criticized because it does not answer the question of the [[origin of life]] but merely places it on another celestial body without explanation. It was also criticized because it could not be tested experimentally. Furthermore, it was suggested that single spores will not survive the physical forces imposed on them in space.<ref>Nussinov, M. D., and S. V. Lysenko. 1983. ''Cosmic vacuum prevents radiopanspermia.'' Orig. Life 13:153-164.</ref> As a result, panspermia fell into oblivion.
 
The concept of panspermia was revived when technology provided the opportunity to study the survival of bacterial spores ín the harsh environment of space.<ref name='Experimental methods'/> Wallis and Wickramasinghe argued in 2004 that the transport of individual bacteria or clumps of bacteria, is overwhelmingly more important than lithopanspermia in terms of numbers of microbes transferred, even accounting for the death rate of unprotected bacteria in transit.<ref>{{cite journal|last=Wickramasinghe|first=M.K.|coauthors=Wickramasinghe, C.|title=Interstellar transfer of planetary microbiota,|journal=Mon. Not.R. Astr. Soc.|year=2004|volume=348|pages=52–57|bibcode=2004MNRAS.348...52W|doi=10.1111/j.1365-2966.2004.07355.x}}</ref> Then it was found that isolated spores of ''[[B. subtilis]]'' were killed by several orders of magnitude if exposed to the full space environment for a mere few seconds. These results clearly negate the original panspermia hypothesis, which requires single spores as space travelers accelerated by the radiation pressure of the Sun, requiring many years to travel between the planets. However, if shielded against solar [[UV]], spores of ''[[Bacillus subtilis]]'' were capable of surviving in space for up to 6 years, especially if embedded in clay or meteorite powder (artificial meteorites). The data support the likelihood of interplanetary transfer of microorganisms within [[meteorite]]s, the so-called '''lithopanspermia''' hypothesis.<ref name='Gerda Horneck'/>
-->
==Science fiction==
* [[Jack Finney]]'s novel ''[[The Body Snatchers]]'' and the subsequent film adaptations describe spores drifting through space to arrive on the surface of Earth, though the premise is most fully discussed in the second version ''[[Invasion of the Body Snatchers (1978 film)]]''.
* Michael Crichton's 1969 novel, ''[[The Andromeda Strain]]'', is based on the panspermiatic premise of a meteor bringing an alien virus to Earth. The phrase "Andromeda Strain" has become a shorthand for alien or mysterious diseases.
* In the ''Star Trek: The Next Generation'' episode, "[[The Chase (Star Trek: The Next Generation)|The Chase]]" (season 6, episode 20, April 26, 1993), the common humanoid form and genetic compatibility of alien species throughout the Alpha Quadrant is revealed to have resulted from directed panspermia by an earlier species of intelligent humanoid progenitors who seeded the many planets with their own DNA.
* In the reimagined ''[[Battlestar Galactica (2004 TV series)|Battlestar Galactica]]'', season 3, episodes 6 and 7 ("[[Torn (Battlestar Galactica)|Torn]]", November 3, 2006; "[[A Measure of Salvation]]", November 10, 2006), a Cylon basestar discovers an ancient beacon and takes it on board, whereupon a deadly virus from the beacon infects the Cylons.  Doctor Cottle determines the Cylon infection to be a three-thousand-year-old strain of human [[Encephalitis|Lymphocytic Encephalitis]].  Admiral Adama and President Roslin speculate that the beacon was accidentally infected prior to placement by ancient human colonists on their way from Kobol to Earth.  Adama remarks, "An entire race almost wiped out because someone forgot to wipe their nose."
* The premise of Gareth Edwards's 2010 film ''[[Monsters (2010 film)|Monsters]]'' is that a NASA deep space probe crashes, bringing back with it an alien species requiring the U.S. and Mexican military to quarantine a large district of the border region.
* The opening sequence of Ridley Scott's 2012 ''Alien'' prequel, ''[[Prometheus (2012 film)|Prometheus]]'' depicts a humanoid species, referred to as 'the Engineers', seeding what is presumably the early Earth by disintegrating the body of one of their members and spilling his DNA into the water of the planet.  At the climax of the film it is revealed that for unknown reasons the Engineers deemed their experiment to have been a failure and intended to end it by eradicating all life on Earth.
*[[Tess Gerritsen]]'s novel, Gravity, involves the exposure of astronauts aboard the  [[Space Shuttle]] and [[International Space Station]], to a chimera based on Archaeons, that were recovered from the Galapagos Rift.
 
==See also==
{{columns-list|2|
* [[Abiogenesis]]
* [[Anthropic principle]]
* [[Astrobiology]]
* [[Drake equation]]
* [[Fermi paradox]]
* [[Fine-tuned Universe]]
* [[Interplanetary contamination]]
* [[Last universal ancestor]]
* [[List of microorganisms tested in outer space]]
* [[Planetary protection]]
* [[Rare Earth hypothesis]]
}}
 
==References==
{{Reflist|colwidth=30em}}
 
==Further reading==
* {{cite journal
|last1= |first1=
|year=
|title=
|journal=[[Nature News]]
|volume= |issue= |pages=
|bibcode=
|doi=10.1038/news040216-20
}}
* {{cite journal
|last1=Warmflash |first1=D.
|last2=Weiss |first2=B.
|date=24 October 2005
|title=Did Life Come from Another World?
|url=http://www.scientificamerican.com/article.cfm?id=did-life-come-from-anothe
|journal=[[Scientific American]]
|volume= |issue= |pages=
|bibcode=
|doi=
}}
* Crick F, 'Life, Its Origin and Nature', Simon and Schuster, 1981, ISBN 0-7088-2235-5
* Hoyle F,  'The Intelligent Universe', Michael Joseph Limited, London 1983, ISBN 0-7181-2298-4
 
==External links==
{{Wiktionary}}
* [http://web.snauka.ru/en/issues/2013/12/30018 A.E.Zlobin, 2013, Tunguska similar impacts and origin of life (mathematical theory of origin of life; incoming of pattern recognition algorithm due to comets)]
* [http://profiles.nlm.nih.gov/SC/B/B/Y/P/_/scbbyp.pdf Francis Crick's notes] for a lecture on directed panspermia, dated 5 November 1976.
 
{{Origin of life}}
{{Astrobiology}}
{{Extraterrestrial life}}
 
[[Category:Panspermia| ]]
[[Category:Astrobiology]]
[[Category:Origin of life]]
[[Category:Biological hypotheses]]
 
{{Link FA|de}}

Latest revision as of 16:25, 1 January 2015

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