Editing Hypothetical types of biochemistry

{{Short description|Possible alternative biochemicals used by life forms}}
[[File:PIA10008 Seas and Lakes on Titan.jpg|thumb|upright=1.5|False-color ''[[Cassini–Huygens|Cassini]]'' radar mosaic of Titan's north polar region; the blue areas are lakes of liquid hydrocarbons.<br />"The existence of lakes of liquid hydrocarbons on Titan opens up the possibility for solvents and energy sources that are alternatives to those in our biosphere and that might support novel life forms altogether different from those on Earth."—NASA Astrobiology Roadmap 2008<ref>{{cite journal|author=David J. Des Marais|display-authors=etal|title=The NASA Astrobiology Roadmap|date=2008|journal=Astrobiology|volume= 8|issue= 4|pages=715–730|doi=10.1089/ast.2008.0819|pmid=18793098| url=https://zenodo.org/record/1065569|bibcode=2008AsBio...8..715D|s2cid=18725684 }}</ref>]]

Several forms of [[biochemistry]] are agreed to be scientifically viable but are not proven to exist at this time.<ref name="McKay 2014">{{cite journal |title=Chance and Necessity in Biochemistry: Implications for the Search for Extraterrestrial Biomarkers in Earth-like Environments |journal=Astrobiology |date=May 27, 2014 |last1=Davila |first1=Alfonso F. |last2=McKay |first2=Christopher P. |volume=14 |issue=6 |pages=534–540 |doi=10.1089/ast.2014.1150 |bibcode=2014AsBio..14..534D |pmid=24867145 |pmc=4060776}}</ref> The kinds of [[life|living organisms currently known on Earth]] all use [[carbon]] compounds for basic structural and [[metabolism|metabolic]] functions, [[water]] as a [[solvent]], and [[DNA]] or [[RNA]] to define and control their form. If [[life]] exists on other [[planet]]s or [[natural satellite|moons]] it may be chemically similar, though it is also possible that there are organisms with quite different chemistries<ref name="WRD-201719">{{cite magazine |last=Singer |first=Emily |title=Chemists Invent New Letters for Nature's Genetic Alphabet |url=https://www.wired.com/2015/07/chemists-invent-new-letters-natures-genetic-alphabet/ |date=July 19, 2015 |magazine=[[Wired (magazine)|Wired]] |access-date=July 20, 2015 }}</ref>{{snd}} for instance, involving other classes of carbon compounds, compounds of another element, or another solvent in place of water.

The possibility of life-forms being based on "alternative" biochemistries is the topic of an ongoing scientific discussion, informed by what is known about extraterrestrial environments and about the chemical behaviour of various elements and compounds. It is of interest in [[synthetic biology]] and is also a [[science fiction theme|common subject in science fiction]].

The element [[silicon]] has been much discussed as a hypothetical alternative to carbon. Silicon is in the same group as carbon on the [[periodic table]] and, like carbon, it is [[tetravalent]]. Hypothetical alternatives to water include [[ammonia]], which, like water, is a [[chemical polarity|polar]] molecule, and cosmically abundant; and non-polar [[hydrocarbon]] solvents such as [[methane]] and [[ethane]], which are known to exist in liquid form on the surface of [[Titan (moon)|Titan]].

==Overview of hypothetical types of biochemistry==
<!-- All information referenced and cited below. -->
{| class="wikitable sortable"
|+  Overview of hypothetical types of biochemistry
!width=100| Type
!width=100| Basis
!width=100| Brief description
! class=unsortable|Details
|-
| Alternative-[[chirality]] biomolecules
| Alternative biochemistry
| Mirror image biochemistry
| Perhaps the least unusual alternative biochemistry would be one with differing [[Left-handed protein|chirality]] of its biomolecules. In known Earth-based life, [[amino acid]]s are almost universally of the {{small|L}} form and [[sugar]]s are of the {{small|D}} form. Molecules using {{small|D}} amino acids or {{small|L}} sugars are possible, though they would be incompatible with organisms using the opposing chirality molecules. [[Gram-positive bacteria]] incorporate {{small|D}}-[[alanine]] into their peptidoglycan layer, created through the actions of [[Epimerase and racemase|racemases]].<ref>{{cite journal |last1=Kovač |first1=Andreja |last2=Majce |first2=Vita |last3=Lenaršič |first3=Roman |last4=Bombek |first4=Sergeja |last5=Bostock |first5=Julieanne M. |last6=Chopra |first6=Ian |last7=Polanc |first7=Slovenko |last8=Gobec |first8=Stanislav |title=Diazenedicarboxamides as inhibitors of d-alanine-d-alanine ligase (Ddl) |journal=Bioorganic & Medicinal Chemistry Letters |date=April 2007 |volume=17 |issue=7 |pages=2047–2054 |doi=10.1016/j.bmcl.2007.01.015 |pmid=17267218 }}</ref>
|-
| Alternative nucleic acids
| Alternative biochemistry
| Different genetic storage
| [[Xeno nucleic acid]]s (XNA) may possibly be used in place of RNA or DNA. XNA is the general term for a nucleic acid with an altered sugar backbone. Examples of XNA are:<ref>{{cite journal| title = Cyanobacteria Produce N-(2-Aminoethyl)Glycine, a Backbone for Peptide Nucleic Acids Which May Have Been the First Genetic Molecules for Life on Earth| year = 2012| doi = 10.1371/journal.pone.0049043| doi-access = free| last1 = Banack| first1 = Sandra Anne| last2 = Metcalf| first2 = James S.| last3 = Jiang| first3 = Liying| last4 = Craighead| first4 = Derek| last5 = Ilag| first5 = Leopold L.| last6 = Cox| first6 = Paul Alan| journal = PLOS ONE| volume = 7| issue = 11| pages = e49043| pmid = 23145061| pmc = 3492184| bibcode = 2012PLoSO...749043B}}</ref>
*[[Threose nucleic acid|TNA]], which uses [[threose]];
*HNA, which uses 1,5-anhydrohexitol;
*[[Glycol nucleic acid|GNA]], which uses [[glycol]];
*CeNA, which uses [[cyclohexene]];
*[[Locked nucleic acid|LNA]], which utilizes a form of ribose that contains an extra linkage between its 4' carbon and 2' oxygen;
*FANA, which uses [[arabinose]] but with a single fluorine atom attached to its 2' carbon;
*PNA, which uses, in place of sugar and phosphate, N-(2-aminoethyl)-glycine units connected by [[peptide bonds]].
In comparison, [[Hachimoji DNA]] changes the base pairs instead of the backbone. These new base pairs are P ([[5-Aza-7-deazaguanine|2-Aminoimidazo[1,2a][1,3,5]triazin-4(1''H'')-one]]), Z ([[6-Amino-5-nitropyridin-2-one]]), B ([[Isoguanine]]), and S (rS = [[Isocytosine]] for RNA, dS = [[1-Methylcytosine]] for DNA).<ref name="SCI-20190222">{{cite journal | vauthors = Hoshika S, Leal NA, Kim MJ, Kim MS, Karalkar NB, Kim HJ, Bates AM, Watkins NE, SantaLucia HA, Meyer AJ, DasGupta S, Ellington AD, SantaLucia J, Georgiadis MM, Benner SA | title = Hachimoji DNA and RNA: A genetic system with eight building blocks | journal = Science | volume = 363 | issue = 6429 | pages = 884–887 | date = February 2019 | pmid = 30792304 | pmc = 6413494 | doi = 10.1126/science.aat0971| bibcode = 2019Sci...363..884H }}</ref><ref name="EA-20190221">{{cite press release |title=Hachimoji -- Expanding the genetic alphabet from four to eight |url=https://www.eurekalert.org/news-releases/466484 |work=EurekAlert! |publisher=American Association for the Advancement of Science |date=21 February 2019 }}</ref>
|-
| Ammonia biochemistry
| Non-water solvents
| Ammonia-based life
| Ammonia is [[Ammonia#In astronomy|relatively abundant]] in the universe and has chemical similarities to water. The possible role of [[liquid ammonia]] as an alternative solvent for life is an idea that goes back at least to 1954, when [[J.&nbsp;B.&nbsp;S. Haldane]] raised the topic at a symposium about life's origin.{{cn|date=October 2024}}
|-
| [[Arsenic biochemistry]]
| Alternative biochemistry
| [[Arsenic]]-based life
| [[Arsenic]], which is chemically similar to [[phosphorus]], while poisonous for most [[organism|life forms]] on Earth, is incorporated into the biochemistry of some organisms.{{cn|date=October 2024}} 
|-
| Borane biochemistry ([[Organoboron chemistry]])
| Alternative biochemistry
| Boranes-based life
| [[Boranes]] are dangerously explosive in Earth's atmosphere, but would be more stable in a [[Reducing atmosphere|reducing environment]] (an atmosphere without oxygen or other oxidizing gases, and which may contain actively reductant gases such as hydrogen, carbon monoxide, methane and hydrogen sulfide). [[Boron]], however, is exceedingly rare in the universe in comparison to its neighbours carbon, nitrogen, and oxygen. On the other hand, structures containing alternating boron and nitrogen atoms share some properties with hydrocarbons.{{cn|date=October 2024}}
|-
|Cosmic necklace-based biology
|Nonplanetary life
|Non-chemical life
|In 2020, Luis A. Anchordoqu and Eugene M. Chudnovsky hypothesized that life composed of magnetic semipoles connected by [[cosmic string]]s could evolve inside stars.<ref name="necklace" />
|-
| [[Dusty plasma]]-based biology
| Nonplanetary life
| Non-chemical life
| In 2007, Vadim N. Tsytovich and colleagues proposed that lifelike behaviors could be exhibited by dust particles suspended in a [[Plasma (physics)|plasma]], under conditions that might exist in space.<ref>{{cite journal | doi = 10.1088/1367-2630/9/8/263| title = From plasma crystals and helical structures towards inorganic living matter| year = 2007| last1 = Tsytovich| first1 = V. N.| last2 = Morfill| first2 = G. E.| last3 = Fortov| first3 = V. E.| last4 = Gusein-Zade| first4 = N. G.| last5 = Klumov| first5 = B. A.| last6 = Vladimirov| first6 = S. V.| journal = New Journal of Physics| volume = 9| issue = 8| page = 263| bibcode = 2007NJPh....9..263T| s2cid = 123459022| doi-access = free}}</ref>
|-
| [[Extremophile]]s
| Alternative environment
| Life in variable environments
| It would be biochemically possible to sustain life in environments that are only periodically consistent with life as we know it, such as extremely high or low temperatures, pressures, or pH; or the presence of high levels of [[salt]] or [[nuclear radiation]].{{cn|date=October 2024}}
|-
| Heteropoly acid biochemistry
| Alternative biochemistry
| Heteropoly acid-based life
| Various metals can form complex structures with oxygen, such as [[heteropoly acid]]s.{{cn|date=October 2024}}
|-
| [[Hydrogen fluoride]] biochemistry
| Non-water solvents
| [[Hydrogen fluoride]]-based life
| Hydrogen fluoride has been considered as a possible solvent for life by scientists such as Peter Sneath.{{cn|date=October 2024}}
|-
| [[Hydrogen sulfide]] biochemistry
| Non-water solvents
| [[Hydrogen sulfide]]-based life
| [[Hydrogen sulfide]] is a [[Hydrogen chalcogenide|chemical analog of water]], but is less polar and a weaker inorganic solvent.{{cn|date=October 2024}}
|-
| Methane biochemistry (Azotosome)
| Non-water solvents
| Methane-based life
| [[Methane]] is [[Methane#Extraterrestrial methane|relatively abundant]] in the Solar System and the Universe, and is known to exist in liquid form on [[Titan (moon)|Titan]], the largest moon of [[Saturn]]. Though highly unlikely, it is considered to be possible for Titan to harbor life. If so, it will most likely be methane-based life.{{cn|date=October 2024}}
|-
| Non-green photosynthesizers
| Other speculations
| Alternate plant life
| Physicists have noted that, although photosynthesis on Earth generally involves green plants, a variety of other-colored plants could also support photosynthesis, essential for most life on Earth, and that other colors might be preferred in places that receive a different mix of stellar radiation than Earth. In particular, [[retinal]] is capable of, and has been observed to, perform photosynthesis.<ref>{{cite book |doi=10.1007/978-1-4615-2962-0_9 |chapter=Comparison of Retinal-Based and Chlorophyll-Based Photosynthesis: A Biothermokinetic Description of Photochemical Reaction Centers |title=Modern Trends in Biothermokinetics |date=1993 |last1=Hellingwerf |first1=Klaas J. |last2=Crielaard |first2=Wim |last3=Westerhoff |first3=Hans V. |pages=45–52 |isbn=978-1-4613-6288-3 }}</ref> Bacteria capable of photosynthesis are known as [[microbial rhodopsin]]s. A plant or creature that uses retinal photosynthesis is always [[Purple Earth hypothesis|purple]].
|-
| [[Shadow biosphere]]
| Alternative environment
| A hidden life biosphere on [[Earth]]
| A shadow biosphere is a hypothetical [[microbe|microbial]] [[biosphere]] of Earth that uses radically different [[biochemistry|biochemical]] and [[molecular biology|molecular]] processes than currently known life. It could exist, for example, deep in the crust or sealed in ancient rocks.{{cn|date=October 2024}}
|-
| Silicon biochemistry ([[Organosilicon]])
| Alternative biochemistry
| Silicon-based life
| Like carbon, silicon can create molecules that are sufficiently large to carry biological information; however, the scope of possible silicon chemistry is far more limited than that of carbon.{{cn|date=October 2024}}
|-
| [[Silicon dioxide]] biochemistry
| Non-water solvents
| [[Silicon dioxide]]-based life
| [[Gerald Feinberg]] and [[Robert Shapiro (chemist)|Robert Shapiro]] have suggested that molten silicate rock could serve as a liquid medium for organisms with a chemistry based on silicon, oxygen, and other elements such as [[aluminium]].{{cn|date=October 2024}}
|-
| Sulfur biochemistry
| Alternative biochemistry
| Sulfur-based life
| The biological use of sulfur as an alternative to carbon is purely hypothetical, especially because sulfur usually forms only linear chains rather than branched ones.{{cn|date=October 2024}}
|}

== Shadow biosphere ==
{{Main|Shadow biosphere}}
[[File:Arecibo message.svg|upright|thumb|The [[Arecibo message]] (1974) sent information into space about basic chemistry of Earth life.]]
A shadow biosphere is a hypothetical [[microbe|microbial]] [[biosphere]] of Earth that uses radically different [[biochemistry|biochemical]] and [[molecular biology|molecular]] processes than currently known life.<ref>{{cite journal | last1 = Davies | first1 = P. C. W. | last2 = Benner | first2 = S.A. | last3 = Cleland | first3 = C.E.|author3-link= Carol Cleland | last4 = Lineweaver | first4 = C.H. | last5 = McKay | first5 = C.P. | last6 = Wolfe-Simon | first6 = F. | s2cid = 5723954 | year = 2009 | title = Signatures of a Shadow Biosphere | journal = Astrobiology | volume = 9 | issue = 2| pages = 241–249 | doi = 10.1089/ast.2008.0251 | bibcode=2009AsBio...9..241D | pmid=19292603}}</ref><ref>{{cite journal |last1=Cleland |first1=Carol E. |author1-link=Carol Cleland |last2=Copley |first2=Shelley D. |date=16 January 2006 |title=The possibility of alternative microbial life on Earth |journal=International Journal of Astrobiology |volume=4 |issue=3–4 |pages=165 |bibcode=2005IJAsB...4..165C |citeseerx=10.1.1.392.6366 |doi=10.1017/S147355040500279X |s2cid=364892}}</ref> Although life on Earth is relatively well-studied, the shadow biosphere may still remain unnoticed because the exploration of the microbial world targets primarily the biochemistry of the macro-organisms.

== Alternative-chirality biomolecules ==
Perhaps the least unusual alternative biochemistry would be one with differing [[Left-handed protein|chirality]] of its biomolecules. In known Earth-based life, [[amino acid]]s are almost universally of the {{small|L}} form and [[sugar]]s are of the {{small|D}} form. Molecules using {{small|D}} amino acids or {{small|L}} sugars may be possible; molecules of such a chirality, however, would be incompatible with organisms using the opposing chirality molecules. Amino acids whose chirality is opposite to the norm are found on Earth, and these substances are generally thought to result from decay of organisms of normal chirality. However, physicist [[Paul Davies]] speculates that some of them might be products of "anti-chiral" life.<ref>{{cite journal |last1=Davies |first1=P.C.W. |last2=Lineweaver |first2=Charles H. |title=Finding a Second Sample of Life on Earth |journal=Astrobiology |date=April 2005 |volume=5 |issue=2 |pages=154–163 |doi=10.1089/ast.2005.5.154 |pmid=15815166 |bibcode=2005AsBio...5..154D }}</ref>

It is questionable, however, whether such a biochemistry would be truly alien. Although it would certainly be an alternative [[stereochemistry]], molecules that are overwhelmingly found in one [[enantiomer]] throughout the vast majority of organisms can nonetheless often be found in another enantiomer in different (often [[Basal (phylogenetics)|basal]]) organisms such as in comparisons between members of [[Archaea]] and other [[Domain (biology)|domains]],{{Citation needed|date=January 2012}} making it an open topic whether an alternative stereochemistry is truly novel.

== Non-carbon-based biochemistries ==
On Earth, all known living things have a carbon-based structure and system. Scientists have speculated about the advantages and disadvantages of using [[Chemical element|element]]s other than carbon to form the molecular structures necessary for life, but no one has proposed a theory employing such atoms to form all the necessary structures. However, as [[Carl Sagan]] argued, it is very difficult to be certain whether a statement that applies to all life on Earth will turn out to apply to all life throughout the universe.<ref>{{cite book| title=Carl Sagan's Cosmic Connection: an Extraterrestrial Perspective| first1=Carl| last1= Sagan|first2=Jerome|last2=Agel|edition=2nd|publisher=Cambridge U.P.| date=2000|isbn= 978-0-521-78303-3|page=41}}</ref> Sagan used the term "[[carbon chauvinism]]" for such an assumption.<ref name="Sagan 2000page=46 46">{{cite book| title=Carl Sagan's Cosmic Connection: an Extraterrestrial Perspective| first=Carl| last= Sagan|edition=2nd|publisher=Cambridge U.P.| date=2000 |page=46}}</ref> He regarded [[silicon]] and [[germanium]] as conceivable alternatives to carbon<ref name="Sagan 2000page=46 46" /> (other plausible elements include but are not limited to [[palladium]] and [[titanium]]); but, on the other hand, he noted that carbon does seem more chemically versatile and is more abundant in the cosmos.<ref>{{cite book| title=Carl Sagan's Cosmic Connection: an Extraterrestrial Perspective| first=Carl| last= Sagan|edition=2nd|publisher=Cambridge U.P.| date=2000|page=47}}</ref>  [[Norman Horowitz]] devised the experiments to determine whether [[life on Mars|life might exist on Mars]] that were carried out by the [[Viking 1|Viking Lander of 1976]], the first U.S. mission to successfully land a probe on the surface of Mars. Horowitz argued that the great versatility of the carbon atom makes it the element most likely to provide solutions, even exotic solutions, to the problems of survival on other planets.<ref name = Horowitz1986>Horowitz, N.H. (1986). Utopia and Back and the search for life in the solar system. New York: W.H. Freeman and Company. {{ISBN|0-7167-1766-2}}.{{pn|date=June 2024}}</ref>  He considered that there was only a remote possibility that non-carbon life forms could exist with genetic information systems capable of self-replication and the ability to evolve and adapt.

=== Silicon biochemistry ===
{{See also|Organosilicon}}
[[File:Silane.png|thumb|right|120px|Structure of [[silane]], analog of [[methane]]]]
[[File:PDMS.svg|thumb|right|Structure of the silicone [[polydimethylsiloxane]] (PDMS)]]
[[File:Diatom2.jpg|thumb|right|upright=1.2|Marine [[diatom]]s{{snd}} carbon-based organisms that extract silicon from sea water, in the form of its oxide (silica) and incorporate it into their cell walls]]
The silicon atom has been much discussed as the basis for an alternative biochemical system, because silicon has many [[chemical property|chemical similarities]] to carbon and is in [[adamantogen|the same group of the periodic table]]. Like carbon, silicon can create molecules that are sufficiently large to carry biological information.<ref name="Pace">{{cite journal|last1 = Pace|first1 = N.&nbsp;R. |title = The universal nature of biochemistry |journal = Proceedings of the National Academy of Sciences of the United States of America |volume = 98 |issue = 3 |pages = 805–808 |date = 2001 |pmid = 11158550 |pmc = 33372 |doi = 10.1073/pnas.98.3.805 |bibcode=2001PNAS...98..805P|doi-access = free }}</ref>

However, silicon has several drawbacks as a carbon alternative.  Carbon is ten times more [[Abundance of the chemical elements|cosmically abundant]] than silicon, and its chemistry appears naturally more complex.<ref name=BC/> By 1998, astronomers had identified 84 carbon-containing molecules in the [[interstellar medium]], but only 8 containing silicon, of which half also included carbon.<ref>{{cite web |url=http://www.faqs.org/faqs/astronomy/faq/part6/section-16.html |title= F.10 Why do we assume that other beings must be based on carbon? Why couldn't organisms be based on other substances? |work=[sci.astro] ET Life (Astronomy Frequently Asked Questions) |access-date=2006-07-21 |first=Joseph |last=Lazio}}</ref>  Even though [[Earth]] and other [[terrestrial planet]]s are exceptionally silicon-rich and carbon-poor (silicon is roughly 925 times [[abundance of elements in Earth's crust|more abundant in Earth's crust]] than carbon), terrestrial life bases itself on carbon. It may avoid silicon because silicon compounds are less varied, unstable in the presence of [[water]], or block the flow of heat.<ref name=BC>{{cite web | url = http://biocab.org/Astrobiology.html | title = Astrobiology | access-date = 2011-01-17 | date = September 26, 2006 | publisher = Biology Cabinet | archive-date = 2010-12-12 | archive-url = https://web.archive.org/web/20101212184044/http://biocab.org/Astrobiology.html | url-status = dead }}</ref>  

Relative to carbon, silicon has a much larger [[atomic radius]], and forms much weaker [[covalent bond]]s to atoms&nbsp;&mdash; except [[oxygen]] and [[fluorine]], with which it forms very strong bonds.<ref name="Pace" />  Almost no [[multiple bond]]s to silicon are stable, although silicon does exhibit varied [[coordination number]].{{sfn|Bains|2004}}  [[Silanes]], silicon analogues to the [[alkane]]s, react rapidly with water, and long-chain silanes spontaneously decompose.<ref name="world-building"/>  Consequently, most terrestrial silicon is "locked up" in [[silica]], and not a wide variety of biogenic precursors.{{sfn|Bains|2004}}   

[[Silicone]]s, which alternate between silicon and [[oxygen]] atoms, are much more stable than silanes, and may even be more stable than the equivalent hydrocarbons in sulfuric acid-rich extraterrestrial environments.<ref name="world-building">{{cite book |first=Stephen |last=Gillette |title=World-Building |publisher=Writer's Digest Books |isbn=978-0-89879-707-7|year=1996 }}</ref>  Alternatively, the weak bonds in silicon compounds may help maintain a rapid pace of life at [[cryogenic]] temperatures.  Polysilanols, the silicon homologues to [[sugar]]s, are among the few compounds soluble in [[liquid nitrogen]].<ref>{{cite web |title=The nature of life |website=Astrobiology |author=William Bains |access-date=2015-03-20 |url=http://www.williambains.co.uk/astrobiology/life2.html|archive-url=https://web.archive.org/web/20190127032142/http://www.williambains.co.uk/astrobiology/life2.html|archive-date=27 Jan 2019}}</ref>{{reliable?|reason=Self-published srcs are OK *if they're by an expert*, but is Bains one?|date=March 2024}}{{sfn|Bains|2004}}

All known silicon [[Macromolecule|macromolecules]] are artificial polymers, and so "monotonous compared with the combinatorial universe of organic macromolecules".<ref name="Pace" />{{sfn|Bains|2004}}  Even so, some Earth life uses [[biogenic silica]]: [[diatom]]s' silicate [[skeleton]]s. [[Graham Cairns-Smith|A. G. Cairns-Smith]] hypothesized that silicate minerals in water [[Abiogenesis#clay|played a crucial role in abiogenesis]], in that biogenic carbon compounds [[macromolecular template|formed around their crystal structures]].<ref>{{Cite book |last=Cairns-Smith |first=A. Graham |title=Seven Clues to the Origin of Life |location=Cambridge |publisher=Cambridge University Press |date=1985 |isbn=978-0-521-27522-4 |url-access=registration |url=https://archive.org/details/sevencluestoorig00cair_0 }}</ref><ref>{{cite book | first=Richard | last=Dawkins | author-link=Richard Dawkins | title=The Blind Watchmaker | publisher=W. W. Norton & Company, Inc | location=New York | orig-year=1986 | year=1996 | isbn=978-0-393-31570-7 | pages=[https://archive.org/details/blindwatchmaker0000dawk/page/148 148–161] | url=https://archive.org/details/blindwatchmaker0000dawk/page/148 }}</ref>  Although not observed in nature, carbon–silicon bonds have been added to biochemistry under [[directed evolution]] (artificial selection): a [[cytochrome c|cytochrome ''c'' protein]] from ''[[Rhodothermus marinus]]'' has been engineered to catalyze new carbon–silicon bonds between hydrosilanes and [[diazo]] compounds.<ref>{{cite journal |last1=Kan |first1=S. B. Jennifer |last2=Lewis |first2=Russell D. |last3=Chen |first3=Kai |last4=Arnold |first4=Frances H. |title=Directed evolution of cytochrome c for carbon–silicon bond formation: Bringing silicon to life |journal=Science |date=25 November 2016 |volume=354 |issue=6315 |pages=1048–1051 |doi=10.1126/science.aah6219 |pmid=27885032 |pmc=5243118 |bibcode=2016Sci...354.1048K }}</ref>

=== Other exotic element-based biochemistries ===
{{See also|Organoboron chemistry}}
* [[Boranes]] are dangerously explosive in Earth's atmosphere, but would be more stable in a [[reducing atmosphere]]. However, boron's low cosmic abundance makes it less likely as a base for life than carbon.
* Various metals, together with oxygen, can form very complex and thermally stable structures rivaling those of organic compounds;{{Citation needed|date=February 2014}} the [[heteropoly acid]]s are one such family. Some metal oxides are also similar to carbon in their ability to form both nanotube structures and diamond-like crystals (such as [[cubic zirconia]]). [[Titanium]], [[aluminium]], [[magnesium]], and [[iron]] are all more abundant in the Earth's crust than carbon. Metal-oxide-based life could therefore be a possibility under certain conditions, including those (such as high temperatures) at which carbon-based life would be unlikely. The Cronin group at Glasgow University reported self-assembly of tungsten [[polyoxometalate]]s into cell-like spheres.<ref name='Cronin'>{{cite magazine | url = https://www.newscientist.com/article/dn20906-lifelike-cells-are-made-of-metal.html | title = Life-like cells are made of metal | access-date = 2014-05-25 | date = September 14, 2011 | magazine = New Scientist}}</ref> By modifying their metal oxide content, the spheres can acquire holes that act as porous membrane, selectively allowing chemicals in and out of the sphere according to size.<ref name='Cronin' />
* [[Sulfur]] is also able to form long-chain molecules, but suffers from the same high-reactivity problems as phosphorus and silanes. The biological use of sulfur as an alternative to carbon is purely hypothetical, especially because sulfur usually forms only linear chains rather than branched ones. (The biological use of sulfur as an electron acceptor is widespread and can be traced back 3.5&nbsp;billion years on Earth, thus predating the use of molecular oxygen.<ref>{{cite journal |last1=Philippot |first1=Pascal |last2=Van Zuilen |first2=Mark |last3=Lepot |first3=Kevin |last4=Thomazo |first4=Christophe |last5=Farquhar |first5=James |last6=Van Kranendonk |first6=Martin J. |title=Early Archaean Microorganisms Preferred Elemental Sulfur, Not Sulfate |journal=Science |date=14 September 2007 |volume=317 |issue=5844 |pages=1534–1537 |doi=10.1126/science.1145861 |pmid=17872441 |bibcode=2007Sci...317.1534P }}</ref> [[Sulfur-reducing bacteria]] can utilize elemental sulfur instead of oxygen, reducing sulfur to [[hydrogen sulfide]].)

== Arsenic as an alternative to phosphorus ==
{{See also|GFAJ-1}}
While [[arsenic]], which is chemically similar to [[phosphorus]], is poisonous for most [[organism|life forms]] on Earth, it is incorporated into the biochemistry of some organisms.<ref>{{cite web|url=http://umbbd.ethz.ch/periodic/elements/as.html |title=Biochemical Periodic Table – Arsenic |publisher=UMBBD |date=2007-06-08 |access-date=2010-05-29}}</ref> Some [[marine algae]] incorporate arsenic into complex organic molecules such as [[arsenosugar]]s and [[arsenobetaine]]s. [[Fungi]] and [[bacteria]] can produce volatile methylated arsenic compounds. [[Arsenate]] reduction and arsenite oxidation have been observed in [[microbes]] (''[[Chrysiogenes arsenatis]]'').<ref>{{cite journal |last1=Niggemyer |first1=Allison |last2=Spring |first2=Stefan |last3=Stackebrandt |first3=Erko |last4=Rosenzweig |first4=R. Frank |title=Isolation and Characterization of a Novel As(V)-Reducing Bacterium: Implications for Arsenic Mobilization and the Genus Desulfitobacterium |journal=Applied and Environmental Microbiology |date=December 2001 |volume=67 |issue=12 |pages=5568–5580 |doi=10.1128/AEM.67.12.5568-5580.2001 |pmid=11722908 |pmc=93345 |bibcode=2001ApEnM..67.5568N }}</ref> Additionally, some [[prokaryote]]s can use arsenate as a terminal electron acceptor during anaerobic growth and some can utilize arsenite as an electron donor to generate energy.

It has been speculated that the earliest life forms on Earth may have used [[arsenic biochemistry]] in place of phosphorus in the structure of their DNA.<ref>{{cite journal |last1=Reilly |first1=Michael |title=Early life could have been based on arsenic |journal=New Scientist |date=April 2008 |volume=198 |issue=2653 |pages=10 |doi=10.1016/S0262-4079(08)61007-6 }}</ref> A common objection to this scenario is that arsenate esters are so much less stable to [[hydrolysis]] than corresponding [[Organophosphate|phosphate esters]] that arsenic is poorly suited for this function.<ref name="Westheimer">{{cite journal |last1=Westheimer |first1=F. H. |title=Why Nature Chose Phosphates |journal=Science |date=6 March 1987 |volume=235 |issue=4793 |pages=1173–1178 |doi=10.1126/science.2434996 |pmid=2434996 |bibcode=1987Sci...235.1173W }}</ref>

The authors of a 2010 [[geomicrobiology]] study, supported in part by NASA, have postulated that a bacterium, named [[GFAJ-1]], collected in the sediments of [[Mono Lake]] in eastern [[California]], can employ such 'arsenic DNA' when cultured without phosphorus.<ref name="nasafund">{{cite web |title=NASA-Funded Research Discovers Life Built With Toxic Chemical |publisher=NASA.gov |date=2 December 2010 |access-date=2010-12-02 |url=http://www.nasa.gov/topics/universe/features/astrobiology_toxic_chemical.html |archive-date=2011-08-28 |archive-url=https://web.archive.org/web/20110828203819/http://www.nasa.gov/topics/universe/features/astrobiology_toxic_chemical.html |url-status=dead }}</ref><ref name="Wolfe-Simon">{{cite journal |last1=Wolfe-Simon |first1=Felisa |last2=Blum |first2=Jodi Switzer |last3=Kulp |first3=Thomas R. |last4=Gordon |first4=Shelley E. |last5=Hoeft |last6=Pett-Ridge |first6=Jennifer |last7=Stolz |first7=John F. |last8=Webb |first8=Samuel M. |last9=Weber |first9=Peter K. |last10=Davies |first10=Paul C. W. |last11=Anbar |first11=Ariel D. |last12=Oremland |first12=Ronald S.|author1-link=Felisa Wolfe-Simon|author12-link=Ronald Oremland |title=A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus |journal=Science |date=2 December 2010 |doi=10.1126/science.1197258 |pmid=21127214 |first5=S. E. |volume=332 |issue=6034 |pages=1163–6 |bibcode=2011Sci...332.1163W |s2cid=51834091 |doi-access=free }}</ref> They proposed that the bacterium may employ high levels of [[Polyhydroxybutyrate|poly-β-hydroxybutyrate]] or other means to reduce the [[Activity (chemistry)|effective concentration]] of water and stabilize its arsenate esters.<ref name="Wolfe-Simon" /> This claim was heavily criticized almost immediately after publication for the perceived lack of appropriate controls.<ref name="Redfield">{{cite web | last=Redfield | first=Rosemary | title=Arsenic-associated bacteria (NASA's claims) | work=rrresearch.blogspot.com/ | date=4 December 2010 | url=http://rrresearch.blogspot.com/2010/12/arsenic-associated-bacteria-nasas.html | access-date=4 December 2010}}{{self-published inline|date=June 2024}}</ref><ref name="Beasties">{{cite web |last=Bradley |first=Alex |title=Arsenate-based DNA: a big idea with big holes |work=scienceblogs.com/webeasties/ |date=5 December 2010 |url=http://scienceblogs.com/webeasties/2010/12/guest_post_arsenate-based_dna.php |access-date=9 December 2010 |url-status=dead |archive-url= https://web.archive.org/web/20101208213626/http://scienceblogs.com/webeasties/2010/12/guest_post_arsenate-based_dna.php |archive-date=8 December 2010  }}</ref> Science writer [[Carl Zimmer]] contacted several scientists for an assessment: "I reached out to a dozen experts ... Almost unanimously, they think the NASA scientists have failed to make their case".<ref name=Zimmer>{{cite news | last=Zimmer | first=Carl | author-link=Carl Zimmer
 | title=Scientists see fatal flaws in the NASA study of arsenic-based life | date=7 December 2010 | url=http://www.slate.com/id/2276919/ | work=[[Slate (magazine)|Slate]] | access-date=7 December 2010}}</ref>
Other authors were unable to reproduce their results and showed that the study had issues with phosphate contamination, suggesting that the low amounts present could sustain extremophile lifeforms.<ref>{{cite web | url = http://www.biotechniques.com/news/Arsenic-Life-Claim-Refuted/biotechniques-332691.html#.UQBYhx3O3zk | title="Arsenic Life" Claim Refuted | access-date = 23 January 2013 | last = Williams | first = Sarah | date = 7 November 2012 | work = BioTechniques | archive-url = https://web.archive.org/web/20160304054417/http://www.biotechniques.com/news/Arsenic-Life-Claim-Refuted/biotechniques-332691.html#.UQBYhx3O3zk | archive-date = 4 March 2016 | url-status = dead }}</ref>
Alternatively, it was suggested that GFAJ-1 cells grow by recycling phosphate from degraded ribosomes, rather than by replacing it with arsenate.<ref>{{cite journal |first=Harris TK and Deutscher MP | last=Basturea GN | title= Growth of a bacterium that apparently uses arsenic instead of phosphorus is a consequence of massive ribosome breakdown | date=17 August 2012 | pmid=22798070 | doi=10.1074/jbc.C112.394403 | volume=287 | issue=34 | journal=J Biol Chem | pages=28816–9 | pmc=3436571| doi-access=free }}</ref>

== Non-water solvents ==

In addition to carbon compounds, all currently known terrestrial life also requires water as a solvent. This has led to discussions about whether water is the only liquid capable of filling that role. The idea that an extraterrestrial life-form might be based on a solvent other than water has been taken seriously in recent scientific literature by the biochemist [[Steven Benner]],<ref>{{cite journal |last1=Benner |first1=Steven A. |last2=Ricardo | first2=Alonso |last3=Carrigan |first3=Matthew A |title=Is there a common chemical model for life in the universe? |date=2004 |journal=[[Current Opinion in Chemical Biology]] |volume=8|pages=676–680 |doi=10.1016/j.cbpa.2004.10.003 |issue=6 |pmid=15556414 }}</ref> and by the astrobiological committee chaired by John A. Baross.<ref name="books.nap.edu">Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life, National Research Council; [http://books.nap.edu/openbook.php?record_id=11919&page=69 The Limits of Organic Life in Planetary Systems]; The National Academies Press, 2007; pages 69–79.</ref> Solvents discussed by the Baross committee include [[ammonia]],<ref name="barossammonia">Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life, National Research Council; [http://books.nap.edu/openbook.php?record_id=11919&page=72 The Limits of Organic Life in Planetary Systems]; The National Academies Press, 2007; p.&nbsp;72.</ref> [[sulfuric acid]],<ref name="barosssulfuric">Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life, National Research Council; [http://books.nap.edu/openbook.php?record_id=11919&page=73 The Limits of Organic Life in Planetary Systems]; The National Academies Press, 2007; p.&nbsp;73.</ref> [[formamide]],<ref name="Planetary Systems p 74">Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life, National Research Council; [http://books.nap.edu/openbook.php?record_id=11919&page=74 The Limits of Organic Life in Planetary Systems]; The National Academies Press, 2007; p.&nbsp;74.</ref> hydrocarbons,<ref name="Planetary Systems p 74" /> and (at temperatures much lower than Earth's) liquid [[nitrogen]], or hydrogen in the form of a [[supercritical fluid]].<ref name="barosscryo">Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life, National Research Council; [http://books.nap.edu/openbook.php?record_id=11919&page=75 The Limits of Organic Life in Planetary Systems]; The National Academies Press, 2007; p.&nbsp;75.</ref>

Water as a solvent limits the forms biochemistry can take. For example, Steven Benner, proposes the [[polyelectrolyte theory of the gene]] that claims that for a genetic [[biopolymer]] such as DNA to function in water, it requires repeated [[Ionic charge|ionic charges.]]<ref>{{cite journal |last1=Benner |first1=Steven A. |last2=Hutter |first2=Daniel |title=Phosphates, DNA, and the Search for Nonterrean Life: A Second Generation Model for Genetic Molecules |journal=Bioorganic Chemistry |date=February 2002 |volume=30 |issue=1 |pages=62–80 |doi=10.1006/bioo.2001.1232 |pmid=11955003 }}</ref> If water is not required for life, these limits on genetic biopolymers are removed.  

Carl Sagan once described himself as both a [[carbon chauvinist]] and a water chauvinist;<ref>{{cite book |author=Sagan, Carl |title=Cosmos |date=2002 |isbn=978-0-375-50832-5 |publisher=Random House |pages=126–127}}</ref> however, on another occasion he said that he was a carbon chauvinist but "not that much of a water chauvinist".<ref name="conversations">{{cite book |author1=Sagan, Carl |author2=Head, Tom |title=Conversations with Carl Sagan |url=https://archive.org/details/conversationswit00saga |url-access=registration |date=2006 |publisher=University Press of Mississippi |isbn=978-1-57806-736-7 |page=[https://archive.org/details/conversationswit00saga/page/10 10]}}</ref>
He speculated on hydrocarbons,<ref name="conversations" />{{rp|11}} [[hydrofluoric acid]],<ref name="Sagan, Carl 2002 128">{{cite book |author=Sagan, Carl |title=Cosmos |date=2002 |isbn=978-0-375-50832-5 |publisher=Random House |page=128}}</ref> and ammonia<ref name="conversations" /><ref name="Sagan, Carl 2002 128" />  as possible alternatives to water.

Some of the properties of water that are important for life processes include:
* A complexity which leads to a large number of permutations of possible reaction paths including acid–base chemistry, H<sup>+</sup> cations, OH<sup>−</sup> anions, hydrogen bonding, van der Waals bonding, dipole–dipole and other polar interactions, aqueous solvent cages, and hydrolysis. This complexity offers a large number of pathways for evolution to produce life, many other solvents{{which|date=November 2018}} have dramatically fewer possible reactions, which severely limits evolution.
* Thermodynamic stability: the free energy of formation of liquid water is low enough (−237.24 kJ/mol) that water undergoes few reactions. Other solvents are highly reactive, particularly with oxygen.
* Water does not combust in oxygen because it is already the combustion product of hydrogen with oxygen. Most alternative solvents are not stable in an oxygen-rich atmosphere, so it is highly unlikely that those liquids could support aerobic life.
* A large temperature range over which it is [[liquid phase|liquid]].
* High solubility of oxygen and carbon dioxide at room temperature supporting the evolution of aerobic aquatic plant and animal life.
* A high [[heat capacity]] (leading to higher environmental temperature stability).
* Water is a room-temperature liquid leading to a large population of quantum transition states required to overcome reaction barriers. Cryogenic liquids (such as liquid methane) have exponentially lower transition state populations which are needed for life based on chemical reactions. This leads to chemical reaction rates which may be so slow as to preclude the development of any life based on chemical reactions.{{citation needed|date=November 2018}}
* Spectroscopic transparency allowing solar radiation to penetrate several meters into the liquid (or solid), greatly aiding the evolution of aquatic life.
* A large [[heat of vaporization]] leading to stable lakes and oceans.
* The ability to dissolve a wide variety of compounds.
* The solid (ice) has lower density than the liquid, so ice floats on the liquid. This is why bodies of water freeze over but do not freeze solid (from the bottom up). If ice were denser than liquid water (as is true for nearly all other compounds), then large bodies of liquid would slowly freeze solid, which would not be conducive to the formation of life.

Water as a compound is cosmically abundant, although much of it is in the form of vapor or ice. Subsurface liquid water is considered likely or possible on several of the outer moons: [[Enceladus]] (where geysers have been observed), [[Europa (moon)|Europa]], [[Titan (moon)|Titan]], and [[Ganymede (moon)|Ganymede]]. Earth and Titan are the only worlds currently known to have stable bodies of liquid on their surfaces.

Not all properties of water are necessarily advantageous for life, however.<ref name="barosswater">Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life, National Research Council; [http://books.nap.edu/openbook.php?record_id=11919&page=70 The Limits of Organic Life in Planetary Systems]; The National Academies Press, 2007; page 70.</ref> For instance, water ice has a high [[albedo]],<ref name="barosswater" /> meaning that it reflects a significant quantity of light and heat from the Sun. During [[ice age]]s, as reflective ice builds up over the surface of the water, the effects of global cooling are increased.<ref name="barosswater" />

There are some properties that make certain compounds and elements much more favorable than others as solvents in a successful biosphere. The solvent must be able to exist in liquid equilibrium over a range of temperatures the planetary object would normally encounter. Because boiling points vary with the pressure, the question tends not to be ''does'' the prospective solvent remain liquid, but ''at what pressure''. For example, [[hydrogen cyanide]] has a narrow liquid-phase temperature range at 1&nbsp;atmosphere, but in an atmosphere with the pressure of [[Venus]], with {{convert|92|bar|atm}} of pressure, it can indeed exist in liquid form over a wide temperature range.

=== Ammonia ===
The [[ammonia]] molecule (NH<sub>3</sub>), like the water molecule, is abundant in the universe, being a compound of hydrogen (the simplest and most common element) with another very common element, nitrogen.<ref name="asimov" /> The possible role of liquid ammonia as an alternative solvent for life is an idea that goes back at least to 1954, when [[J.&nbsp;B.&nbsp;S. Haldane]] raised the topic at a symposium about life's origin.<ref name="haldane">{{Cite journal |author=J. B. S. Haldane |date=1954 |title=The Origins of Life |journal=New Biology |volume=16 |pages=12–27}} cited in {{cite web |url=http://www.daviddarling.info/encyclopedia/A/ammonialife.html/ |title=Ammonia-based life |last=Darling |first=David |archive-url=https://web.archive.org/web/20121018095038/http://www.daviddarling.info/encyclopedia/A/ammonialife.html |archive-date=2012-10-18 |url-status=dead |access-date=2012-10-01 }}</ref>

Numerous chemical reactions are possible in an ammonia solution, and liquid ammonia has chemical similarities with water.<ref name="asimov" /><ref name="ddammonia">{{cite web | url = http://www.daviddarling.info/encyclopedia/A/ammonialife.html | title = ammonia-based life | access-date = 2012-10-01 | last = Darling | first = David}}</ref> Ammonia can dissolve most organic molecules at least as well as water does and, in addition, it is capable of dissolving many elemental metals. Haldane made the point that various common water-related organic compounds have ammonia-related analogs; for instance the ammonia-related [[amine]] group (−NH<sub>2</sub>) is analogous to the water-related [[hydroxyl]] group (−OH).<ref name="ddammonia" />

Ammonia, like water, can either accept or donate an H<sup>+</sup> ion. When ammonia accepts an H<sup>+</sup>, it forms the [[ammonium]] cation (NH<sub>4</sub><sup>+</sup>), analogous to [[hydronium]] (H<sub>3</sub>O<sup>+</sup>). When it donates an H<sup>+</sup> ion, it forms the [[metal amides|amide]] anion (NH<sub>2</sub><sup>−</sup>), analogous to the [[hydroxide]] anion (OH<sup>−</sup>).<ref name="barossammonia" /> Compared to water, however, ammonia is more inclined to accept an H<sup>+</sup> ion, and less inclined to donate one; it is a stronger [[nucleophile]].<ref name="barossammonia" /> Ammonia added to water functions as an [[Arrhenius base]]: it increases the concentration of the anion hydroxide. Conversely, using a [[Acid–base reaction#Solvent system definition|solvent system definition]] of acidity and basicity, water added to liquid ammonia functions as an acid, because it increases the concentration of the cation ammonium.<ref name="ddammonia" /> The carbonyl group (C=O), which is much used in terrestrial biochemistry, would not be stable in ammonia solution, but the analogous [[imine]] group (C=NH) could be used instead.<ref name="barossammonia" />

However, ammonia has some problems as a basis for life. The [[hydrogen bonds]] between ammonia molecules are weaker than those in water, causing ammonia's [[heat of vaporization]] to be half that of water, its [[surface tension]] to be a third, and reducing its ability to concentrate non-polar molecules through a [[hydrophobic]] effect. Gerald Feinberg and Robert Shapiro have questioned whether ammonia could hold prebiotic molecules together well enough to allow the emergence of a self-reproducing system.<ref>{{cite book |title=Life Beyond Earth |url=https://archive.org/details/lifebeyondearthi0000fein |url-access=registration |first=Gerald |last= Feinberg |author2=Robert Shapiro |publisher=Morrow |date=1980 |isbn=978-0-688-03642-3}} cited in {{cite web | url = http://www.daviddarling.info/encyclopedia/A/ammonialife.html/ | title = ammonia-based life | access-date = 2012-10-01 | last = Darling | first = David | url-status = dead | archive-url = https://web.archive.org/web/20121018095038/http://www.daviddarling.info/encyclopedia/A/ammonialife.html | archive-date = 2012-10-18 }}</ref> Ammonia is also flammable in oxygen and could not exist sustainably in an environment suitable for [[aerobic metabolism]].<ref name="dsmammonia">{{cite book |title=Life in the Universe: Expectations and Constraints |url=https://archive.org/details/lifeuniverseexpe00schu |url-access=limited |first1=Dirk |last1= Schulze-Makuch |first2=Louis Neal |last2=Irwin |publisher=Springer |edition=2 |date=2008 |isbn= 978-3-540-76816-6 |page=[https://archive.org/details/lifeuniverseexpe00schu/page/n130 119]}}</ref>

[[File:Layers of titan.jpg|thumb|right|upright=1.5|Titan's theorized internal structure, subsurface ocean shown in blue]]
A [[biosphere]] based on ammonia would likely exist at temperatures or air pressures that are extremely unusual in relation to life on Earth. Life on Earth usually exists between the melting point and [[boiling point]] of water, at a pressure designated as [[normal pressure]], between {{convert|0|and|100|°C|K|abbr=on|lk=out}}. When also held to normal pressure, ammonia's melting and boiling points are {{convert|-78|°C|K|abbr=on}} and {{convert|-33|°C|K|abbr=on}} respectively. Because chemical reactions generally proceed more slowly at lower temperatures, ammonia-based life existing in this set of conditions might metabolize more slowly and evolve more slowly than life on Earth.<ref name="dsmammonia" /> On the other hand, lower temperatures could also enable living systems to use chemical species that would be too unstable at Earth temperatures to be useful.<ref name="asimov" />

A set of conditions where ammonia is liquid at Earth-like temperatures would involve it being at a much higher pressure. For example, at 60&nbsp;[[Atmosphere (unit)|atm]] ammonia melts at {{convert|-77|°C|K|abbr=on}} and boils at {{convert|98|°C|K|abbr=on}}.<ref name="barossammonia" />

Ammonia and ammonia–water mixtures remain liquid at temperatures far below the freezing point of pure water, so such biochemistries might be well suited to planets and moons orbiting outside the water-based [[habitability zone]]. Such conditions could exist, for example, under the surface of [[Saturn]]'s largest moon [[Titan (moon)|Titan]].<ref>{{cite web |url=http://www.es.ucl.ac.uk/research/planetary/undergraduate/dom/titan/titan.htm |title=Exobiological Implications of a Possible Ammonia-Water Ocean Inside Titan |last=Fortes |first=A. D. |date=1999 |access-date=7 June 2010}}</ref>

=== Methane and other hydrocarbons ===
[[Methane]] (CH<sub>4</sub>) is a simple hydrocarbon: that is, a compound of two of the most common elements in the cosmos: hydrogen and carbon. It has a cosmic abundance comparable with ammonia.<ref name="asimov" /> Hydrocarbons could act as a solvent over a wide range of temperatures, but would lack [[chemical polarity|polarity]]. Isaac Asimov, the [[biochemist]] and science fiction writer, suggested in 1981 that poly-[[lipids]] could form a substitute for proteins in a non-polar solvent such as methane.<ref name="asimov" /> Lakes composed of a mixture of hydrocarbons, including methane and [[ethane]], have been detected on the surface of Titan by the [[Cassini–Huygens|''Cassini'' spacecraft]].

There is debate about the effectiveness of methane and other hydrocarbons as a solvent for life compared to water or ammonia.<ref name="methanesolvent">Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life, National Research Council; [http://books.nap.edu/openbook.php?record_id=11919&page=74 The Limits of Organic Life in Planetary Systems]; The National Academies Press, 2007; page 74.</ref><ref name="Polyesters">{{cite journal |title=Solubility of Polyethers in Hydrocarbons at Low Temperatures. A Model for Potential Genetic Backbones on Warm Titans |journal=Astrobiology |date=March 2015 |last1=McLendon |first1=Christopher |last2=Opalko |first2=F. Jeffrey |volume=15 |issue=3 |pages= 200–206 |doi=10.1089/ast.2014.1212 |bibcode=2015AsBio..15..200M |pmid=25761113}}</ref><ref>{{cite news |last=Hadhazy |first=Adam |url=http://www.space.com/29389-alien-life-hydrocarbon-exoplanets-ether-dna.html?adbid=10152808883996466&adbpl=fb&adbpr=17610706465&short_code=2ze6t |title=Alien Life on Oily Exoplanets Could Have Ether-based 'DNA' |work=Astrobiology Magazine |publisher=Space.com |date=13 May 2015 |access-date=2015-05-21 }}</ref> Water is a stronger solvent than the hydrocarbons, enabling easier transport of substances in a cell.<ref name=methlife>{{cite web |title=What is Consuming Hydrogen and Acetylene on Titan? |publisher=NASA/JPL |date=2010 |access-date=2010-06-06 |url=http://www.jpl.nasa.gov/news/news.cfm?release=2010-190 |url-status=dead |archive-url=https://web.archive.org/web/20110629185640/http://www.jpl.nasa.gov/news/news.cfm?release=2010-190 |archive-date=June 29, 2011 }}</ref> However, water is also more chemically reactive and can break down large organic molecules through hydrolysis.<ref name=methanesolvent /> A life-form whose solvent was a hydrocarbon would not face the threat of its biomolecules being destroyed in this way.<ref name=methanesolvent /> Also, the water molecule's tendency to form strong hydrogen bonds can interfere with internal hydrogen bonding in complex organic molecules.<ref name="barosswater" /> Life with a hydrocarbon solvent could make more use of hydrogen bonds within its biomolecules.<ref name="methanesolvent" /> Moreover, the strength of hydrogen bonds within biomolecules would be appropriate to a low-temperature biochemistry.<ref name="methanesolvent" />

Astrobiologist [[Christopher McKay (planetary scientist)|Chris McKay]] has argued, on thermodynamic grounds, that if life does exist on Titan's surface, using hydrocarbons as a solvent, it is likely also to use the more complex hydrocarbons as an energy source by reacting them with hydrogen, [[redox|reducing]] ethane and [[acetylene]] to methane.<ref name="mckay">{{cite journal |journal = Icarus |volume= 178 |issue = 1 |pages = 274–276 |date= 2005 |doi = 10.1016/j.icarus.2005.05.018 |title = Possibilities for methanogenic life in liquid methane on the surface of Titan |author1=McKay, C. P. |author2=Smith, H. D. |bibcode=2005Icar..178..274M|url= https://zenodo.org/record/1259025 }}</ref> Possible evidence for this form of [[life on Titan]] was identified in 2010 by Darrell Strobel of [[Johns Hopkins University]]; a greater abundance of molecular hydrogen in the upper atmospheric layers of Titan compared to the lower layers, arguing for a downward diffusion at a rate of roughly 10<sup>25</sup> molecules per second and disappearance of hydrogen near Titan's surface. As Strobel noted, his findings were in line with the effects Chris McKay had predicted if [[methanogenic]] life-forms were present.<ref name="methlife" /><ref name="mckay" /><ref name="strobel">{{cite journal |last1=Strobel |first1=Darrell F. |title=Molecular hydrogen in Titan's atmosphere: Implications of the measured tropospheric and thermospheric mole fractions |journal=Icarus |date=August 2010 |volume=208 |issue=2 |pages=878–886 |doi=10.1016/j.icarus.2010.03.003 |bibcode=2010Icar..208..878S }}</ref> The same year, another study showed low levels of acetylene on Titan's surface, which were interpreted by Chris McKay as consistent with the hypothesis of organisms reducing acetylene to methane.<ref name="methlife" /> While restating the biological hypothesis, McKay cautioned that other explanations for the hydrogen and acetylene findings are to be considered more likely: the possibilities of yet unidentified physical or chemical processes (e.g. a non-living surface [[catalyst]] enabling acetylene to react with hydrogen), or flaws in the current models of material flow.<ref name="life?">{{cite web |title=Have We Discovered Evidence For Life On Titan |author=Mckay, Chris |date=2010 |url=http://astronomy.nmsu.edu/tharriso/ast105/making_sense.php.html |work=[[New Mexico State University]]<!---[http://artsci.nmsu.edu/ College of Arts and Sciences], [http://astronomy.nmsu.edu/ Department of Astronomy]---> |access-date=2014-05-15 |archive-url=https://web.archive.org/web/20160309224810/http://astronomy.nmsu.edu/tharriso/ast105/making_sense.php.html |archive-date=2016-03-09 |url-status=dead }}</ref> He noted that even a non-biological catalyst effective at 95&nbsp;K would in itself be a startling discovery.<ref name="life?" />

==== Azotosome ====
A hypothetical [[cell membrane]] termed an azotosome, capable of functioning in liquid [[methane]] in Titan conditions was computer-modeled in an article published in February 2015. Composed of [[acrylonitrile]], a small molecule containing carbon, hydrogen, and nitrogen, it is predicted to have stability and flexibility in liquid methane comparable to that of a [[phospholipid bilayer]] (the type of cell membrane possessed by all life on Earth) in liquid water.<ref name=azotosomepaper>{{cite journal |last1=Stevenson |first1=James |last2=Lunine |first2=Jonathan |last3=Clancy |first3=Paulette |title=Membrane alternatives in worlds without oxygen: Creation of an azotosome |journal=Science Advances |date=27 Feb 2015 |volume=1 |issue=1 |doi=10.1126/sciadv.1400067 |pmid=26601130 |pmc=4644080 |pages=e1400067 |bibcode=2015SciA....1E0067S}}</ref><ref name=azotosomemodel>[http://phys.org/news/2015-02-life-saturn-moon-titan.html Life 'not as we know it' possible on Saturn's moon Titan].</ref> An analysis of data obtained using the Atacama Large Millimeter / submillimeter Array (ALMA), completed in 2017, confirmed substantial amounts of acrylonitrile in Titan's atmosphere.<ref name="SP-20170728">{{cite web |last=Wall |first=Mike |title=Saturn Moon Titan Has Molecules That Could Help Make Cell Membranes |url=https://www.space.com/37653-saturn-moon-titan-cell-membrane-molecules.html |date=28 July 2017 |work=[[Space.com]] |access-date=29 July 2017 }}</ref><ref name="SA-20170728">{{cite journal |author=Palmer, Maureen Y.|display-authors=etal |title=ALMA detection and astrobiological potential of vinyl cyanide on Titan |date=28 July 2017 |journal=[[Science Advances]] |volume =3 |number=7 |doi= 10.1126/sciadv.1700022 |pmc=5533535 |bibcode=2017SciA....3E0022P |pmid=28782019 |page=e1700022}}</ref> Later studies questioned whether acrylonitrile would be able to self-assemble into azotosomes.<ref>{{Cite journal|last1=Sandström|first1=H.|last2=Rahm|first2=M.|date=January 2020|title=Can polarity-inverted membranes self-assemble on Titan?|journal=Science Advances|volume=6 |issue=4 |pages=eaax0272 |language=EN|doi=10.1126/sciadv.aax0272|pmc=6981084|pmid=32042894|bibcode=2020SciA....6..272S }}</ref>

=== Hydrogen fluoride ===
[[Hydrogen fluoride]] (HF), like water, is a polar molecule, and due to its polarity it can dissolve many ionic compounds. At [[atmospheric pressure]], its melting point is {{convert|189.15|K|C}}, and its boiling point is {{convert|292.69|K|C}}; the difference between the two is a little more than 100&nbsp;K. HF also makes hydrogen bonds with its neighbor molecules, as do water and ammonia. It has been considered as a possible solvent for life by scientists such as Peter Sneath<ref name="sneath">{{cite book |title=Planets and Life |last=Sneath |first=P.&nbsp;H.&nbsp;A. |date=1970 |publisher=Thames and Hudson}} cited in {{cite book |title=Extraterrestrial Encounter |last=Boyce |first=Chris |date=1981 |publisher=New English Library |pages=125, 182}}</ref> and Carl Sagan.<ref name="Sagan, Carl 2002 128" />

HF is dangerous to the systems of molecules that Earth-life is made of, but certain other organic compounds, such as [[paraffin wax]]es, are stable with it.<ref name="Sagan, Carl 2002 128" /> Like water and ammonia, liquid hydrogen fluoride supports an acid–base chemistry. Using a solvent system definition of acidity and basicity, [[nitric acid]] functions as a base when it is added to liquid HF.<ref>{{cite book |title=Chemistry in Nonaqueous Ionizing solvents: Inorganic Chemistry in Liquid Hydrogen Cyanide and Liquid hydrogen Fluoride |first1=Gerhart |last1= Jander |first2=Hans |last2= Spandau |first3=C.&nbsp;C. |last3=Addison |publisher=Pergamon Press |location=N.Y. |date=1971 |volume=II}} cited in {{cite book |title=Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization |first=Robert A. |last= Freitas |publisher=Xenology Research Institute |location=Sacramento, CA |date=1979 |chapter=8.2.2 |chapter-url=http://www.xenology.info/Xeno/8.2.2.htm}}</ref>

However, hydrogen fluoride is cosmically rare, unlike water, ammonia, and methane.<ref>{{cite book |title=Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization |first=Robert A. |last= Freitas |publisher=Xenology Research Institute |location=Sacramento, CA |date=1979 |chapter=8.2.2 |chapter-url=http://www.xenology.info/Xeno/8.2.2.htm}}</ref>

=== Hydrogen sulfide ===
[[Hydrogen sulfide]] is the closest [[Hydrogen chalcogenide|chemical analog to water]],<ref>{{cite web | url=http://www.daviddarling.info/encyclopedia/S/solvent.html | title = solvent | access-date = 2012-10-12 | last = Darling | first = David}}</ref> but is less polar and is a weaker inorganic solvent.<ref>{{cite book| title=Ionizing Solvents| first1=J.| last1= Jander| first2=C.| last2=Lafrenz | publisher=John Wiley & Sons Ltd., Verlag Chemie| location=Weinheim/Bergstr.| date=1970|volume=I}} cited in {{cite book| title=Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization| first=Robert A.| last= Freitas| publisher=Xenology Research Institute| location=Sacramento, CA| date=1979 |chapter=8.2.2 |chapter-url=http://www.xenology.info/Xeno/8.2.2.htm}}</ref> Hydrogen sulfide is quite plentiful on Jupiter's moon [[Io (moon)|Io]] and may be in liquid form a short distance below the surface; astrobiologist [[Dirk Schulze-Makuch]] has suggested it as a possible solvent for life there.<ref>{{cite web |url=http://www.astrobio.net/exclusive/3518/the-chance-for-life-on-io | title = The Chance for Life on Io | access-date = 2013-05-25 | last = Choi | first = Charles Q.| date = 2010-06-10 |archive-url=https://web.archive.org/web/20121027183552/http://www.astrobio.net/exclusive/3518/the-chance-for-life-on-io |archive-date=2012-10-27 |url-status=usurped}}</ref> On a planet with hydrogen sulfide oceans, the source of the hydrogen sulfide could come from volcanoes, in which case it could be mixed in with a bit of [[hydrogen fluoride]], which could help dissolve minerals. Hydrogen sulfide life might use a mixture of carbon monoxide and carbon dioxide as their carbon source. They might produce and live on [[sulfur monoxide]], which is analogous to oxygen (O<sub>2</sub>). Hydrogen sulfide, like hydrogen cyanide and ammonia, suffers from the small temperature range where it is liquid, though that, like that of hydrogen cyanide and ammonia, increases with increasing pressure.

=== Silicon dioxide and silicates ===
[[Silicon dioxide]], also known as silica and quartz, is very abundant in the universe and has a large temperature range where it is liquid. However, its melting point is {{convert|1,600|to|1,725|C|F}}, so it would be impossible to make organic compounds in that temperature, because all of them would decompose. Silicates are similar to silicon dioxide and some have lower melting points than silica. Feinberg and Shapiro have suggested that molten silicate rock could serve as a liquid medium for organisms with a chemistry based on silicon, oxygen, and other elements such as [[aluminium]].<ref>{{cite book |author1=David W. Koerner |author2=Simon LeVay |title=Here Be Dragons : The Scientific Quest for Extraterrestrial Life |date=2000 |isbn=978-0-19-803337-0 |publisher=Oxford U.P. |pages=202}}</ref>

=== Other solvents or cosolvents ===
[[File:Sulfuric-acid-Givan-et-al-1999-3D-balls.png|thumb|Sulfuric acid (H<sub>2</sub>SO<sub>4</sub>)]]
Other solvents sometimes proposed:
* [[Supercritical fluid]]s: [[supercritical carbon dioxide]] and supercritical hydrogen.<ref name="Supercritical Carbon Dioxide">{{cite journal |last1=Budisa |first1=Nediljko |last2=Schulze-Makuch |first2=Dirk |title=Supercritical Carbon Dioxide and Its Potential as a Life-Sustaining Solvent in a Planetary Environment |journal=Life |date=8 August 2014 |volume=4 |issue=3 |pages=331–340 |doi=10.3390/life4030331 |pmc=4206850 |pmid=25370376|bibcode=2014Life....4..331B |doi-access=free }}</ref>
* Simple hydrogen compounds: [[hydrogen chloride]].<ref name="wardsolvents">{{Cite book | last1 = Ward | first1 = Peter D. | last2 = Benner | first2 = Steven A. | contribution = Alien biochemistries | date = 2007 | title = Planets and Life | editor-last = Sullivan | editor-first = Woodruff T. | editor2-last = Baross | editor2-first = John A. | page = 540 | place = Cambridge | publisher = Cambridge | isbn = 978-0-521-53102-3}}</ref>
* More complex compounds: [[sulfuric acid]],<ref name="barosssulfuric" /> [[formamide]],<ref name="Planetary Systems p 74" /> [[methanol]].<ref name="wardsolvents" />
* Very-low-temperature fluids: [[liquid nitrogen]]<ref name="barosscryo" /> and [[liquid hydrogen|hydrogen]].<ref name="barosscryo" />
* High-temperature liquids: [[sodium chloride]].<ref>[http://www.astrobio.net/news-exclusive/the-methane-habitable-zone/ The methane habitable zone].</ref>

Sulfuric acid in liquid form is strongly polar. It remains liquid at higher temperatures than water, its liquid range being 10&nbsp;°C to 337&nbsp;°C at a pressure of 1&nbsp;atm, although above 300&nbsp;°C it slowly decomposes. Sulfuric acid is known to be abundant in the [[atmosphere of Venus|clouds of Venus]], in the form of [[aerosol]] droplets. In a biochemistry that used sulfuric acid as a solvent, the [[alkene]] group (C=C), with two carbon atoms joined by a double bond, could function analogously to the carbonyl group (C=O) in water-based biochemistry.<ref name="barosssulfuric" />

A proposal has been made that life on Mars may exist and be using a mixture of water and [[hydrogen peroxide]] as its solvent.<ref>{{cite journal | title = A Possible Biogenic Origin for Hydrogen Peroxide on Mars | journal = International Journal of Astrobiology | volume = 6 | issue = 2 | pages = 147 | date = 2007-05-22 | first = Joop M. | last = Houtkooper |author2=Dirk Schulze-Makuch | doi = 10.1017/S1473550407003746 | arxiv = physics/0610093 |bibcode = 2007IJAsB...6..147H | s2cid = 8091895 }}</ref>
A 61.2% (by mass) mix of water and hydrogen peroxide has a freezing point of −56.5&nbsp;°C and tends to [[super-cool]] rather than crystallize. It is also [[hygroscopic]], an advantage in a water-scarce environment.<ref>{{cite journal |url=http://www.cosis.net/abstracts/EPSC2007/00439/EPSC2007-J-00439.pdf |archive-url=https://web.archive.org/web/20070926102728/http://www.cosis.net/abstracts/EPSC2007/00439/EPSC2007-J-00439.pdf |archive-date=2007-09-26 |url-status=live |journal=EPSC Abstracts |volume= 2 |id= EPSC2007-A-00439 |date=2007 |title=The H<sub>2</sub>O<sub>2</sub>–H<sub>2</sub>O Hypothesis: Extremophiles Adapted to Conditions on Mars? |first=Joop M. |last= Houtkooper |author2=Dirk Schulze-Makuch |bibcode=2007epsc.conf..558H |pages=558}}</ref><ref>{{cite web |url=http://www.planetary.org/blog/article/00001109/ |title=Europlanet : Life's a bleach |date=2007-08-24 |first=Doug |last=Ellison |publisher=Planetary.org |access-date=2007-08-25 |archive-date=2010-09-25 |archive-url=https://web.archive.org/web/20100925041103/http://www.planetary.org/blog/article/00001109/ |url-status=dead }}</ref>

Supercritical carbon dioxide has been proposed as a candidate for alternative biochemistry due to its ability to selectively dissolve organic compounds and assist the functioning of enzymes and because "super-Earth"- or "super-Venus"-type planets with dense high-pressure atmospheres may be common.<ref name="Supercritical Carbon Dioxide" />

== Other speculations ==
=== Non-green photosynthesizers ===
Physicists have noted that, although photosynthesis on Earth generally involves green plants, a variety of other-colored plants could also support photosynthesis, essential for most life on Earth, and that other colors might be preferred in places that receive a different mix of stellar radiation than Earth.<ref>{{cite web|url=http://www.nasa.gov/centers/goddard/news/topstory/2007/spectrum_plants.html |title=NASA – NASA Predicts Non-Green Plants on Other Planets |publisher=Nasa.gov |date=2008-02-23 |access-date=2010-05-29}}</ref><ref name="Kiang">{{Cite journal | last = Kiang | first = Nancy Y. |author2=Segura, Antígona |author2-link=Antígona Segura |author3=Tinetti, Giovanna |author4=Jee, Govind|author5-link=Robert E. Blankenship |author5=Blankenship, Robert E. |author6=Cohen, Martin |author7=Siefert, Janet |author8=Crisp, David |author9= Meadows, Victoria S.  | title = Spectral signatures of photosynthesis. II. Coevolution with other stars and the atmosphere on extrasolar worlds | journal = [[Astrobiology (journal)|Astrobiology]] | volume = 7 | issue = 1 | pages = 252–274 | date = 2007-04-03 | doi = 10.1089/ast.2006.0108 | bibcode=2007AsBio...7..252K | pmid=17407410|arxiv = astro-ph/0701391 | s2cid = 9172251 }}</ref>
These studies indicate that blue plants would be unlikely; however yellow or red plants may be relatively common.<ref name="Kiang" />

=== Variable environments ===
Many Earth plants and animals undergo major biochemical changes during their life cycles as a response to changing environmental conditions, for example, by having a [[spore]] or [[hibernation]] state that can be sustained for years or even millennia between more active life stages.<ref name="Christmas in Yellowstone">{{cite web |url=https://www.pbs.org/wnet/nature/yellowstone/hibernate.html |title=Christmas in Yellowstone |publisher=Pbs.org |access-date=2010-05-29 |archive-date=2008-10-08 |archive-url=https://web.archive.org/web/20081008005741/http://www.pbs.org/wnet/nature/yellowstone/hibernate.html |url-status=dead }}</ref>  Thus, it would be biochemically possible to sustain life in environments that are only periodically consistent with life as we know it.

For example, frogs in cold climates can survive for extended periods of time with most of their body water in a frozen state,<ref name="Christmas in Yellowstone" /> whereas desert frogs in Australia can become inactive and dehydrate in dry periods, losing up to 75% of their fluids, yet return to life by rapidly rehydrating in wet periods.<ref>{{Cite journal|title=Main and Bentley, Ecology, "Water Relations of Australian Burrowing Frogs and Tree Frogs" (1964)|journal = Ecology|volume = 45|issue = 2|pages = 379–382|jstor = 1933854|last1 = Main|first1 = A. R.|last2 = Bentley|first2 = P. J.|year = 1964|doi = 10.2307/1933854}}</ref> Either type of frog would appear biochemically inactive (i.e. not living) during dormant periods to anyone lacking a sensitive means of detecting low levels of metabolism.

=== Alanine world and hypothetical alternatives ===
[[File:Early genetic code.svg|thumb|Early stage of the genetic code (GC-Code) with "alanine world" and its possible alternatives.]]
The [[genetic code]] may have evolved during the transition from the [[RNA world]] to a [[protein]] world.<ref>{{cite journal | first=Abdrew J. | last=Doig | title=Frozen, but no accident – why the 20 standard amino acids were selected | date=7 December 2016 | doi=10.1111/febs.13982 | volume=284 | issue=9 | journal=FEBS J. | pages=1296–1305| pmid=27926995 | doi-access=free }}</ref> The [[Alanine#Alanine world hypothesis|Alanine World Hypothesis]] postulates that the evolution of the genetic code (the so-called GC phase<ref>{{cite journal | first1=Hyman | last1=Hartman | first2=Temple F. | last2=Smith | title=The Evolution of the Ribosome and the Genetic Code | date=20 May 2014 | doi=10.3390/life4020227 | volume=4 | issue=2 | journal=Life | pages=227–249| pmid=25370196 | pmc=4187167 | bibcode=2014Life....4..227H | doi-access=free }}</ref>) started with only four basic [[amino acid]]s: [[alanine]], [[glycine]], [[proline]] and [[ornithine]] (now [[arginine]]).<ref>{{cite journal|last1=Kubyshkin|first1=Vladimir|last2=Budisa|first2=Nediljko|title=The Alanine World Model for the Development of the Amino Acid Repertoire in Protein Biosynthesis|journal=Int. J. Mol. Sci.|date=24 September 2019|volume=20|issue=21|pages=5507|doi=10.3390/ijms20215507|pmid=31694194|pmc=6862034|doi-access=free}}</ref> The evolution of the genetic code ended with 20 [[proteinogenic amino acid]]s. From a chemical point of view, most of them are Alanine-derivatives particularly suitable for the construction of [[Alpha helix|α-helices]] and [[Beta sheet|β-sheets]]{{snd}} basic [[Protein secondary structure|secondary structural]] elements of modern proteins. Direct evidence of this is an experimental procedure in [[molecular biology]] known as [[alanine scanning]].

A hypothetical "Proline World" would create a possible alternative life with the genetic code based on the proline chemical scaffold as the [[Protein primary structure|protein backbone]]. Similarly, a "Glycine World" and "Ornithine World" are also conceivable, but nature has chosen none of them.<ref>{{cite journal|last1=Kubyshkin|first1=Vladimir|last2=Budisa|first2=Nediljko|title=Anticipating alien cells with alternative genetic codes: away from the alanine world!|journal=Curr. Opin. Biotechnol.|date=3 July 2019|volume=60|pages=242–249|doi=10.1016/j.copbio.2019.05.006|pmid=31279217|doi-access=free}}</ref> Evolution of [[life]] with Proline, Glycine, or Ornithine as the basic structure for protein-like [[polymer]]s ([[foldamer]]s) would lead to parallel biological worlds. They would have morphologically radically different [[body plan]]s and [[genetics]] from the living organisms of the known [[biosphere]].<ref>{{cite journal|last1=Budisa|first1=Nediljko|last2=Kubyshkin|first2=Vladimir|last3=Schmidt|first3=Markus|title=Xenobiology: A Journey towards Parallel Life Forms|journal=ChemBioChem|date=22 April 2020|volume=21|issue=16|pages=2228–2231|doi=10.1002/cbic.202000141|pmid=32323410|doi-access=free}}</ref>

== Nonplanetary life ==
{{main|Non-planetary abiogenesis}}
=== Dusty plasma-based ===
{{See also|Dusty plasma}}
In 2007, Vadim N. Tsytovich and colleagues proposed that lifelike behaviors could be exhibited by dust particles suspended in a [[Plasma (physics)|plasma]], under conditions that might exist in space.<ref name="dust1">{{cite press release |title=Physicists Discover Inorganic Dust With Lifelike Qualities |url=https://www.sciencedaily.com/releases/2007/08/070814150630.htm |work=ScienceDaily |publisher=Institute of Physics |date=15 August 2007 }}</ref><ref name="dust2">{{cite journal| title=From plasma crystals and helical structures towards inorganic living matter| journal= New J. Phys.| volume= 9| issue= 263| doi=10.1088/1367-2630/9/8/263|first=V N| last= Tsytovich| author2= G E Morfill, V E Fortov, N G Gusein-Zade, B A Klumov and S V Vladimirov| date=14 August 2007| page=263|bibcode = 2007NJPh....9..263T | last3=Fortov| first3=V E| last4=Gusein-Zade| first4=N G| last5=Klumov| first5=B A| last6=Vladimirov| first6=S V| doi-access=free}}</ref> Computer models showed that, when the dust became charged, the particles could self-organize into microscopic helical structures, and the authors offer "a rough sketch of a possible model of...helical grain structure reproduction".

=== Cosmic necklace-based ===
In 2020, Luis A. Anchordoqu and Eugene M. Chudnovsky of the [[City University of New York]] hypothesized that cosmic necklace-based life composed of magnetic monopoles connected by [[cosmic string]]s could evolve inside stars.<ref name="necklace">{{cite journal |last1=A. Anchordoqui |first1=Luis |last2=M. Chudnovsky |first2=Eugene |title=Can Self-Replicating Species Flourish in the Interior of a Star? |journal=Letters in High Energy Physics |date=31 March 2020 |volume=2020 |page=166 |doi=10.31526/lhep.2020.166 |bibcode=2020LHEP....3..166A }}</ref> This would be achieved by a stretching of cosmic strings due to the star's intense gravity, thus allowing it to take on more complex forms and potentially form structures similar to the RNA and DNA structures found within carbon-based life. As such, it is theoretically possible that such beings could eventually become intelligent and construct a civilization using the power generated by the star's nuclear fusion. Because such use would use up part of the star's energy output, the luminosity would also fall. For this reason, it is thought that such life might exist inside stars observed to be cooling faster or dimmer than current cosmological models predict.

=== Life on a neutron star ===
[[Frank Drake]] suggested in 1973 that intelligent life could inhabit [[neutron star]]s.<ref>{{cite journal|last=Drake|first=F.D.|date=December 1973|title=Life on a Neutron Star: An Interview with Frank Drake|journal=Astronomy|pages=5–8|url=https://www.gwern.net/docs/science/1973-drake.pdf |archive-url=https://web.archive.org/web/20210315021022/https://www.gwern.net/docs/science/1973-drake.pdf |archive-date=2021-03-15 |url-status=live}}</ref> Physical models in 1973 implied that Drake's creatures would be microscopic.{{citation needed|date=April 2024}}

== Scientists who have published on this topic ==
Scientists who have considered possible alternatives to carbon-water biochemistry include:
*'''[[J. B. S. Haldane]]''' (1892–1964), a geneticist noted for his work on [[abiogenesis]].<ref name="haldane" />
*'''[[Axel Firsoff|V. Axel Firsoff]]''' (1910–1981), British astronomer.<ref>{{Cite journal|author=V. Axel Firsoff|date=January 1962|title=An Ammonia-Based Life|journal=Discovery|volume=23|pages=36–42}} cited in {{cite web|url=http://www.daviddarling.info/encyclopedia/A/ammonialife.html/|title=ammonia-based life|last=Darling|first=David|archive-url=https://web.archive.org/web/20121018095038/http://www.daviddarling.info/encyclopedia/A/ammonialife.html|archive-date=2012-10-18|url-status=dead|access-date=2012-10-01}}</ref>
*'''[[Isaac Asimov]]''' (1920–1992), biochemist and science fiction writer.<ref name="asimov">{{Cite journal| journal=Cosmic Search|title=Not as We Know it – the Chemistry of Life|date=Winter 1981|issue= 9 (Vol 3 No 1)|author=Isaac Asimov|publisher=North American AstroPhysical Observatory|url=http://www.bigear.org/CSMO/HTML/CS09/cs09all.htm}}</ref>
*'''[[Fred Hoyle]]''' (1915–2001), astronomer and science fiction writer.
*'''[[Norman Horowitz]]''' (1915–2005), [[California Institute of Technology|Caltech]] geneticist who devised the first experiments carried out to detect life on Mars.<ref name = Horowitz1986 />
*'''[[George C. Pimentel]]''' (1922–1989), American chemist, [[University of California, Berkeley]].<ref name="Shklovskii 1977 229">{{cite book|title=Intelligent Life in the Universe|last=Shklovskii|first=I.S.|author2=Carl Sagan|date=1977|publisher=Picador|page=229}}</ref>
*'''[[Peter Sneath]]''' (1923–2011), microbiologist, author of the book ''Planets and Life''.<ref name="sneath" />
*'''[[Gerald Feinberg]]''' (1933–1992), physicist and '''[[Robert Shapiro (chemist)|Robert Shapiro]]''' (1935–2011), chemist, co-authors of the book ''Life Beyond Earth''.<ref>{{cite book|title=Life Beyond Earth|url=https://archive.org/details/lifebeyondearthi0000fein|url-access=registration|last=Feinberg|first=Gerald|author2=Robert Shapiro|date=1980|publisher=Morrow|isbn=978-0-688-03642-3}}{{pn|date=June 2024}}</ref><ref>A detailed review of this book is: {{Cite journal|author=John Gribbin|date=2 Oct 1980|title=Life beyond Earth|journal=New Scientist|page=xvii}}</ref>
*'''[[Carl Sagan]]''' (1934–1996), astronomer,<ref name="Shklovskii 1977 229" /> science popularizer, and [[SETI]] proponent.
*'''[[Jonathan Lunine]]''' (born 1959), American planetary scientist and physicist.
*'''[[Robert Freitas]]''' (born 1952), specialist in nano-technology and nano-medicine.<ref>{{cite book|url=http://www.xenology.info/Xeno.htm|title=Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization|last=Freitas|first=Robert A.|date=1979|publisher=Xenology Research Institute|location=Sacramento, CA}}</ref><ref>This work is acknowledged the partial basis of the article {{cite web|url=http://www.daviddarling.info/encyclopedia/A/ammonialife.html/|title=ammonia-based life|last=Darling|first=David|archive-url=https://web.archive.org/web/20121018095038/http://www.daviddarling.info/encyclopedia/A/ammonialife.html|archive-date=2012-10-18|url-status=dead|access-date=2012-10-01}}</ref>
* '''[[John Baross]]''' (born 1940), oceanographer and astrobiologist, who chaired a committee of scientists under the [[United States National Research Council]] that published a report on life's limiting conditions in 2007.<ref name="baross">Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life, National Research Council; [http://books.nap.edu/openbook.php?record_id=11919 The Limits of Organic Life in Planetary Systems]; The National Academies Press, 2007.</ref><ref>Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life, National Research Council; [http://books.nap.edu/openbook.php?record_id=11919&page=R10 The Limits of Organic Life in Planetary Systems]; The National Academies Press, 2007; page 5</ref>

== See also ==
{{div col|colwidth=30em}}
* [[Abiogenesis]]
* [[Astrobiology]]
* [[Carbon chauvinism]]
* [[Carbon-based life]]
* [[Earliest known life forms]]
* [[Extraterrestrial life]]
* [[Hachimoji DNA]]
* [[Iron–sulfur world hypothesis]]
*[[Life origination beyond planets]]
* [[Mirror life]]
* [[Nexus for Exoplanet System Science]]
* [[Non-cellular life]]
* [[Non-proteinogenic amino acids]]
* [[Nucleic acid analogues]]
* [[Planetary habitability]]
* [[Shadow biosphere]]
{{div col end}}

== References ==
{{reflist|30em}}

== Further reading ==
* {{Cite journal| journal=Astrobiology |title=Many Chemistries Could Be Used to Build Living Systems | date=2004 |volume=4 |pages= 137–167 |first=William |last=Bains |s2cid=27477952 | doi = 10.1089/153110704323175124| pmid=15253836| issue=2 |bibcode = 2004AsBio...4..137B }}

== External links ==
* [http://www.faqs.org/faqs/astronomy/faq/part6/section-16.html Astronomy FAQ]
* [http://www.daviddarling.info/encyclopedia/A/ammonialife.html Ammonia-based life]
* [http://www.daviddarling.info/encyclopedia/S/siliconlife.html Silicon-based life]
{{Astrobiology}}
{{Extraterrestrial life}}
{{Molecules detected in outer space}}

{{Portal bar|Astronomy|Biology|Space}}

[[Category:Astrobiology]]
[[Category:Science fiction themes]]
[[Category:Biological hypotheses]]
[[Category:Scientific speculation]]