Kebolehdiaman planet: Perbezaan antara semakan
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Sungguhpun sebahagian besar bintang terdiri daripada [[hidrogen]] dan [[helium]], namun terdapat variasi yang ketara dalam jumlah kandungan unsur berat ([[logam]]). Kadar logam yang tinggi dalam bintang dihubungkaitkan dengan berapa banyaknya bahan berat yang ada pada mulanya dalam {{link-en|cakera protoplanet|protoplanetary disk}}. Mengikut teori pembentukan [[sistem planet]], Jika jumlah logam sedikit maka kemungkinan terbentuknya planet adalah makin tipis. Sebarang planet yang terbentuk di keliling bintang yang kurang logam barangkali rendah jisim dan oleh itu tidak sesuai untuk kehidupan. Kajian-kajian [[spektroskopi]] ke atas sistem-sistem yang terdapat [[eksoplanet]] setakat ini mengesahkan hubung kait antara kandungan logam yang tinggi dan pembentuk planet, yang mana bintang mana yang mempunyai planet yang setidak-tidaknya serupa dengan planet yang sedia diketahui, ternyata lebih kaya dengan logam jika dibandingkan dengan bintang yang tidak diiringi planet.<ref>{{cite web |last1=Santos |first1=Nuno C. |last2=Israelian |first2=Garik |last3=Mayor |first3=Michael |date=2003 |title=Confirming the Metal-Rich Nature of Stars with Giant Planets |work=Proceedings of 12th Cambridge Workshop on Cool Stars, Stellar Systems, and The Sun |publisher=[[University of Colorado at Boulder|University of Colorado]] |url=http://origins.colorado.edu/cs12/proceedings/oral/tuesday/santosxx.pdf |access-date=11 August 2007 }}</ref> Hubungan antara kelogaman tinggi dan pembentukan planet ini juga bermaksud kemungkinan besar sistem-sistem yang boleh didiami terdapat pada bintang generasi muda kerana bintang yang terbentuk awal-awal dalam sejarah alam semesta adalah kurang kandungan logam.
== Ciri-ciri planet ==
[[File:Earthlike moon extrasolar gas giant.jpg|thumb|right|Bulan yang mengitari gergasi gas mungkin boleh didiami.<ref name="moonlife">{{cite web|url=http://library.thinkquest.org/C003763/index.php?page=interview13 |title=An interview with Dr. Darren Williams |date=2000 |work=Astrobiology: The Living Universe |access-date=5 August 2007 |url-status=dead |archive-url=https://web.archive.org/web/20070828084412/http://library.thinkquest.org/C003763/index.php?page=interview13 |archive-date=28 August 2007 }}</ref>]]
Tanda-tanda kebolehdiaman dan [[biopenunjuk]] harus ditafsirkan dalam konteks planet dan alam sekitar.<ref name='NASA strategy 2015'/> Sama ada planet itu boleh didiami atau tidak bergantung pada turutan peristiwa yang menjurus ke arah pembentukannya, yang mungkin merangkumi penghasilan molekul-molekul organik dalam [[awan molekul|awanan molekul]] dan [[cakera protoplanet]], penyampaian bahan-bahan sewaktu dan selepas [[Tokokan (astrofizik)|tokokan]], dan lokasi orbit dalam sistem cakerawalanya.<ref name='NASA strategy 2015'/> Tanggapan terutama mengenai planet boleh didiami adalah bahawa ia bersifat [[Planet bumian|bumian (terestrial)]]. Planet sedemikian terletak lebih kurang dalam satu [[peringkat magnitud]] [[jisim Bumi]], terutamanya terbentuk daripada batu [[silikat]] dan tidak menokok lapisan luaran bergas [[hidrogen]] dan [[helium]] yang terdapat pada [[gergasi gas]]. Kemungkinan bahawa hidupan boleh berkembang di puncak awanan planet gergasi gas belum dikesampingkan sama sekali,{{refn |group="lower-alpha" |Dalam buku ''[[Evolving the Alien]]'', [[Jack Cohen (saintis)|Jack Cohen]] dan [[Ian Stewart (ahli matematik)|Ian Stewart]] menilai kemunasabahan senario-senario di mana mungkin terbentuknya hidupan di puncak awanan planet-planet seakan Musytari. Begitu juga, [[Carl Sagan]] berhujah bahawa mungkin terdapat hidupan di awanan [[Musytari]] itu sendiri.<ref name="Sagan, C. 1976" /><ref name="Darling" />}} tetapi dianggap tidak mungkin kerana tiadanya permukaan, malah gravitinya pun melampau.<ref>{{cite web |url=http://motivate.maths.org/conferences/conf58/c58_talk4.shtml |title=Could there be life in the outer solar system? |date=2002 |work=Millennium Mathematics Project, Videoconferences for Schools |publisher=[[University of Cambridge]] |access-date=5 August 2007 }}</ref> Akan tetapi, satelit semula jadi (bulan) yang mengitari planet gergasi gas masih calon yang sah untuk menempatkan kehidupan.<ref name="moonlife" />
Pada Februari 2011, misi [[Kepler (kapal angkasa)|Balai Cerap Angkasa Kepler]] mengeluarkan {{link-en|Senarai eksoplanet yang ditemui menggunakan kapal angkasa Kepler|List of exoplanets discovered using the Kepler spacecraft|senarai 1235 calon planet luar suria}}, termasuklah 54 yang mungkin terletak dalam zon boleh didiami.<ref name=borucki>{{cite journal |date=2011 |title=Characteristics of planetary candidates observed by Kepler, II: Analysis of the first four months of data |journal=[[The Astrophysical Journal]] |volume=736 |issue=1 |pages=19 |arxiv=1102.0541 |bibcode= 2011ApJ...736...19B |doi=10.1088/0004-637X/736/1/19|last1=Borucki |first1=William J. |last2=Koch |first2=David G. |last3=Basri |first3=Gibor |last4=Batalha |first4=Natalie |last5=Brown |first5=Timothy M. |last6=Bryson |first6=Stephen T. |last7=Caldwell |first7=Douglas |last8=Christensen-Dalsgaard |first8=Jørgen |last9=Cochran |first9=William D. |last10=Devore |first10=Edna |last11=Dunham |first11=Edward W. |last12=Gautier |first12=Thomas N. |last13=Geary |first13=John C. |last14=Gilliland |first14=Ronald |last15=Gould |first15=Alan |last16=Howell |first16=Steve B. |last17=Jenkins |first17=Jon M. |last18=Latham |first18=David W. |last19=Lissauer |first19=Jack J. |last20=Marcy |first20=Geoffrey W. |last21=Rowe |first21=Jason |last22=Sasselov |first22=Dimitar |last23=Boss |first23=Alan |last24=Charbonneau |first24=David |last25=Ciardi |first25=David |last26=Doyle |first26=Laurance |last27=Dupree |first27=Andrea K. |last28=Ford |first28=Eric B. |last29=Fortney |first29=Jonathan |last30=Holman |first30=Matthew J. |s2cid=15233153 |display-authors=29 }}</ref><ref>{{cite web |url=http://kepler.nasa.gov/news/nasakeplernews/index.cfm?FuseAction=ShowNews&NewsID=98 |title=NASA Finds Earth-size Planet Candidates in Habitable Zone, Six Planet System |date=2 February 2011 |publisher=[[NASA]] |access-date=2 February 2011 }}</ref> Enam daripada calon dalam zon adalah lebih kecil daripada dua kali saiz Bumi.<ref name=borucki/> Satu lagi kajian terbaru mendapati bahawa salah satu calon tersebut (KOI 326.01) lebih besar dan panas daripada yang dilaporkan terlebih dahulu.<ref name=Grant>{{cite web |last=Grant |first=Andrew |title=Exclusive: "Most Earth-Like" Exoplanet Gets Major Demotion—It Isn't Habitable |url=http://blogs.discovermagazine.com/80beats/2011/03/08/exclusive-most-earth-like-exoplanet-gets-major-demotion-it-isnt-habitable/|publisher=[[Discover Magazine]] |date=8 March 2011 |access-date=9 March 2011 }}</ref> Berdasarkan dapatan ini, pasukan Kepler menganggarkan bahawa terdapat sekurang-kurangnya 50 bilion planet dalam Bimasakti, dan sekurang-kurangnya 500 juta daripadanya terletak dalam zon boleh didiami.<ref name="BorensteinS">{{cite news |first=Seth |last=Borenstein |title=Cosmic census finds crowd of planets in our galaxy |agency=Associated Press |date=19 February 2011 |url=http://apnews.excite.com/article/20110219/D9LG45NO0.html |access-date=19 February 2011 }}</ref>
Dalam menganalisis persekitaran mana yang berkemungkinan menampung kehidupan, selalunya terdapat perbezaan antara organisma unisel yang serba ringkas seperti [[bakteria]] dan [[arkaea]], dan makhluk-makhluk metazoan yang kompleks (seperti haiwan). Dalam apa juga salasilah hidupan sekalipun, organisma unisel mesti sentiasa mendahului organisma multisel, bahkan di mana adanya organisma unisel tidak terjamin akan adanya juga makhluk yang lebih kompleks.<ref group=lower-alpha>Semakin ramai pihak yang bersepakat bahawa mikroorganisma unisel sebenarnya umum terdapat di seantero jagat, lebih-lebih lagi memandangkan terdapat hidupan [[ekstremofil]] di Bumi yang mekar di persekitaran yang pernah dianggap mustahil didiami. Kemungkinan wujudnya hidupan multisel kompleks pula jauh lebih banyak dipertikaikan. Dalam karya ''[[Hipotesis Bumi Langka|Rare Earth: Why Complex Life Is Uncommon in the Universe]]'', [[Peter Ward (ahli paleontologi)|Peter Ward]] dan [[Donald E. Brownlee|Donald Brownlee]] berhujah bahawa hidupan mikrob barangkali meluas sementara hidupan kompleks sangat sukar hendak ditemui dan mungkin hanya terdapat di Bumi kita. Teori ini disokong sedikit sebanyak oleh pengetahuan semasa kita tentang sejarah Bumi, bahawa organisma multisel dipercayai sudah muncul sewaktu [[letupan Kambria]] sekitar 600 juta tahun dahulu, tetapi lebih 3 bilion tahun selepas hidupan mula-mula menjelma. Bahawasanya hidupan Bumi kekal unisel begitu lama menggariskan kenyataan bahawa langkah muktamad ke arah organisma kompleks tidak semestinya perlu berlaku.</ref> Ciri-ciri planet yang dihuraikan di bawah adalah dianggap mustahak untuk hidupan secara umumnya, tetapi dalam apa jua keadaan sekalipun, organisma multisel lebih banyak tuntutan daripada hidupan unisel.
=== Jisim ===
[[File:Mars sunset PIA00920.jpg|thumb|Planet [[Marikh]] dengan atmosferanya yang [[Penipisan|tipis]], jika terletak di jarak dari Matahari yang sama dengan Bumi sekalipun namun ia tetap lebih sejuk daripada Bumi.]]
Planet jisim rendah nyata kurang sesuai untuk hidupan atas dua sebab:
# Kurangnya [[graviti]] menyukarkan pemeliharaan [[Atmosfera planet|atmosfera]]. [[Molekul|Molekul-molekul]] juzuknya lebih berkemungkinan mencapai [[halaju lepas]] dan melesap ke angkasa jika ditimpa [[angin suria]] atau diganggu pelanggaran. Planet yang tidak tebal atmosferanya akan kekurangan jirim yang diperlukan untuk mencapai [[biokimia]] primal (untuk memulakan kehidupan), kekurangan tebatan (untuk membendung lepasan tenaga dari permukaan), kepincangan [[pemindahan haba]] seluruh permukaan (contohnya, Planet [[Marikh]] dengan atmosferanya yang [[Penipisan|tipis]], jika terletak di jarak dari Matahari yang sama dengan Bumi sekalipun namun ia tetap lebih sejuk daripada Bumi), dan kurangnya perlindungan daripada [[meteoroid]] dan [[sinaran]] berfrekuensi tinggi. Lebih-lebih lagi, jika atmosferanya kurang daripada 0.006 kali atmosfera Bumi, maka air tidak boleh wujud dalam bentuk cecair kerana tidak tercapainya [[tekanan atmosfera]] yang dikehendaki, iaitu 4.56 [[Torr|mm Hg]] (608 Pa) (atau 0.18 [[inci raksa|inci Hg]]).<!--http://zebu.uoregon.edu/~soper/Mars/water.html--> Umumnya, makin rendah tekanan maka makin kecil julat suhu air cecair.
# Makin kecil [[diameter]] sesebuah planet maka makin tinggi nisbah permukaan dengan isipadunya. Jasad-jasad seumpama ini sering cepat kehilangan tenaga yang tertinggal dari pembentukannya dan oleh itu ciri-ciri [[geologi]]nya pun mati, ketiadaan [[gunung berapi]], [[gempa bumi]] dan [[tektonik plat|aktiviti tektonik]] yang membekalkan bahan-bahan penampung kehidupan kepada permukaan dan juga bahan-bahan penyederhana suhu [[karbon dioksida]] ke dalam atmosfera. Nampaknya tektonik plat memainkan peranan amat penting di Bumi kerana bukan sahaja prosesnya mengitar semula bahan-bahan kimia dan mineral yang penting, bahkan juga memupuk [[biokepelbagaian]] menerusi pembentukan benua dan pertambahan rencam alam sekitar di samping membantu mewujudkan sel-sel perolakan yang diperlukan untuk menjana [[medan magnet Bumi]].<ref>[[#Ward|Ward]], pp. 191–220</ref>
"Jisim rendah" bukanlah faktor yang mutlak sepenuhnya; Bumi rendah jisimnya berbanding planet-planet [[gergasi gas]] dalam Sistem Suria tetapi mempunyai diameter, jisim, dan kepadatan tertinggi daripada semua jasad bumian.<ref group=lower-alpha>Terdapat "jurang jisim" dalam Sistem Suria antara Bumi dan dua gergasi gas terkecil iaitu [[Uranus]] dan [[Neptun]], masing-masing bersamaan 13 dan 17 kali jisim Bumi. Ini pun hanya kemungkinan kerana tidak terdapat halangan geofizik kepada pembentukan jasad-jasad perantara (jika dibandingkan dengan misalnya [[OGLE-2005-BLG-390Lb]] dan [[Super-Bumi]]), malah haruslah terdapat planet-planet dalam lingkungan 2 hingga 12 kali jisim Bumi di seluruh galaksi. Jika sistem bintangnya mengizinkan, maka planet-planetnya pun sesuai untuk hidupan kerana cukup besar untuk kekal dinamik secara dalaman dan memelihara atmosfera selama berbilion-bilion tahun tetapi tidaklah besar sehingga dapat menokok lindungan bergas yang akan mengehadkan kemungkinan pembentukan hidupan.</ref> Bumi cukup besar untuk memelihara atmosfera melalui graviti sahaja, dan juga cukup besar sehingga teras leburnya kekal sebagai 'enjin haba' yang memacu kepelbagaian geologi permukaan (satu lagi komponen utama pemanasan haba ialah penguraian unsur-unsur [[radioaktif]] dalam teras planet), berbanding dengan Marikh yang rata-rata 'mati bumi' dan kehilangan sebahagian besar atmosferanya.<ref>{{cite web |url=http://csmres.jmu.edu/geollab/Fichter/PlateTect/heathistory.html |title=The Heat History of the Earth |work=Geolab |publisher=[[James Madison University]] |access-date=11 May 2007 }}</ref> Maka wajarlah disimpulkan bahawa had jisim minimum bagi kebolehdiaman terletak antara jisim Marikh dan Bumi atau Zuhrah; telah disebutkan 0.3 kali jisim Bumi sebagai garis pemisah kasar untuk planet-planet boleh didiami.<ref>{{cite journal |last1=Raymond |first1=Sean N. |last2=Quinn |first2=Thomas |last3=Lunine |first3=Jonathan I. |date=January 2007 |title=High-resolution simulations of the final assembly of Earth-like planets 2: water delivery and planetary habitability |journal=Astrobiology |arxiv=astro-ph/0510285 |doi=10.1089/ast.2006.06-0126 |volume=7 |pages=66–84 |pmid=17407404 |bibcode=2007AsBio...7...66R |issue=1 |s2cid=10257401 |url=http://cds.cern.ch/record/895337 |type=Submitted manuscript }}</ref> Bagaimanapun, kajian Pusat Astrofizik Harvard-Smithsonian pada tahun 2008 mengusulkan bahawa garis pemisah tersebut mungkin lebih tinggi, dan Bumi mungkin sebetulnya terletak dekat dengan paras minimum kebolehdiaman kerana jika jisim lebih rendah daripada itu, maka mustahil terjadi tektonik plat. Planet Zuhrah yang jisimnya 85% jisim Bumi tidak menunjukkan sebarang tanda-tanda aktiviti tektonik. Sebaliknya, "[[super-Bumi]]" atau planet bumian yang lebih tinggi jisimnya berbanding Bumi akan mempunyai tahap tektonik plat yang lebih tinggi dan oleh itu nyata sekali terletak dalam julat boleh didiami.<ref>{{cite web |title=Earth: A Borderline Planet for Life? |work=Harvard-Smithsonian Center for Astrophysics |date=2008 |url=http://www.cfa.harvard.edu/press/2008/pr200802.html |access-date=4 June 2008 }}</ref>
Juga terdapat kekecualian daripada kesimpulan di atas asalkan disebabkan keadaan luar biasa: salah satu bulan [[Musytari]] bernama [[Io (bulan)|Io]] (yang lebih kecil daripada mana-mana planet bumian) mempunyai dinamik gunung berapi disebabkan tegasan-tegasan graviti yang diaruh oleh orbitnya, sementara jirannya [[Europa (bulam)|Europa]] mungkin mempunyai lautan cecair atau lecah ais di bawah lindungan beku yang juga disebabkan kuasa yang dijana daripada mengitari gergasi gas. Sementara itu, satelit [[Zuhal]] yang bernama [[Titan (bulan)|Titan]] mempunyai sedikit peluang untuk menampung hidupan kerana memelihara atmosfera yang tebal dan mmempunyai laut-laut [[metana]] cecair di permukaan. Tindak balas organik-kimia yang hanya memerlukan tenaga minimum boleh terjadi di dalam laut-laut ini, tetapi barangkali tidak wujudnya sebarang sistem hidupan yang berasaskan tindak balas yang begitu minimum. Jasad-jasad satelit tersebut biarpun kes terkecuali tetapi dapat membuktikan bahawa jisim sebagai syarat kebolehdiaman planet belum boleh dikira sebagai mutlak berdasarkan takat pengetahuan masa kini.<ref>{{Cite news |url=https://www.bbc.co.uk/news/science-environment-15863549 |title=Most liveable alien worlds ranked|access-date=2017-08-16 |date=November 23, 2011 |publisher=BBC Science & Environment|work=BBC News}}</ref>
Makin besar planetnya maka makin besar jugalah atmosferanya. Kombinasi halaju lepas tinggi untuk menakung atom ringan-ringan serta aktiviti peluahan gas yang berleluasa daripada peningkatan tektonik plat boleh menambah tekanan atmosfera dan suku di permukaan secara mendadak berbanding Bumi. Kesan rumah hijau dipertingkat dalam atmosfera begitu berat ini sering membayangkan bahawa zon boleh didiami bagi planet-planet begitu besar haruslah lebih jauh dari bintang pusat (berbanding jarak Bumi dari Matahari).
Akhir sekali, planet besar berkemungkinan mempunyai teras besi yang besar, membolehkan adanya [[medan magnet]] untuk [[magnetosfera|melindungi]] planet daripada [[angin najam]] dan [[sinaran kosmos]] yang boleh menghilangkan atmosfera planet dan menimpakan zarah-zarah [[Pengionan|terion]] ke atas segala hidupan yang ada. Jisim bukanlah satu-satunya kriteria untuk menghasilkan medan magnet kerana planetnya juga mesti berputar cukup laju untuk menjana [[Teori dinamo|kesan dinamo]] di dalam terasnya<ref>{{cite web |url=http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magearth.html |title=Magnetic Field of the Earth |last=Nave |first=C. R. |work=[[HyperPhysics]] |publisher=[[Georgia State University]] |access-date=11 May 2007 }}</ref>, tetapi ia tetap memainkan peranan penting dalam proses.
=== Radius ===
Ukuran jejari (radius) bagi eksoplanet yang berpotensi boleh didiami dikatakan tergolong dalam 0.5 hingga 2.5 kali jejari Bumi.<ref name="phl.upr.edu"/>
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=== Orbit and rotation ===
As with other criteria, stability is the critical consideration in evaluating the effect of orbital and rotational characteristics on planetary habitability. [[Orbital eccentricity]] is the difference between a planet's farthest and closest approach to its parent star divided by the sum of said distances. It is a ratio describing the shape of the elliptical orbit. The greater the eccentricity the greater the temperature fluctuation on a planet's surface. Although they are adaptive, living organisms can stand only so much variation, particularly if the fluctuations overlap both the [[freezing point]] and [[boiling point]] of the planet's main biotic solvent (e.g., water on Earth). If, for example, Earth's oceans were alternately boiling and freezing solid, it is difficult to imagine life as we know it having evolved. The more complex the organism, the greater the temperature sensitivity.<ref>[[#Ward|Ward]], pp. 122–123.</ref> The Earth's orbit is almost perfectly circular, with an eccentricity of less than 0.02; other planets in the Solar System (with the exception of [[Mercury (planet)|Mercury]]) have eccentricities that are similarly benign. Nonetheless, there may be scientific support, based on studies reported in March 2020, for considering that parts of the planet Mercury may have been habitable, and perhaps that actual [[life form]]s, albeit likely primitive [[microorganism]]s, may have existed on the planet after all.<ref name="NYT-20200324">{{cite news |last=Hall |first=Shannon |title=Life on the Planet Mercury? 'It's Not Completely Nuts' - A new explanation for the rocky world's jumbled landscape opens a possibility that it could have had ingredients for habitability. |url=https://www.nytimes.com/2020/03/24/science/mercury-life-water.html |date=24 March 2020 |work=[[The New York Times]] |access-date=26 March 2020 }}</ref><ref name="SR-20200316">{{cite journal |author=Roddriquez, J. Alexis P. |display-authors=et al. |title=The Chaotic Terrains of Mercury Reveal a History of Planetary Volatile Retention and Loss in the Innermost Solar System |date=16 March 2020 |journal=[[Scientific Reports]] |volume=10 |issue=4737 |pages=4737 |doi=10.1038/s41598-020-59885-5 |pmid=32179758 |pmc=7075900 |bibcode=2020NatSR..10.4737R |doi-access=free }}</ref>
Habitability is also influenced by the architecture of the planetary system around a star. The evolution and stability of these systems are determined by gravitational dynamics, which drive the orbital evolution of terrestrial planets. Data collected on the orbital eccentricities of extrasolar planets has surprised most researchers: 90% have an orbital eccentricity greater than that found within the Solar System, and the average is fully 0.25.<ref>{{cite web |url=https://www.astrobio.net/meteoritescomets-and-asteroids/elusive-earths/ |title=Elusive Earths |last=Bortman |first=Henry |date=22 June 2005 |publisher=Astrobiology Magazine |access-date=8 June 2020 }}</ref> This means that the vast majority of planets have highly eccentric orbits and of these, even if their average distance from their star is deemed to be within the HZ, they nonetheless would be spending only a small portion of their time within the zone.
A planet's movement around its [[rotation|rotational axis]] must also meet certain criteria if life is to have the opportunity to evolve. A first assumption is that the planet should have moderate [[season]]s. If there is little or no [[axial tilt]] (or obliquity) relative to the perpendicular of the [[ecliptic]], seasons will not occur and a main stimulant to biospheric dynamism will disappear. The planet would also be colder than it would be with a significant tilt: when the greatest intensity of radiation is always within a few degrees of the equator, warm weather cannot move poleward and a planet's climate becomes dominated by colder polar weather systems.
If a planet is radically tilted, seasons will be extreme and make it more difficult for a [[biosphere]] to achieve [[homeostasis]]. The axial tilt of the Earth is higher now (in the [[Quaternary]]) than it has been in the past, coinciding with reduced polar [[glacier|ice]], warmer temperatures and ''less'' seasonal variation. Scientists do not know whether this trend will continue indefinitely with further increases in axial tilt (see [[Snowball Earth]]).
The exact effects of these changes can only be computer modelled at present, and studies have shown that even extreme tilts of up to 85 degrees do not absolutely preclude life "provided it does not occupy continental surfaces plagued seasonally by the highest temperature."<ref>{{cite press release |title=Planetary Tilt Not A Spoiler For Habitation |publisher=[[Penn State University]] |date =25 August 2003 |url =http://www.psu.edu/ur/2003/planetarytilt.html |access-date=11 May 2007 }}</ref> Not only the mean axial tilt, but also its variation over time must be considered. The Earth's tilt varies between 21.5 and 24.5 degrees over 41,000 years. A more drastic variation, or a much shorter periodicity, would induce climatic effects such as variations in seasonal severity.
Other orbital considerations include:
* The planet should rotate relatively quickly so that the day-night cycle is not overlong. If a day takes years, the temperature differential between the day and night side will be pronounced, and problems similar to those noted with extreme orbital eccentricity will come to the fore.
* The planet also should rotate quickly enough so that a magnetic dynamo may be started in its iron core to produce a magnetic field.
* Change in the direction of the axis rotation ([[precession]]) should not be pronounced. In itself, precession need not affect habitability as it changes the direction of the tilt, not its degree. However, precession tends to accentuate variations caused by other orbital deviations; see [[Milankovitch cycles]]. Precession on Earth occurs over a 26,000-year cycle.
The Earth's [[Moon]] appears to play a [[Earth#Moon|crucial role]] in moderating the Earth's climate by stabilising the axial tilt. It has been suggested that a chaotic tilt may be a "deal-breaker" in terms of habitability—i.e. a satellite the size of the Moon is not only helpful but required to produce stability.<ref>{{cite journal |last1=Lasker |first1=J. |last2=Joutel |first2=F. |last3=Robutel |first3=P. |date=July 1993 |title=Stabilization of the earth's obliquity by the moon |journal=[[Nature (journal)|Nature]] |volume=361 |issue=6413 |pages=615–617 |bibcode=1993Natur.361..615L |doi=10.1038/361615a0 |s2cid=4233758 }}</ref> This position remains controversial.<ref group=lower-alpha>According to prevailing theory, the formation of the Moon commenced when a Mars-sized body struck the Earth in a glancing collision late in its formation, and the ejected material coalesced and fell into orbit (see [[giant impact hypothesis]]). In ''Rare Earth'' Ward and Brownlee emphasize that such impacts ought to be rare, reducing the probability of other Earth-Moon type systems and hence the probability of other habitable planets. Other moon formation processes are possible, however, and the proposition that a planet may be habitable in the absence of a moon has not been disproven.</ref>
In the case of the Earth, the sole Moon is sufficiently massive and orbits so as to significantly contribute to [[ocean tides]], which in turn aids the dynamic churning of Earth's large liquid water oceans. These lunar forces not only help ensure that the oceans do not stagnate, but also play a critical role in Earth's dynamic climate.<ref>{{cite web|last1=Dorminey|first1=Bruce|title=Without the Moon, Would There Be Life on Earth?|url=https://www.scientificamerican.com/article/moon-life-tides/|website=scientificamerican.com|publisher=Scientific American|access-date=2018-05-01|date=2009-04-29|quote="Europa must have big tides, so it's my favorite for microbial life," says Max Bernstein, an astrochemist and program scientist at NASA Headquarters in Washington, D.C. "Europa is considered by many as the best place to find life in the solar system."}}</ref><ref>File:Tidalwaves1.gif</ref>
===Geology===
[[File:Earth poster.svg|thumb|Geological cross section of Earth]]
[[File:Simple model of the Earth's magnetic field.ogv|thumb|A visualization showing a simple model of [[Earth's magnetic field]].]]
Concentrations of [[radionuclide]]s in rocky planet mantles may be critical for the habitability of Earth-like planets as such planets with higher abundances likely lack [[Dynamo theory|a persistent dynamo]] for a significant fraction of their lifetimes and those with lower concentrations [[Earth's internal heat budget#Radiogenic heat|may often be geologically inert]]. Planetary dynamos create strong [[magnetic field]]s which may often be necessary for life to develop or persist as they shield planets from [[solar wind]]s and [[cosmic radiation]]. The electromagnetic [[emission spectra]] of stars could be used to identify those which are more likely to host habitable Earth-like planets. As of 2020 radionuclides are thought to be produced by rare stellar processes such as [[neutron star merger]]s.<ref>{{cite news |last1=Woo |first1=Marcus |title=Stellar Smashups May Fuel Planetary Habitability, Study Suggests |url=https://www.scientificamerican.com/article/stellar-smashups-may-fuel-planetary-habitability-study-suggests/ |access-date=9 December 2020 |work=Scientific American |language=en}}</ref><ref>{{cite journal |last1=Nimmo |first1=Francis |last2=Primack |first2=Joel |last3=Faber |first3=S. M. |last4=Ramirez-Ruiz |first4=Enrico |last5=Safarzadeh |first5=Mohammadtaher |title=Radiogenic Heating and Its Influence on Rocky Planet Dynamos and Habitability |journal=The Astrophysical Journal |date=10 November 2020 |volume=903 |issue=2 |pages=L37 |doi=10.3847/2041-8213/abc251 |url=https://iopscience.iop.org/article/10.3847/2041-8213/abc251 |access-date=9 December 2020 |language=en |issn=2041-8213|arxiv=2011.04791|bibcode=2020ApJ...903L..37N |s2cid=226289878 }}</ref> Additional geological characteristics may be essential or major factors in the habitability of natural celestial bodies – including some that may shape the body's heat and magnetic field. Some of these are unknown or not well understood and being investigated by [[planetary scientist]]s, geochemists and others.<ref>{{cite news |title=The existence of a magnetic field beyond 3.5 billion years ago is still up for debate |url=https://phys.org/news/2020-04-magnetic-field-billion-years-debate.html |access-date=28 December 2020 |work=phys.org |language=en}}</ref>{{additional citation needed|date=December 2020|quote=Scientists know that today the Earth's magnetic field is powered by the solidification of the planet's liquid iron core. The cooling and crystallization of the core stirs up the surrounding liquid iron, creating powerful electric currents that generate a magnetic field stretching far out into space. This magnetic field is known as the geodynamo.<br/><br/>Multiple lines of evidence have shown that the Earth's magnetic field existed at least 3.5 billion years ago. However, the planet's core is thought to have started solidifying just 1 billion years ago, meaning that the magnetic field must have been driven by some other mechanism prior to 1 billion years ago. Pinning down exactly when the magnetic field formed could help scientists figure out what generated it to begin with.}}
==== Geochemistry ====
{{See|Geochemistry}}
It is generally assumed that any extraterrestrial life that might exist will be based on the same fundamental [[biochemistry]] as found on Earth, as the four elements most vital for life, [[carbon]], [[hydrogen]], [[oxygen]], and [[nitrogen]], are also the most common chemically reactive elements in the universe. Indeed, simple biogenic compounds, such as very simple [[amino acid]]s such as [[glycine]], have been found in [[meteorite]]s and in the [[interstellar medium]].<ref>{{cite web |title=Organic Molecule, Amino Acid-Like, Found In Constellation Sagittarius |publisher=ScienceDaily |date=2008 |url=https://www.sciencedaily.com/releases/2008/03/080326161658.htm |access-date=20 December 2008 }}</ref> These four elements together comprise over 96% of Earth's collective [[biomass]]. Carbon has an unparalleled ability to bond with itself and to form a massive array of intricate and varied structures, making it an ideal material for the complex mechanisms that form living [[Cell (biology)|cells]]. Hydrogen and oxygen, in the form of water, compose the solvent in which biological processes take place and in which the first reactions occurred that led to [[Abiogenesis|life's emergence]]. The energy released in the formation of powerful [[covalent bond]]s between carbon and oxygen, available by oxidizing organic compounds, is the fuel of all complex life-forms. These four elements together make up [[amino acids]], which in turn are the building blocks of [[protein]]s, the substance of living tissue. In addition, neither [[sulfur]], required for the building of proteins, nor [[phosphorus]], needed for the formation of [[DNA]], [[RNA]], and the adenosine phosphates essential to [[metabolism]], is rare.
Relative abundance in space does not always mirror differentiated abundance within planets; of the four life elements, for instance, only [[oxygen]] is present in any abundance in the Earth's [[crust (geology)|crust]].<ref>{{cite web |url=http://www.daviddarling.info/encyclopedia/E/elbio.html |title=Elements, biological abundance |last=Darling |first=David |author-link=David Darling (astronomer) |publisher=The Encyclopedia of Astrobiology, Astronomy, and Spaceflight |access-date=11 May 2007 }}</ref> This can be partly explained by the fact that many of these elements, such as [[hydrogen]] and [[nitrogen]], along with their simplest and most common compounds, such as [[carbon dioxide]], [[carbon monoxide]], [[methane]], [[ammonia]], and water, are gaseous at warm temperatures. In the hot region close to the Sun, these volatile compounds could not have played a significant role in the planets' geological formation. Instead, they were trapped as gases underneath the newly formed crusts, which were largely made of rocky, involatile compounds such as [[silica]] (a compound of [[silicon]] and oxygen, accounting for oxygen's relative abundance). [[Outgassing]] of volatile compounds through the first volcanoes would have contributed to the formation of the planets' [[Celestial body atmosphere|atmospheres]]. The [[Miller–Urey experiment]] showed that, with the application of energy, simple inorganic compounds exposed to a primordial atmosphere can react to synthesize [[amino acid]]s.<ref>{{cite web |url=http://zebu.uoregon.edu/internet/l3.html |title=How did chemistry and oceans produce this? |work=The Electronic Universe Project |publisher=[[University of Oregon]] |access-date=11 May 2007 }}</ref>
Even so, [[Volcanism|volcanic]] outgassing could not have accounted for the amount of water in Earth's oceans.<ref>{{cite web |url=http://zebu.uoregon.edu/internet/l2.html |title=How did the Earth Get to Look Like This? |work=The Electronic Universe Project |publisher=[[University of Oregon]] |access-date=11 May 2007 }}</ref> The vast majority of the water—and arguably carbon—necessary for life must have come from the outer Solar System, away from the Sun's heat, where it could remain solid. [[Comet]]s impacting with the Earth in the Solar System's early years would have deposited vast amounts of water, along with the other volatile compounds life requires onto the early Earth, providing a kick-start to the [[origin of life]].
Thus, while there is reason to suspect that the four "life elements" ought to be readily available elsewhere, a habitable system probably also requires a supply of long-term orbiting bodies to seed inner planets. Without comets there is a possibility that life as we know it would not exist on Earth.
=== Microenvironments and extremophiles ===
[[File:Atacama.png|thumb|The [[Atacama Desert]] in [[South America]] provides an analog to [[Mars]] and an ideal environment to study the boundary between sterility and habitability.]]
One important qualification to habitability criteria is that only a tiny portion of a planet is required to support life. Astrobiologists often concern themselves with "micro-environments", noting that "we lack a fundamental understanding of how evolutionary forces, such as [[genetic mutation|mutation]], [[natural selection|selection]], and [[genetic drift]], operate in micro-organisms that act on and respond to changing micro-environments."<ref>{{cite web|url=http://astrobiology.arc.nasa.gov/roadmap/g5.html |title=Understand the evolutionary mechanisms and environmental limits of life |work=Astrobiology: Roadmap |publisher=[[NASA]] |date=September 2003 |access-date=6 August 2007 |url-status=dead |archive-url=https://web.archive.org/web/20110126083203/http://astrobiology.arc.nasa.gov/roadmap/g5.html |archive-date=26 January 2011 }}</ref> [[Extremophile]]s are Earth organisms that live in niche environments under severe conditions generally considered [[wikt:inimical#Adjective|inimical]] to life. Usually (although not always) unicellular, extremophiles include acutely [[Alkaliphile|alkaliphilic]] and [[acidophile (organisms)|acidophilic]] organisms and others that can survive water temperatures above 100 °C in [[hydrothermal vents]].
The discovery of life in extreme conditions has complicated definitions of habitability, but also generated much excitement amongst researchers in greatly broadening the known range of conditions under which life can persist. For example, a planet that might otherwise be unable to support an atmosphere given the solar conditions in its vicinity, might be able to do so within a deep shadowed rift or volcanic cave.<ref>{{cite web |first=Stephen |last=Hart |url=http://www.space.com/scienceastronomy/astrobio_caves_030617-1.html |title=Cave Dwellers: ET Might Lurk in Dark Places |publisher=[[Space.com]] |date=17 June 2003 |archive-url=https://web.archive.org/web/20030620142504/http://www.space.com/scienceastronomy/astrobio_caves_030617-1.html |archive-date=20 June 2003 |access-date=6 August 2007 <!-- alternate copy: http://www.nasa.gov/vision/universe/solarsystem/cave_slime.html --> <!--}}</ref> Similarly, craterous terrain might offer a refuge for primitive life. The [[Lawn Hill crater]] has been studied as an astrobiological analog, with researchers suggesting rapid sediment infill created a protected microenvironment for microbial organisms; similar conditions may have occurred over the geological history of [[Mars]].<ref>{{cite journal |last1=Lindsay |first1=J |last2=Brasier |first2=M |title=Impact Craters as biospheric microenvironments, Lawn Hill Structure, Northern Australia |journal=Astrobiology |volume=6 |issue=2 |date=2006 |pages=348–363 |doi=10.1089/ast.2006.6.348 |pmid=16689651 |bibcode=2006AsBio...6..348L |url=https://ora.ox.ac.uk/objects/uuid:c9796531-6943-4302-9733-8e8616adf78a }}</ref>
Earth environments that ''cannot'' support life are still instructive to astrobiologists in defining the limits of what organisms can endure. The heart of the [[Atacama desert]], generally considered the driest place on Earth, appears unable to support life, and it has been subject to study by NASA and [[European Space Agency|ESA]] for that reason: it provides a Mars analog and the moisture gradients along its edges are ideal for studying the boundary between sterility and habitability.<ref>{{cite web|first=Christopher |last=McKay |date=June 2002 |url=http://quest.nasa.gov/challenges/marsanalog/egypt/AtacamaAdAstra.pdf |title=Too Dry for Life: The Atacama Desert and Mars |publisher=[[NASA]] |work=Ames Research Center |access-date=26 August 2009 |url-status=dead |archive-url=https://web.archive.org/web/20090826151945/http://quest.nasa.gov/challenges/marsanalog/egypt/AtacamaAdAstra.pdf |archive-date=26 August 2009 }}</ref> The Atacama was the subject of study in 2003 that partly replicated experiments from the [[Viking (spacecraft)|Viking]] landings on Mars in the 1970s; no [[DNA]] could be recovered from two soil samples, and incubation experiments were also negative for [[biosignature]]s.<ref>{{cite journal |title=Mars-Like Soils in the Atacama Desert, Chile, and the Dry Limit of Microbial Life |journal=Science |date=7 November 2003 |first1=Rafael |last1=Navarro-González |first2=Christopher P. |last2=McKay |volume=302 |issue=5647 |pages=1018–1021 |doi=10.1126/science.1089143 |jstor=3835659 |pmid=14605363 |bibcode=2003Sci...302.1018N |s2cid=18220447 }}</ref>
===Ecological factors===
The two current ecological approaches for predicting the potential habitability use 19 or 20 environmental factors, with emphasis on water availability, temperature, presence of nutrients, an energy source, and protection from solar ultraviolet and [[Cosmic ray|galactic cosmic radiation]].<ref name='D.C.Golden'>{{cite journal|last1=Schuerger|first1=Andrew C.|last2=Golden|first2=D.C.|last3=Ming|first3=Doug W.|title=Biotoxicity of Mars soils: 1. Dry deposition of analog soils on microbial colonies and survival under Martian conditions|journal=Planetary and Space Science|date=November 2012|volume=72|issue=1|pages=91–101|doi=10.1016/j.pss.2012.07.026|bibcode = 2012P&SS...72...91S }}</ref><ref name=Beaty>{{citation |first=David W. |last=Beaty |title=Findings of the Mars Special Regions Science Analysis Group |journal=Astrobiology |volume=6 |issue=5 |pages=677–732 |editor-last=the Mars Exploration Program Analysis Group (MEPAG) |date=14 July 2006 |url=http://mepag.jpl.nasa.gov/reports/MEPAG_SR-SAG_final1.pdf |access-date=6 June 2013 |display-authors=etal|bibcode=2006AsBio...6..677M |doi=10.1089/ast.2006.6.677 |pmid=17067257 }}</ref>
{| class="wikitable"
|-
! style="align: center; background: lavender;" colspan="2" | '''Some habitability factors'''<ref name=Beaty/>
|-
|[[Water on Mars|Water]] || {{·}} Activity of liquid water <br /> {{·}} Past or future liquid (ice) inventories <br /> {{·}} [[Salinity]], [[pH]], and [[Reduction potential|Eh]] of available water
|-
|Chemical environment || '''Nutrients:''' <br /> {{·}} C, H, N, O, P, S, essential metals, essential micronutrients <br /> {{·}}[[Nitrogen fixation|Fixed nitrogen]] <br /> {{·}}Availability/mineralogy <br /> '''Toxin abundances and lethality:''' <br /> {{·}} [[Heavy metal (chemistry)|Heavy metals]] (e.g. Zn, Ni, Cu, Cr, As, Cd, etc.; some are essential, but toxic at high levels) <br /> {{·}} Globally distributed oxidizing soils
|-
|Energy for [[metabolism]] || '''Solar''' (surface and near-surface only) <br /> '''Geochemical''' (subsurface) <br /> {{·}} [[Oxidizing agent|Oxidants]] <br /> {{·}} [[Reducing agent|Reductants]] <br /> {{·}} [[Redox gradient]]s
|-
|Conducive <br /> physical conditions || {{·}}Temperature <br /> {{·}}Extreme diurnal temperature fluctuations <br /> {{·}}Low pressure (is there a low-pressure threshold for terrestrial [[Anaerobic organism|anaerobes]]?) <br /> {{·}}Strong [[ultraviolet germicidal irradiation]] <br /> {{·}}[[Cosmic ray|Galactic cosmic radiation]] and [[Solar proton event|solar particle events]] (long-term accumulated effects) <br /> {{·}} Solar UV-induced volatile oxidants, e.g. [[Superoxide|O <sub>2</sub><sup>−</sup>]], O<sup>−</sup>, [[Hydrogen peroxide|H<sub>2</sub>O<sub>2</sub>]], O<sub>3</sub> <br /> {{·}}Climate and its variability (geography, seasons, diurnal, and eventually, obliquity variations) <br /> {{·}}Substrate (soil processes, rock microenvironments, dust composition, shielding) <br /> {{·}}High [[Carbon dioxide|CO<sub>2</sub>]] concentrations in the global atmosphere <br /> {{·}}Transport ([[Aeolian processes|aeolian]], ground water flow, surface water, glacial)
|}
-->
== Catatan ==
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