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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"/>
=== Orbit anddan rotationputaran === ▼AsSeperti withdengan otherkriteria criterialain, stabilityadalah ispenting theuntuk criticalmengambil considerationkira indalam evaluatingmenilai thekesan effectciri-ciri oforbit orbitaldan andputaran rotationalterhadap characteristicskebolehdiaman on planetary habitabilityplanet. [[ OrbitalKesipian eccentricityorbit]] isadalah theperbezaan differenceantara betweenjarak apaling planet'sjauh farthestdan andpaling closestdekat approachdengan tobintang itsinduknya, parentdibahagikan stardengan dividedjumlah bycampuran thekedua-dua sumjarak oftersebut; saidia distances.merupakan Itnisbah isyang amenghuraikan ratiobentuk describingorbit the shape of the elliptical orbitbujur. TheMakin greaterbesar thekesipiannya eccentricitymaka themakin greaterketaralah theperubahan temperaturesuhu fluctuation on apermukaan planet 's surface. AlthoughOrganisma theyhidup aresejauh adaptive,mana livingmampu organismsmenyesuaikan candiri standpun onlyada sohadnya, much variation,lebih-lebih particularlylagi ifjika theperubahan fluctuationsitu overlapbertindan bothdengan thekedua-dua [[ freezingtakat pointbeku]] anddan [[ boilingtakat pointdidih]] ofbahan thepelarut planet'sutama main biotic solventplanetnya ( e.g.misalnya, waterair ondi EarthBumi). If, for example, Earth's oceansAndaikan werejika alternatelylautan boilingBumi andmendidih freezingbersilih solidmembeku, itsukar isdibayangkan difficulthidupan todapat imagineberkembang lifedi as we know it having evolvedsini. TheMakin morekompleks complexorganisma theitu, organism,makin thepekalah greateria thekepada temperatureperubahan sensitivitysuhu.<ref>[[#Ward|Ward]], pp. 122–123.</ref> TheOrbit Earth'sBumi orbitberbentuk isbulatan almosthampir perfectlysempurna circular,dengan withkesipian ankurang eccentricity of less thandaripada 0.02; otherplanet planetslain indalam theSistem Solar SystemSuria ( with the exception ofkecuali [[ Mercury (planet)|MercuryUtarid]]) havepun eccentricitieslebih thatkurang are similarly benignsama. Nonetheless, there may be scientificSungguhpun supportbegitu, basedberdasarkan onkajian studiesyang reporteddiwartakan inpada MarchMac 2020, forterdapat consideringkemungkinan thatberasas partssaintifik ofbahawa thesesetengah kawasan di planet MercuryUtarid maypernah haveboleh been habitabledidiami, andbahkan perhapsmungkin thatjuga actualpernah wujudnya [[ life formhidupan]] s, albeitsebenar likelywalaupun primitivesekadar [[ microorganismmikroorganisma]] s, may have existed on the planet after allprimitif.<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> ▼▲=== 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>
HabitabilityKebolehdiaman isplanet alsojuga influenceddipengaruhi byoleh thereka architecturebina ofsistem thecakerawala planetarydi systemkeliling around a starbintangnya. ThePerkembangan evolutiondan andkestabilan stabilitysistemnya ofditentukan theseoleh systemsdinamik aregraviti determinedyang bymendorong gravitationalperkembangan dynamics,orbit whichplanet drivebumian. theData orbitalyang evolutiondikumpul oftentang terrestrialkesipian planets.orbit Dataplanet collectedluar onsuria thetelah orbitalmemeranjatkan eccentricitiesramai ofpengkaji, extrasolaryang planets has surprised most researchers:mana 90% havedaripadanya anmempunyai orbitalkesipian eccentricityorbit greateryang thanlebih thattinggi founddaripada withinyang theterdapat Solardalam SystemSistem Suria, and the averagedan ispuratanya fullygenap 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> ThisIni meansbermakna thatmajoriti thebesar vastplanet majoritymempunyai oforbit planetsyang haveamat highly eccentric orbits and of thesesipi, evendan ifsungguhpun theirjarak averagepurata distancedari frombintangnya theirdikira stardi isdalam deemed to be within the HZZBD, they nonetheless would be spending only a smallnamun portionia oftidak theirbanyak timemeluangkan withinmasa thedalam zonezonnya.
[[Putaran]] planet pada paksinya juga mesti memenuhi kriteria tertentu untuk memberi hidupan peluang untuk berkembang. Anggapan utamanya, planet harus mempunyai peralihan [[musim]] yang sederhana. Jika tidak terdapat [[kecondongan paksi]] (senget) yang ketara berbanding dengan garis serenjang [[satah ekliptik]], maka musim-musim tidak akan terbentuk, tidak akan jadilah perangsang utama untuk menjana kedinamikan biosfera (lingkungan hidupan), dan planetnya juga akan lebih sejuk daripada planet yang mempunyai kecondongan ketara; jika keamatan sinaran tertinggi sentiasa dalam darjah yang terlalu sedikit dari khatulistiwa, maka cuaca panas tidak boleh mencapai kawasan kutub dan iklim planet akan didominasi oleh sistem cuaca yang serba sejuk seperti di kutub.
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.
Jika planet terlalu condong pula, maka musim-musimnya juga melampau dan menyukarkan [[biosfera]] dari mencapai [[homeostasis]]. Kecondongan paksi Bumi kini (dalam [[Kuaterner]]) lebih tinggi daripada dahulu, sejajar dengan berkurangannya [[glasier|ais]] kutub, suhu semakin panas dan ''berkurangannya'' variasi musim. Para saintis tidak pasti sama ada perkembangan sebegini akan berlarutan untuk selamanya jika kecondongan paksi bertambah lagi (lihat [[Bumi Bola Salji]]).
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]]).
TheKesan-kesan exactsebenar effectsperubahan ofini thesehanya changesdapat candiunjurkan onlydengan bekomputer computerpada modelledmasa at presentini, andmalah studieskajian-kajian havemenunjukkan shownbahawa thatkecondongan evenpaksi extremesejauh tilts85 ofdarjah uppun totidak 85semestinya degreesmenidakkan dokewujudan nothidupan absolutelyselagi precludeia lifetidak "providedmelibatkan it does not occupypermukaan-permukaan continentalbenua surfacesyang plagueddilanda seasonallyoleh bysuhu thetertinggi highestsecara temperaturebermusim."<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> NotBukan sahaja onlymin the(nilai meanhitung axialpanjang) tiltkecondongan paksi, butbahkan alsojuga itsvariasi variation(julat overanjakan) timesepanjang mustmasa bemesti considered.diambil Thekira. Earth'sKecondongan tiltBumi variesbervariasi betweenantara 21.5 anddan 24.5 degreesdarjah overselama 41,000 yearstahun. AVariasi moreyang drasticlebih variation,drastik oratau akeberkalaan muchyang shorterlebih periodicity,pendek wouldakan inducemengaruh climatickesan-kesan effectspada suchiklim asseperti variationsvariasi indalam seasonalkeamatan severitymusim.
Lain-lain faktor orbit untuk diambil kira termasuk:
Other orbital considerations include:
* Planet seharusnya berputar cukup cepat supaya kitaran siang malam tidak terlalu lama. Jika satu hari mengambil masa bertahun-tahun, maka akan terdapat perbezaan ketara antara suhu siang dan malam, maka akan timbul masalah-masalah yang serupa dengan planet yang melampau sipi orbitnya.
* 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.
* Planet juga seharusnya berputar cukup cepat supaya 'dinamo magnetik' dapat dihidupkan di dalam teras besinya untuk menghasilkan medan magnet.
* The planet also should rotate quickly enough so that a magnetic dynamo may be started in its iron core to produce a magnetic field.
* Pertukaran hala putaran paksi ([[liukan]]) janganlah terlalu ketara. Liukan itu sendiri tidak harus mempengaruhi kebolehdiaman kerana ia mengubah kecondongan dari segi halanya dan bukan darjahnya. Akan tetapi, liukan seringkali menyerlahkan variasi yang disebabkan oleh sisihan orbit yang lain; baca [[kitaran Milankovitch]]. Satu kitaran liukan Bumi mengambil masa 26,000 tahun.
* 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 [[MoonBulan]] appearsBumi tokelihatan playmemainkan aperanan [[Earth#Moon|crucialpenting role]]dalam inmenyederhanakan moderatingiklim theBumi Earth'sdengan climatemenstabilkan by stabilising the axialkecondongan tiltpaksi. ItTelah hasdiusulkan beenbahawa suggestedkecendungan thatyang acamuk chaoticboleh tiltmenjadi maypenentu besegala adari "deal-breaker"segi inkebolehdiaman; termsmaksudnya, ofsatelit habitability—i.e.sebesar aBulan satellitebukan thesahaja sizemembantu of the Moon is not only helpful butmalah requiredwajib tountuk producemewujudkan stabilitykestabilan.<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> ThisPendirian positionini remainsmasih controversialdipertikaikan.<ref group=lower-alpha>AccordingMenurut toteori prevailingyang theoryditerima umum, thepembentukan formationBulan ofbermula theapabila Moonsebuah commencedjasad whensebesar aMarikh Mars-sizedmenghentam bodyBumi strucksecara thepelanggaran Earthterkena insipi adi glancingakhir collisionpembentukan lateBumi, indan itsbahan-bahan formation,yang andterpancut thedari ejectedpelanggaran materialini coalesced andbertaut-taut felldan intomemasuki orbit (seelihat [[gianthipotesis impacthentaman hypothesisbesar]]). InDalam buku ''Rare Earth'', Ward anddan Brownlee emphasizemenekankan thatbahawa suchhentaman-hentaman impactssebegini oughtsepatutnya tojarang be rareberlaku, reducingoleh theitu probabilitymengurangkan ofkebarangkalian otheradanya Earthsistem-Moonsistem typeseakan systems'Bulan andBumi', hencedan theoleh probabilityitu ofberkurang otherjuga habitablekebarangkalian planets.wujudnya Otherplanet-planet moonboleh formationdidiami processesyang arelain. possible,Namun howeverbegitu, andproses-proses thepembentukan propositionbulan thatlain aadalah planetmungkin, maybahkan beusul habitablebahawa inplanet theboleh absencedidiami of a moontanpa hasbulan notbelum beenlagi disprovendisangkal.</ref>
InBagi theBumi, caseBulan ofyang thetunggal Earth,mempunyai thejisim soleyang Mooncukup isdan sufficientlymengorbit massivepada andkedudukan orbitsdan sokadar asyang tocukup significantlyuntuk contributemenyumbang tobesar kepada [[oceanpasang tidessurut]], whichlautan inyang turnmembantu aidsBumi themenggodak dynamiclautan churningair ofcecair Earth's large liquid watersecara oceansdinamik. ThesePengaruh lunardari forcesBulan notini onlybukan helpsahaja ensurememastikan thatair thelautan oceanstidak domati, notbahkan stagnate,juga butmemainkan alsoperanan playyang aamat criticalpenting roledalam inperikliman Earth'sBumi dynamicyang climatedinamik.<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>
===GeologyGeologi===
[[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.}}
==== GeochemistryGeokimia ====
{{See|GeochemistryGeokimia}}
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.
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