Kebolehdiaman planet: Perbezaan antara semakan
Kandungan dihapus Kandungan ditambah
Teg: Suntingan sumber 2017 |
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Baris 150:
|Keadaan fizikal<br />yang mengizinkan || {{·}}Suhu <br />{{·}}Turun naik suhu harian ketara <br />{{·}}Tekanan rendah (adakah terdapat ambang tekanan rendah untuk [[organisma anaerob|anaerob]] bumian?) <br />{{·}}[[Penyinaran germisid ultraungu|Sinaran pembasmi kuman ultraungu]] yang kuat <br />{{·}}[[Sinaran kosmos|Sinaran kosmos galaksi]] dan [[Peristiwa proton suria|peristiwa zarah suria]] (kesan terkumpul jangka panjang) <br />{{·}} Bahan pengoksidaan meruap aruhan ultraungu suria, cth. [[Superoksida|O <sub>2</sub><sup>−</sup>]], O<sup>−</sup>, [[Hidrogen peroksida|H<sub>2</sub>O<sub>2</sub>]], [[Ozon|O<sub>3</sub>]] <br />{{·}}Iklim dan keberubahan iklim (geografi, peralihan musim, kitaran harian, dan variasi kecondongan) <br />{{·}}Substrat (proses tanih, mikropersekitaran berbatu, komposisi debu, perlindungan angkasa) <br />{{·}}Kepekatan [[Karbon dioksida|CO<sub>2</sub>]] tinggi dalam atmosfera segenap bumi<br />{{·}}Angkutan ([[Proses aeolian|angin]], air tanah, air permukaan, glasier)
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== Sistem bintang alternatif ==
Dalam menentukan kebolehjadian hidupan luar angkasa, para ahli astronomi selama ini menumpukan perhatian kepada bintang-bintang yang setaraf dengan Matahari. Akan tetapi, sistem-sistem planet yang menyerupai Sistem Suria kita terbukti jarang didapati, maka mereka pun mula menerokai kemungkinan wujudnya hidupan di sistem-sistem yang jauh berbeza daripada sistem kita.
=== Sistem bintang ganda ===
{{Utama|Kebolehdiaman sistem bintang ganda}}
Sering dianggarkan bahawa lebih 50% daripada semua sistem najam adalah sistem [[bintang ganda]], tetapi mungkin sebahagiannya bias pensampelan kerana bintang-bintang yang serba cerah dan masif (berjisim amat besar) selalunya ditemui dalam bentuk ganda ({{lang|en|binary}}) dan oleh itu paling mudah dicerap dan dikatalogkan; analisis lebih jitu membayangkan bahawa sistem bintang malap-malap yang lebih umum ditemui adalah berbintang tunggal, dan sebanyak dua pertiga daripada semua sistem najam yang ada adalah berbintang tunggal.<ref>{{cite press release |title=Most Milky Way Stars Are Single |publisher=[[Harvard-Smithsonian Center for Astrophysics]] |date=30 January 2006 |url =http://cfa-www.harvard.edu/press/2006/pr200611.html |archive-url=https://web.archive.org/web/20070813062958/http://cfa-www.harvard.edu/press/2006/pr200611.html |archive-date=13 August 2007 |url-status=dead |access-date=5 June 2007 }}</ref>
Jarak antara bintang dalam sistem ganda mungkin dalam kurang dari satu hingga ratusan [[unit astronomi]] (UA, purata jarak antara Bumi dan Matahari). Bagi yang jaraknya ratusan UA, kesan gravitinya tidak akan ketara pada planet yang mengorbit bintang yang sesuai, maka potensi kebolehdiaman tidak akan terjejas melainkan orbitnya terlebih sipi (contohnya [[Nemesis (bintang andaian)|Nemesis]]). Jika jaraknya terlalu dekat pula, maka orbit yang stabil mustahil terbentuk. Jika jarak planet dengan bintang utamanya melebihi satu perlima jarak terdekatnya dengan bintang yang lain, maka kestabilan orbit tidak terjamin.<ref>{{cite web|url=http://www.solstation.com/habitable.htm |title=Stars and Habitable Planets |work=www.solstation.com |publisher=Sol Company |access-date=5 June 2007 |url-status=dead |archive-url=https://web.archive.org/web/20110628175616/http://www.solstation.com/habitable.htm |archive-date=28 June 2011 }}</ref> Tidak jelas sama ada planet boleh terbentuk dalam sistem bintang ganda kerana daya gravitinya mungkin mengganggu pembentukan planet. Hasil teori oleh [[Alan Boss]] di [[Carnegie Institution]] menunjukkan bahawa gergasi gas boleh terbentuk di keliling bintang dalam sistem bintang ganda sama seperti sistem bintang tunggal.<ref>{{cite press release|title=Planetary Systems can from around Binary Stars |publisher=[[Carnegie Institution]] |url=http://carnegieinstitution.org/news_releases/news_0601_10.html |author=Boss, Alan |date=January 2006 |access-date=5 June 2007 |url-status=dead |archive-url=https://web.archive.org/web/20110515225714/http://carnegieinstitution.org/news_releases/news_0601_10.html |archive-date=15 May 2011 }}</ref>
Satu kajian terhadap [[Alpha Centauri]], iaitu sistem najam yang paling dekat dengan Matahari kita, mengusulkan bahawa bintang ganda tidak semestinya disingkirkan dalam pencarian planet yang boleh didiami. Planet-planet Centauri A dan B mempunyai jarak paling dekat 11 UA (min 23 UA), dan kedua-duanya pun haruslah mempunyai zon boleh didiami yang sesuai. Kajian kestabilan orbit jangka panjang secara simulasi planet dalam sistem menunjukkan bahawa sebarang planet dalam sekitar 3 UA dari mana-mana bintang mungkin kekal cukup stabil (iaitu [[paksi semimajor]] menyisih kurang dari 5% sepanjang 32 000 kala ganda). Zon boleh didiami berterusan (ZBDT untuk 4.5 bilion tahun) bagi [[Alpha Centauri|Centauri A]] diberi anggaran konservatif dalam 1.2 hingga 1.3 UA, dan Centauri B pula 0.73 hingga 0.74—kedua-duanya di dalam kawasan cukup stabil.<ref>{{cite journal |last1=Wiegert |first1=Paul A. |last2=Holman |first2=Matt J. |date=April 1997 |title=The stability of planets in the Alpha Centauri system |journal=The Astronomical Journal |volume=113 |issue=4 |pages=1445–1450 |bibcode=1997AJ....113.1445W |doi=10.1086/118360 |arxiv=astro-ph/9609106 |s2cid=18969130 }}</ref>
=== Sistem kerdil merah ===
{{main|Kebolehdiaman sistem kerdil merah}}
[[File:BrownDwarfs Comparison 01.png|300px|thumb|Perbandingan saiz bintang dan suhu [[fotosfera]]. Sebarang planet yang mengitari kerdil merah seperti yang ditunjukkan dalam rajah ini ([[Gliese 229A]]) haruslah dekat untuk mencapai suhu seakan Bumi, mungkin mengaruh penguncian pasang surut. Penghargaan: MPIA/V. Joergens.]]
Menentukan kebolehdiaman bintang [[kerdil merah]] mungkin boleh membantu dalam menentukan betapa mungkin meluasnya hidupan di alam semesta, kerana kerdil merah merangkumi 70 hingga 90 peratus daripada semua bintang di galaksi.
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==== Saiz ====
Astronomers for many years ruled out red dwarfs as potential abodes for life. Their small size (from 0.08 to 0.45 solar masses) means that their [[nuclear reaction]]s proceed exceptionally slowly, and they emit very little light (from 3% of that produced by the Sun to as little as 0.01%). Any planet in orbit around a red dwarf would have to huddle very close to its parent star to attain Earth-like surface temperatures; from 0.3 AU (just inside the orbit of [[Mercury (planet)|Mercury]]) for a star like [[Lacaille 8760]], to as little as 0.032 AU for a star like [[Proxima Centauri]]<ref>{{cite web |title=Habitable zones of stars |url=http://exobio.ucsd.edu/Space_Sciences/zones.htm |work=NASA Specialized Center of Research and Training in Exobiology |publisher=[[University of Southern California]], San Diego |url-status=dead |archive-date=21 November 2000 |archive-url=https://web.archive.org/web/20001121121500/http://exobio.ucsd.edu/Space_Sciences/zones.htm |access-date=11 May 2007 }}</ref> (such a world would have a year lasting just 6.3 days). At those distances, the star's gravity would cause tidal locking. One side of the planet would eternally face the star, while the other would always face away from it. The only ways in which potential life could avoid either an inferno or a deep freeze would be if the planet had an atmosphere thick enough to transfer the star's heat from the day side to the night side, or if there was a gas giant in the habitable zone, with a [[habitable moon]], which would be locked to the planet instead of the star, allowing a more even distribution of radiation over the planet. It was long assumed that such a thick atmosphere would prevent sunlight from reaching the surface in the first place, preventing [[photosynthesis]].
[[File:Gliese 667 Cc sunset.jpg|thumb|left|300px|An artist's impression of [[GJ 667 Cc]], a potentially habitable planet orbiting a red dwarf constituent in a [[star system|trinary star system]].]]This pessimism has been tempered by research. Studies by Robert Haberle and Manoj Joshi of [[NASA]]'s [[Ames Research Center]] in California have shown that a planet's atmosphere (assuming it included greenhouse gases [[carbon dioxide|CO<sub >2</sub>]] and [[Water (molecule)|H<sub>2</sub>O]]) need only be {{convert|100|mb|atm|2}}, for the star's heat to be effectively carried to the night side.<ref>{{cite journal |last1=Joshi |first1=M. M. |last2=Haberle |first2=R. M. |last3=Reynolds |first3=R. T. |date=October 1997 |title=Simulations of the Atmospheres of Synchronously Rotating Terrestrial Planets Orbiting M Dwarfs: Conditions for Atmospheric Collapse and the Implications for Habitability |journal=Icarus |volume=129 |issue=2 |pages=450–465 |doi=10.1006/icar.1997.5793 |bibcode=1997Icar..129..450J |url=http://crack.seismo.unr.edu/ftp/pub/gillett/joshi.pdf |access-date=4 April 2011 |archive-url=https://web.archive.org/web/20110814012947/http://crack.seismo.unr.edu/ftp/pub/gillett/joshi.pdf |archive-date=14 August 2011 |url-status=dead }}</ref> This is well within the levels required for photosynthesis, though water would still remain frozen on the dark side in some of their models. Martin Heath of [[Greenwich Community College]], has shown that seawater, too, could be effectively circulated without freezing solid if the ocean basins were deep enough to allow free flow beneath the night side's ice cap. Further research—including a consideration of the amount of photosynthetically active radiation—suggested that tidally locked planets in red dwarf systems might at least be habitable for higher plants.<ref>{{cite journal |last1=Heath |first1=Martin J. |last2=Doyle |first2=Laurance R. |last3=Joshi |first3=Manoj M. |author4=Haberle, Robert M. |date=1999 |title=Habitability of Planets Around Red Dwarf Stars |journal=Origins of Life and Evolution of the Biosphere |volume=29 |issue=4 |pages=405–424 |url=http://www.as.utexas.edu/astronomy/education/spring02/scalo/heath.pdf |doi=10.1023/A:1006596718708 |pmid=10472629 |bibcode=1999OLEB...29..405H |s2cid=12329736 |access-date=11 August 2007 }}</ref>
==== Other factors limiting habitability ====
Size is not the only factor in making red dwarfs potentially unsuitable for life, however. On a red dwarf planet, photosynthesis on the night side would be impossible, since it would never see the sun. On the day side, because the sun does not rise or set, areas in the shadows of mountains would remain so forever. [[Photosynthesis]] as we understand it would be complicated by the fact that a red dwarf produces most of its radiation in the [[infrared]], and on the Earth the process depends on visible light. There are potential positives to this scenario. Numerous terrestrial ecosystems rely on [[chemosynthesis]] rather than photosynthesis, for instance, which would be possible in a red dwarf system. A static primary star position removes the need for plants to steer leaves toward the sun, deal with changing shade/sun patterns, or change from photosynthesis to stored energy during night. Because of the lack of a day-night cycle, including the weak light of morning and evening, far more energy would be available at a given radiation level.
Red dwarfs are far more variable and violent than their more stable, larger cousins. Often they are covered in [[starspot]]s that can dim their emitted light by up to 40% for months at a time, while at other times they emit gigantic flares that can double their brightness in a matter of minutes.<ref>{{cite web |first=Ken |last=Croswell |url=https://www.newscientist.com/article/mg16922754.200-red-willing-and-able.html |title=Red, willing and able |date=27 January 2001 |type=[http://www.kencroswell.com/reddwarflife.html Full reprint] |publisher=[[New Scientist]] |access-date=5 August 2007 }}</ref> Such variation would be very damaging for life, as it would not only destroy any complex organic molecules that could possibly form biological precursors, but also because it would blow off sizeable portions of the planet's atmosphere.
For a planet around a red dwarf star to support life, it would require a rapidly rotating magnetic field to protect it from the flares. A tidally locked planet rotates only very slowly, and so cannot produce a geodynamo at its core. The violent flaring period of a red dwarf's life cycle is estimated to only last roughly the first 1.2 billion years of its existence. If a planet forms far away from a red dwarf so as to avoid tidal locking, and then migrates into the star's habitable zone after this turbulent initial period, it is possible that life may have a chance to develop.<ref>{{cite web |title=AstronomyCast episode 40: American Astronomical Society Meeting, May 2007 |work=Universe Today |last1=Cain |first1=Fraser |last2=Gay |first2=Pamela |author-link2=Pamela L. Gay |url=http://www.astronomycast.com/2007/06/episode-40-american-astronomical-society-meeting-may-2007/ |date=2007 |access-date=17 June 2007 }}</ref> However, given its age, at 7–12 billion years of age, Barnard's Star is considerably older than the Sun. It was long assumed to be quiescent in terms of stellar activity. Yet, in 1998, astronomers observed an intense [[stellar flare]], surprisingly showing that Barnard's Star is, despite its age, a [[flare star]].<ref name=Flare>{{cite web
| first=Ken | last=Croswell |date=November 2005
| url=http://www.astronomy.com/news/2005/11/a-flare-for-barnards-star
| access-date=2006-08-10 | title=A Flare for Barnard's Star
| work=Astronomy Magazine | publisher=Kalmbach Publishing Co }}</ref>
==== Longevity and ubiquity ====
Red dwarfs have one advantage over other stars as abodes for life: far greater longevity. It took 4.5 billion years before humanity appeared on Earth, and life as we know it will see suitable conditions for 1<ref>{{cite press release |title='The end of the world' has already begun, UW scientists say |publisher=[[University of Washington]] |url=http://www.washington.edu/news/2003/01/13/the-end-of-the-world-has-already-begun-uw-scientists-say/|author=Hines, Sandra |date=13 January 2003 |access-date=5 June 2007 }}</ref> to 2.3<ref name=pnas1_24_9576>{{cite journal |last1=Li |first1=King-Fai |last2=Pahlevan |first2=Kaveh |last3=Kirschvink |first3=Joseph L. |last4=Yung |first4=Yuk L. |date=2009 |title=Atmospheric pressure as a natural climate regulator for a terrestrial planet with a biosphere |journal=Proceedings of the National Academy of Sciences |volume=106 |issue=24 |pages=9576–9579 |url=http://www.gps.caltech.edu/~kfl/paper/Li_PNAS2009.pdf |doi=10.1073/pnas.0809436106 |pmid=19487662 |pmc=2701016 |bibcode=2009PNAS..106.9576L |access-date=19 July 2009 }}</ref> {{nowrap |billion years}} more. Red dwarfs, by contrast, could live for trillions of years because their nuclear reactions are far slower than those of larger stars, meaning that life would have longer to evolve and survive.
While the likelihood of finding a planet in the habitable zone around any specific red dwarf is slight, the total amount of habitable zone around all red dwarfs combined is equal to the total amount around Sun-like stars given their ubiquity.<ref>{{cite web |url=http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=1694 |title=M Dwarfs: The Search for Life is On, Interview with Todd Henry |date=29 August 2005 |publisher=Astrobiology Magazine |access-date=5 August 2007 }}</ref> Furthermore, this total amount of habitable zone will last longer, because red dwarf stars live for hundreds of billions of years or even longer on the main sequence.<ref name="Red Dwarfs Universe Today">{{cite web |last=Cain |first=Fraser |title=Red Dwarf Stars |url=http://www.universetoday.com/24670/red-dwarf-stars/ |publisher=Universe Today |date=4 February 2009}}</ref> However, combined with the above disadvantages, it is more likely that red dwarf stars would remain habitable longer to microbes, while the shorter-lived yellow dwarf stars, like the Sun, would remain habitable longer to animals.
=== Bintang masif ===
Recent research suggests that very large stars, greater than ~100 solar masses, could have planetary systems consisting of hundreds of Mercury-sized planets within the habitable zone. Such systems could also contain [[brown dwarfs]] and low-mass stars (~0.1–0.3 solar masses).<ref>{{cite journal |title=The outcome of the protoplanetary disk of very massive stars, January 2011 |journal=New Astronomy |last1=Kashi |first1=Amit |last2=Soker |first2=Noam |doi=10.1016/j.newast.2010.06.003 |bibcode=2011NewA...16...27K |date=2011 |volume=16 |issue=1 |pages=27–32 |arxiv=1002.4693 |citeseerx=10.1.1.770.1250 |s2cid=119255193 }}</ref> However the very short lifespans of stars of more than a few solar masses would scarcely allow time for a planet to cool, let alone the time needed for a stable biosphere to develop. Massive stars are thus eliminated as possible abodes for life.<ref>[[Stellar mass#Age]]</ref>
However, a massive-star system could be a progenitor of life in another way – the [[supernova]] explosion of the massive star in the central part of the system. This supernova will disperse heavier elements throughout its vicinity, created during the phase when the massive star has moved off of the main sequence, and the systems of the potential low-mass stars (which are still on the main sequence) within the former massive-star system may be enriched with the relatively large supply of the heavy elements so close to a supernova explosion. However, this states nothing about what types of planets would form as a result of the supernova material, or what their habitability potential would be.
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== Catatan ==
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