近日,，一個國際科學(xué)家小組評選出哪些衛(wèi)星和行星最有可能存在地外生命,。這些最適于居住的地外星球包括土衛(wèi)六“泰坦”和系外行星Gliese 581g,，后者位于天秤座，距地球大約20.5光年,。評選結(jié)果刊登在《天體生物學(xué)》（Astrobiology）雜志上,。
在所發(fā)表的研究論文中，研究人員提出了兩個不同的指數(shù),，一個是與地球相似度指數(shù),，一個是行星適居性指數(shù)。
與地球相似度指數(shù)用于評比行星和衛(wèi)星與地球的相似程度,，評比時參考體積,、密度,、與母星之間的距離等因素。行星適居性指數(shù)關(guān)注的則是其他一系列因素,，例如是否擁有多巖或者冰凍地表,，是否擁有大氣層或者磁場。此外,，這一指數(shù)還關(guān)注任何生物體能獲取的能量,，或者通過母星的光照，或者通過所謂的潮汐擾動過程,。最后,，行星適居性指數(shù)還要考慮是否存在有機(jī)化合物，至關(guān)重要的化學(xué)反應(yīng)能否獲得液態(tài)溶劑支持等,。
與地球相似度指數(shù)的評比中,，得分最高的是Gliese 581g(一些天文學(xué)家對其是否存在尚存在分歧)，得分為0.89,。天文學(xué)家對Gliese 581系統(tǒng)進(jìn)行了深入研究,。這個系統(tǒng)由4顆，也可能是5顆行星環(huán)繞一顆紅矮星構(gòu)成,。此外,，系外行星HD 69830 d的得分也相對較高，為0.60,。這顆行星體積與天王星相當(dāng),，位于船尾座。據(jù)信,，HD 69830 d位于所謂的“金發(fā)區(qū)”,，這一區(qū)域與母星不遠(yuǎn)不近，表面溫度既不太高,，也不太低,。
行星適居性指數(shù)的評比結(jié)果和與地球相似度指數(shù)截然不同，得分最高的是土衛(wèi)六“泰坦”,，適居性指數(shù)達(dá)到0.64,。（生物谷Bioon.com)
doi:10.1089/ast.2010.0592
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PMID:
A Two-Tiered Approach to Assessing the Habitability of Exoplanets
Dirk Schulze-Makuch, Abel Méndez, Alberto G. Fairén, Philip von Paris, Carol Turse, Grayson Boyer, Alfonso F. Davila, Marina Resendes de Sousa António, David Catling, and Louis N. Irwin
In the next few years, the number of catalogued exoplanets will be counted in the thousands. This will vastly expand the number of potentially habitable worlds and lead to a systematic assessment of their astrobiological potential. Here, we suggest a two-tiered classification scheme of exoplanet habitability. The first tier consists of an Earth Similarity Index (ESI), which allows worlds to be screened with regard to their similarity to Earth, the only known inhabited planet at this time. The ESI is based on data available or potentially available for most exoplanets such as mass, radius, and temperature. For the second tier of the classification scheme we propose a Planetary Habitability Index (PHI) based on the presence of a stable substrate, available energy, appropriate chemistry, and the potential for holding a liquid solvent. The PHI has been designed to minimize the biased search for life as we know it and to take into account life that might exist under more exotic conditions. As such, the PHI requires more detailed knowledge than is available for any exoplanet at this time. However, future missions such as the Terrestrial Planet Finder will collect this information and advance the PHI. Both indices are formulated in a way that enables their values to be updated as technology and our knowledge about habitable planets, moons, and life advances. Applying the proposed metrics to bodies within our Solar System for comparison reveals two planets in the Gliese 581 system, GJ 581 c and d, with an ESI comparable to that of Mars and a PHI between that of Europa and Enceladus.
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