近日，一个国际科学家小组评选出哪些卫星和行星最有可能存在地外生命。这些最适于居住的地外星球包括土卫六“泰坦”和系外行星Gliese 581g，后者位于天秤座，距地球大约20.5光年。评选结果刊登在《天体生物学》（Astrobiology）杂志上。
在所发表的研究论文中，研究人员提出了两个不同的指数，一个是与地球相似度指数，一个是行星适居性指数。
与地球相似度指数用于评比行星和卫星与地球的相似程度，评比时参考体积、密度、与母星之间的距离等因素。行星适居性指数关注的则是其他一系列因素，例如是否拥有多岩或者冰冻地表，是否拥有大气层或者磁场。此外，这一指数还关注任何生物体能获取的能量，或者通过母星的光照，或者通过所谓的潮汐扰动过程。最后，行星适居性指数还要考虑是否存在有机化合物，至关重要的化学反应能否获得液态溶剂支持等。
与地球相似度指数的评比中，得分最高的是Gliese 581g(一些天文学家对其是否存在尚存在分歧)，得分为0.89。天文学家对Gliese 581系统进行了深入研究。这个系统由4颗，也可能是5颗行星环绕一颗红矮星构成。此外，系外行星HD 69830 d的得分也相对较高，为0.60。这颗行星体积与天王星相当，位于船尾座。据信，HD 69830 d位于所谓的“金发区”，这一区域与母星不远不近，表面温度既不太高，也不太低。
行星适居性指数的评比结果和与地球相似度指数截然不同，得分最高的是土卫六“泰坦”，适居性指数达到0.64。（生物谷Bioon.com)
doi:10.1089/ast.2010.0592
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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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