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.
This research output has an Altmetric Attention Score of 93. This is our high-level measure of the quality and quantity of online attention that it has received. This Attention Score, as well as the ranking and number of research outputs shown below, was calculated when the research output was last mentioned on 17 May 2026.
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#1
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Altmetric has tracked 33,928,890 research outputs across all sources so far. Compared to these this one has done particularly well and is in the 98th percentile: it's in the top 5% of all research outputs ever tracked by Altmetric.
So far Altmetric has tracked 1,563 research outputs from this source. They typically receive a lot more attention than average, with a mean Attention Score of 41.8. This one has done particularly well, scoring higher than 90% of its peers.
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We're also able to compare this research output to 16 others from the same source and published within six weeks on either side of this one. This one has done particularly well, scoring higher than 93% of its contemporaries.
