A Two-Tiered Approach to Assessing the Habitability of Exoplanets | Abstract
[1507.06293v1] Earth Similarity Index with two free parameters
Earth Similarity Index -- how much some planet or moon resembles the Earth.
This is an attempt to quantify how similar a celestial body is to the Earth, as a way of identifying possibly habitable ones, at least ones where living things can inhabit the surface. It uses:
- Mean density
- Escape velocity
- Surface temperature
Averaging the first two values gives an interior similarity index, and the second two gives a surface similarity index. The overall index is the average of these two indices.
The PHL page has a formula for this ESI, and it also has a simplified one, one that uses only a planet's radius and its star's light flux at it.
The most Earthlike Solar-System object is Mars, with interior = 0.82, surface = 0.60, total = 0.70. That is using actual surface temperatures. Using a naive estimate of proportional to 1/sqrt(distance) gives srf = 0.60, tot = 0.71.
Venus is int = 0.98 and srf = 0.20 (actual), 0.78 (naive), tot = 0.44 (a), 0.87 (n). That planet's atmospheric greenhouse effect is what makes the difference. That is another complication in assessing Earth similarity, because a little more received light may give a planet a Venus-like runaway greenhouse effect.
Mercury and the Moon have int = 0.84 and 0.67, and srf = 0.42 and 0.46 (actual), 0.39 and 0.69 (naive), tot = 0.60 and 0.56 (a), 0.57 and 0.68 (n)
The next one is Io, with int = 0.35 and srf = 0.19 (actual), 0.18 (naive), tot = 0.36(a), 0.35 (n). The remaining Solar-System large bodies are worse, often much worse.
Turning to exoplanets, the Wikipedia article's list of high scorers are all planets of red dwarfs, with the exception of Kepler-62e. That planet's star is a K5 star, between the Sun and red-dwarfhood.