Earth Similarity Index

Serious discussion of science, skepticism, and evolution
Post Reply
User avatar
lpetrich
Posts: 14453
Joined: Mon Feb 23, 2009 6:53 pm
Location: Lebanon, OR, USA

Earth Similarity Index

Post by lpetrich » Wed Jan 17, 2018 11:44 am

Earth Similarity Index (ESI) - Planetary Habitability Laboratory @ UPR Arecibo
A Two-Tiered Approach to Assessing the Habitability of Exoplanets | Abstract
[1507.06293v1] Earth Similarity Index with two free parameters
(Wikipedia)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:
  • Radius
  • Mean density
  • Escape velocity
  • Surface temperature
though for better comparison, it should be extended to include possible tidal locking of rotation, stellar activity (can strip off atmospheres), and planetary magnetic fields (can protect atmospheres).

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.

User avatar
lpetrich
Posts: 14453
Joined: Mon Feb 23, 2009 6:53 pm
Location: Lebanon, OR, USA

Post by lpetrich » Wed Jan 17, 2018 12:28 pm

Turning to TRAPPIST-1, I find:
Planet | ESI | Int | Srf | Simp b | 0.69 | 0.85 | 0.56 | 0.56 c | 0.80 | 0.90 | 0.71 | 0.73 d | 0.88 | 0.87 | 0.88 | 0.90 e | 0.71 | 0.66 | 0.75 | 0.85 f | 0.65 | 0.67 | 0.63 | 0.68 g | 0.64 | 0.73 | 0.56 | 0.58 h | 0.46 | 0.57 | 0.38 | 0.46

Planet d is thus the most Earthlike of that star's planet.

Looking at Proxima Centauri b, I find tot = 0.91, int = 0.98, srf = 0.84, simp = 0.85

I neglected to post the simplified values for the Solar System, so here they are. Venus: 0.78, Mars: 0.65, Moon: 0.60, Mercury: 0.39, Io: 0.23, with the other values smaller.


Of all the planets that have been found so far, it seems like most of the more Earthlike ones have been discovered orbiting red dwarfs. Such planets have complications like tidal locking and their stars being very active.

But it is around stars like those that it is easiest to discover Earthlike planets.

Post Reply