I’ve lost count of the number of times I’ve read that the “first Earth-like exoplanet” has been discovered. With nearly 2,000 exoplanets found to date, it is no wonder so many of them will resemble our planet in some way. But which exoplanets are similar enough to the Earth that they could actually be habitable?
Many of the claims about the habitability of exoplanets are greatly exaggerated. The exoplanet GJ 1132 b was just announced by the MEarth project, as “arguably the most important planet ever found outside the solar system.” While it’s one of the nearest exoplanets yet discovered, it’s hardly Earth-like—situated close to its host star with a scalding surface temperature of several hundred degrees Celsius.
Similarly, Tau Ceti e and Kepler-186f have both been touted as Earth twins, but there are other exoplanets out there that are rather more Earth-like.
A good way to estimate how habitable a planet is the Earth Similarity Index (ESI). This number is calculated from the exoplanet’s radius, density, surface temperature, and escape velocity, which is the minimum speed needed to break free from the planet’s surface. For many exoplanets, we don’t have all these measurements, so some of them have to be estimated based on the best available information. The ESI ranges from 0 to 1 and anything with an ESI above 0.8 may be considered “Earth-like.” In our solar system, Mars scores 0.64 (the same as Kepler 186f) while Venus comes in at 0.78 (the same as Tau Ceti e).
Here are the five top candidates for an Earth-twin, based on their ESI values.
1. Kepler-438b
Kepler-438b (ESI=0.88) has the highest ESI of any exoplanet known. Discovered in 2015 around a red dwarf star, significantly smaller and cooler than our sun, it has a radius only 12 percent larger than Earth’s. It orbits the star, which is 470 light years from Earth, every 35 days and is in its habitable zone, the region around a star which is neither too hot nor too cold for orbiting planets to support liquid water on the surface.
As with other discoveries by Kepler around faint stars, the planet’s mass has not been measured, but if its composition is rocky, it may be only 1.4 times that of the Earth’s with a surface temperature between 0 degrees C (Celsius) (32 degrees Fahrenheit (F)) and 60 degrees C (140 degrees F). However, the ESI is not a foolproof method for classifying the Earth-like nature of a planet. It has recently been found that Kepler 438b’s host star regularly sends out powerful flares of radiation, which may render the planet uninhabitable after all.
2. Gliese 667 Cc
Gliese 667 Cc (ESI=0.85) was discovered in 2011 orbiting a red dwarf in the Gliese 667 triple star system, just 24 light years away. It was found by the radial velocity method, which is a measure of the small movement a star makes as it responds to the gravitational tug of the planet. The planet’s mass has been estimated at 3.8 times the Earth’s, but we don’t know its size. This is because the planet does not pass in front of the star, which would allow us to measure the planet’s radius. With an orbital period of 28 days, it sits in the habitable zone of this cool star, with a possible surface temperature of around 5 degrees C (41 degrees F).
3. Kepler-442b
Kepler-442b (ESI=0.84) is a planet 1.3 times the size of the Earth discovered in 2015. It is orbiting a star cooler than the sun, about 1100 light years away. Its orbital period of 112 days places it in its star’s habitable zone, but with a surface temperature that could be as low as -40 degrees C (-40 degrees F). However, by comparison, the temperature on Mars can be -125 degrees C (-193 F) near its poles in the winter. Once again, the exoplanet’s mass is not known, but if it has a rocky composition, it may be only 2.3 times the mass of the Earth.
