New Rankings List Gliese 581g As the Most Habitable Exoplanet

This artist's concept shows the inner planets of the Gliese 581 system. Planet G, shown in foreground, is thought to be in its star's habitable zone. Unfortunately, we won't be able to visit other solar systems for a very, very long time, according to a new study. Lynette Cook/via NASA

Behold, by decree, the most habitable exoplanet known to date. Some people think Gliese 581g may not exist, sure, but if it does it’s probably nice and Earthy warm.

Gliese 581 is one of our favorite exoplanet systems, anchored by a red dwarf star only 20 light years from Earth. Astronomers think it has at least four, maybe five exoplanets, one or maybe two of which is possibly habitable. Gliese 581c is too close to its star, giving it Venus-like boiling temperatures that would prevent liquid water. Gliese 581d may be too far, staying too chilly to support water or life — although models suggest it could have a dense carbon-rich atmosphere to keep it warm. (It’s also on this new maybe-habitable exoplanet list.)

Gliese 581g was another story. It’s only one and a half, maybe two times the size of Earth and thought to exist smack in the star’s “Goldilocks zone.” It receives the same light flux from its star that Earth does from the sun.

Upon closer inspection, some astronomers thought 581g didn’t even exist. But new evidence from the team who discovered it says that it does, proven by the planets’ circular, rather than elliptical, orbits.

This debate is likely far from settled, but the University of Puerto Rico at Arecibo’s Planetary Habitability Laboratory says this world is the most habitable one out there, topping a brand-new list of the top five most habitable exoplanets.

Exoplanet habitability is a budding science, with several competing theories about how one might ascertain a planet’s likelihood to support life. The Earth Similarity Index is one way to do that, with planets falling on a spectrum from zero (not Earth-like) to one (very Earth-like). Gliese 581g has an ESI rating of 0.92, the highest of all — and way better than Mars, by the way, which rings in at 0.66.

A new paper describing Gliese 581g appears in the journal Astronomical Notes.