New data suggests the exoplanet Gliese 581g is the best candidate so far of a potential habitable exoplanet. The nearby star Gliese 581 — located about 20 light years away from Earth in the constellation Libra — has four planets; the outermost planet, Gliese 581d, was already suspected habitable.

This will be the first evidence for any two potential habitable exoplanets that are orbiting the same star.

Based on the new data, Gliese 581g probably has a radius not larger than 1.5 times Earth radii. It receives about the same light flux as Earth does from the Sun due to its closer orbital position around a dim red dwarf star. These factors combine to make Gliese 581g the most Earth-like planet known with an Earth Similarity Index, a measure of Earth-likeness from zero to one, of 0.92 and higher than the previously top candidate Gliese 667Cc, discovered last year.

Gliese 581g Q&A on Space.com