Closest yet habitable planet candidate around a Sun-like star

HD 40307g is a super-Earth at least seven times more massive than Earth orbiting the star at the right distance to support liquid water. The planet could have a size between 1.9 and 2.5 Earth radii depending on composition, either rocky or water-rich, respectively. HD 40307g receives on average about 67% of the light Earth receives from the Sun.

There is no information about the type and composition of the atmosphere of HD 40307g. Average temperatures might be near 9°C (48°F) assuming a similar scaled-up terrestrial atmosphere. It might also experience strong seasonal surface temperature shifts between -17° to 52°C due to its orbital eccentricity. Nevertheless, these extremes are tolerable by most complex life, as we know it.

Planetary Habitability Laboratory press release. The quoted bounds on radii scale respectively to 1.93 and 1.12 Earth gravities. The system is 42 light years (13 parsecs) away. This is further than the exoplanet candidate Gliese 667Cc which orbits an M dwarf (22 light years; see sidebar) but much closer than Kepler-22b, the first exo-Earth candidate in the habitable zone of a Sun-like star.


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