Gliese 581

Gliese 581 is a red dwarf star about 20.5 light years from Earth. So far, that's unremarkable. Astronomers using the HARPS telescope, which is designed specifically to look for low mass exoplanets, however, have discovered the lightest exoplanet yet found. Gliese 581 e is a so-called Super Earth, but it has only 1.9 Earth masses. It also orbits the star in only a bit more than three Earth days, which means it likely does not support life.

The system also has at least three more planets, two of which would also seem to meet the Super Earth definition, which tops out at ten Earth masses; the other planet known in the system is even larger.

Using a ground-based technology to find an exoplanet so near Earth's size is a significant achievement, but the bigger story may be the emerging Gliese 581 planetary system. Red dwarfs account for roughly three quarters of all stars in the galaxy, and now we know they can be home to substantial planetary families. They are also stable in energy output for extraordinarily long periods. If life evolving around a red dwarf is unlikely, a stable star surrounded by immense natural resources might be extremely attractive to a civilization seeking to expand beyond its home star system and into the galaxy, one star system after another.