There have been roughly a dozen extrasolar planets found of between 10 and 20 Earth masses, such as those orbiting the stars Mu Arae, 55 Cancri and GJ 436. These planets have been nicknamed "Neptunes" because they roughly approximate that planet's mass (17 Earths).
and five of the six planets orbiting the nearby red dwarf Gliese 581. Gliese 581 d is roughly 7.7 times Earth's mass, while Gliese 581 c is five times Earth's mass and was initially thought to be the first terrestrial planet found within a star's habitable zone. However, more detailed studies revealed that it was slightly too close to its star to be habitable, and that the farther planet in the system, Gliese 581 d, though it is much colder than Earth, could potentially be habitable if its atmosphere contained sufficient greenhouse gases.