Super-Earth - HD 40307g

Twice as big in volume as the Earth, HD 40307 g straddles the line between "Super-Earth" and "mini-Neptune" and scientists aren't sure if it has a rocky surface or one that's buried beneath thick layers of gas and ice. One thing is certain though: at eight times the Earth's mass, its gravitational pull is much, much stronger.

Credit

NASA/JPL-Caltech