Они имели в виду, что не газовый гигант, так что и Земля тоже "каменистая" в этом смысле :)
"Prof Kane explained: "There seems to be a transition that occurs at about 1.5 times the Earth's radius, such that if the planet is larger, then it starts to develop a very substantial atmosphere very similar to what we see in the gas giant planets in our own Solar System.
"And so anything less than 1.5 is probably more like a rocky planet that we are familiar with.""
http://www.bbc.com/news/science-environment-27054366