Editor’s note: A version of this story appeared in CNN’s Wonder Theory science newsletter. To get it in your inbox, sign up for free here . CNN  —  As scientists search for worlds that may be habitable for life, they’ve discovered a type that is common in the universe — but doesn’t exist in our own solar system. These enigmatic planets are called sub-Neptunes, which are larger than Earth but smaller than Neptune. An April study catapulted one such world, named K2-18b, into the spotlight.