Dave5

While the exact composition of these worlds remains unclear, estimates of their surface temperatures from previous studies point them being significantly colder than Earth. They’re also each less dense than Earth, despite being roughly the same size as our planet.

“Our analyses predict that these 17 worlds may have ice-covered surfaces but receive enough internal heating from the decay of radioactive elements and tidal forces from their host stars to maintain internal oceans,” Lynnae Quick of NASA’s Goddard Space Flight Center said in a statement. 

https://www.space.com/nasa-17-exoplanets-liquid-oceans-beneath-surface