In this illustration, exoplanet Kepler-138 d is in the foreground and Kepler-138 c is on the left. In the background, Kepler 138 b is passing in front of its parent star. The low densities of Kepler-138 c and Kepler-138 d indicate that they must be composed largely of water. Credit: NASA, ESA, Leah Hustak (STScI)
In this illustration, exoplanet Kepler-138 d is in the foreground and Kepler-138 c is on the left. In the background, Kepler 138 b is passing in front of its parent star. The low densities of Kepler-138 c and Kepler-138 d indicate that they must be composed largely of water.