Free Astronomy Magazine March-April 2023

MARCH-APRIL 2023 constitute the bulk of gas-giant planets like Jupiter). The most com- mon candidate material is water. “We previously thought that plan- ets that were a bit larger than Earth were big balls of metal and rock, like scaled-up versions of Earth, and that’s why we called them super- Earths,” explained Benneke. “How- ever, we have now shown that these two planets, Kepler-138 c and d, are quite different in nature and that a large fraction of their entire volume is likely composed of water. It is the best evidence yet for water worlds, a type of planet that was theorised by astronomers to exist for a long time.“ With volumes more than three times that of Earth and masses twice as big, planets c and d have much lower densities than Earth. I n this illustration, the super-Earth Kepler-138 d is in the foreground. To the left, the planet Kepler-138 c, and in the background the planet Kepler 138 b, seen in silhouette transiting its central star. The low density of Ke- pler-138 c and Kepler-138 d — which are nearly identical in size — means that they must be composed largely of water. [NASA, ESA, L. Hustak (STScI)]