Hubble Helps Discover a New Type of Planet Largely Composed of Water

In this illustration the super-Earth, Kepler-138d is in the foreground. To the left, the planet Kepler-138c, and in the background the planet Kepler 138b, seen in silhouette transiting its central star. Kepler 138 is a red dwarf star located 218 light-years away. The low density of Kepler-138c and Kepler-138d — which are nearly identical in size — means that they must be composed largely of water. They are both twice Earth’s mass but have roughly half of Earth’s density, and therefore cannot be solid rock. This is based on measurements of their mass vs. physical diameter. They are considered a new class of “water-planet,” unlike any major planet found in our solar system. Kepler-138 b is one of the smallest exoplanets known, having the mass of the planet Mars and the density of rock. [Image Description: Illustration of three planets and their star on the black background of space speckled with dots of different colors. The planets are various sizes, indicating different distances from the viewer and from the star. The largest, foreground planet, is at the upper right, and is grayish white with bands of clouds.]