
Artistic representation of the planetary system of the star Kepler-138. We see Kepler-138 d in the foreground, and closer to the star, Kepler-138 c. These two planets are probably composed mainly of water. The small planet Kepler-138 b is seen transiting in front of the star. A fourth planet, Kepler-138 e, is further away and not visible in the image. (Image: STSCI.)