Illustration comparing the sizes of exoplanets GJ 1214b (second from left) and GJ 436b (right) with Earth (left) and Neptune (third from left). Exoplanets of this size are called super-Earths and have masses between those of gas giants like Neptune and smaller, rocky planets like Earth. Spectroscopic observations by the NASA/ESA Hubble Space Telescope suggest that these worlds could be smothered in clouds that mask information about the planets' underlying chemical compositions. This illustration is speculative and does not necessarily reflect their true appearance.