Size comparison of HAT-P-11b (gray) with Neptune Astronomers have detected water vapor in the atmosphere of an exoplanet, an extrasolar planet, which does not orbit the Sun. The planet is the size of Neptune and about four times the size of Earth. It orbits a star some 130 light years from Earth. HAT-P-11b’s atmosphere was also found to contain copious amounts of hydrogen. Atmospheres are telltales...