In a first for astronomers studying worlds beyond our solar system, data from the Hubble Space Telescope have revealed
water vapor in the atmosphere of an Earth-size planet. Although this
exoplanet orbits a star that is smaller than our sun, it falls within what’s known as the star’s habitable zone, the range of orbital distances where it would be warm enough for liquid water to exist on a planet’s surface.
The discovery, announced this week in two independent studies, comes from years of observations of the exoplanet K2-18b, a super-Earth that’s about 111 light-years from our solar system. Discovered in 2015 by
NASA’s Kepler spacecraft, K2-18b is very unlike our home world: It’s more than eight times the mass of Earth, which means it’s either an icy giant like Neptune or a rocky world with a thick, hydrogen-rich atmosphere.
