PARIS - Water has been discovered for the first time in the atmosphere of an exoplanet with Earth-like temperatures that could support life as we know it, scientists revealed on Wednesday.
An artist's impression of planet K2-18b. (Photo: NASA/REUTERS)
Eight times the mass of Earth and twice as big, the planet K2-18b orbits in its star's "habitable zone" at a distance-neither too far nor too close-where water can exist in liquid form, according to the report in the journal Nature Astronomy.
"This planet is the best candidate we have outside our solar system" in the search for signs of life, said co-author Giovanna Tinetti, an astronomer at University College London, or UCL. "We cannot assume that it has oceans on the surface but it is a real possibility."
Of the more than 4,000 exoplanets detected to date, this is the first known to combine a rocky surface and an atmosphere with water.
Most exoplanets with atmospheres are giant balls of gas, and the handful of rocky planets for which data is available seem to have no atmosphere at all.
Even if they did, most Earth-like planets are too far from their stars to have liquid water or so close that any H2O has evaporated.
Discovered in 2015, K2-18b is one of hundreds of so-called "super-Earths"-planets with less than ten times the mass of ours-spotted by NASA's Kepler spacecraft.
Future space missions are expected to detect hundreds more in the coming decades.
"Finding water in a potentially habitable world other than Earth is incredibly exciting," said lead-author Angelos Tsiaras, also from UCL.
K2-18b orbits a red dwarf star about 110 light years distant-a million billion kilometers-in the Leo constellation of the Milky Way, and is probably bombarded by more destructive radiation than Earth.