Washington: Marking a new milestone in the search for water on distant planets, astronomers have found water vapour in the atmosphere of the exoplanet HAT P-11b, that is about four times the radius of Earth.
Astronomers look for water on exoplanets because it is a precondition for life - though the presence of water alone is not enough for life to arise.
The findings also suggest that astronomers' ideas about how planets formed appear to hold true for other planetary systems.
"The finding of water vapour and hydrogen on HAT P-11b is a key piece of the puzzle, consistent with astronomers' main ideas on the formation of planets," said Drake Deming, a professor at the University of Maryland in the US.
The exoplanet HAT P-11b was discovered by the Hungarian-made Automated Telescope (HAT) network.
The researchers observed HAT P-11b using two NASA telescopes - the Hubble Space Telescope, which measures visible and near-infrared light, and the Spitzer Space Telescope, which records only infrared light - between July 2011 and December 2012.
The team compared those data to observations by NASA's Kepler Space Telescope, which was launched to look for exoplanets and continuously records images of the portion of the sky where HAT P-11b is located.
It is about 26 times Earth's mass and compared with planets in our solar system, HAT P-11b is closest in size to Neptune.
But it is much closer to its host star and therefore much hotter, about 878 degrees Kelvin.
It probably has a rocky core, wrapped in a thick, gaseous envelope of about 90 percent hydrogen.
Its atmosphere is cloudless at high altitude, but contains the signature of water vapour.
The findings appeared in the journal Nature.