This could also be possible that the water vapour got detected only in the habitable zone of the planet while there would be a shortage of habitable atmosphere.
"Given the high level of activity of its red dwarf star, K2-18b may be more hostile to life as we know it than Earth, as it is likely to be exposed to more high-energy radiation".
This week, astronomers announced new hope for extraterrestrial habitability: an exoplanet some 110 light-years away from Earth that harbors water in its atmosphere.
During the study, researchers made use of archival data collected by the Hubble Space Telescope between 2016 and 2017 and found clear signs of water vapour in the exoplanet's atmosphere.
Angelos Tsiaras, an astronomer at UCL, said the team is focusing its attention on identifying exoplanets with conditions similar to those on Earth.
"It's got water in its air but it's nearly certainly got no surface ... where you could find that water pooling as a liquid in lakes and oceans, and that's of course, what we need for life, at least as far as we're aware", he said.
The planet orbits a red dwarf star and is estimated to have a temperature of -73 to 46 degrees Celsius.
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While light from the sun takes several minutes to reach Earth, light from K2-18b's star takes a century to reach our planet, "so for us to travel there is impossible", he said.
They also observed the signatures of hydrogen and helium in the atmosphere, two of the most abundant elements in the universe. "It brings us closer to the answer to the fundamental question: Is earth unique?" The London data suggest water vapor makes up anywhere between 0.01% and 50% of the atmosphere - "quite a big range", Waldmann acknowledged. Researchers believe that other molecules including nitrogen and methane may be present but, with current observations they remain undetectable. Further studies will also aim to estimate cloud coverage and the percentage of atmospheric water present.
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.
The British scientists used the data gathered by NASA's Hubble Space Telescope.
The prospective launch of Nasa's much delayed James Webb Space Telescope (JWST) in 2021, and the European Space Agency's Ariel mission seven years later, will enable astronomers to study in detail the atmospheres of the varied worlds that have been detected so far.
Depending on what future observations yield, K2-18b could turn out to be a planet with a dense, rocky core swaddled by a thick atmosphere, like Neptune-not exactly the most life-friendly of places.