K2-18b was discovered in 2015 and is one of hundreds of super-Earths - planets with a mass between Earth and Neptune - found by NASA's Kepler spacecraft.
The team's analysis of eight of these transits, as well as transits observed by the Kepler and Spitzer space telescopes, will appear in the Astronomical Journal.
Now we have for the first time managed to detect water vapour in the atmosphere of an exoplanet that is potentially habitable. The lead researcher, Prof Giovanna Tinetti of University College London (UCL) depicted the disclosure as "mind-blowing".
"Of course, K2-18b is not a second Earth", Dr Tsiaras added, "because it is a planet that is much bigger and has a different atmospheric composition".
Researchers do not think K2-18b is the only super-earth like this and expect to find others with similar characteristics among more than 4000 exoplanets that have already been discovered.
The planet orbits the cool dwarf star K2-18, which is 110 light years from Earth in the constellation of Leo. These observations, plus the fact that K2-18b is parked inside the habitable zone - the sweet spot in a star system where water could persist in liquid form - warranted further study.
Twice as large as our own planet and eight times as massive, K2-18b possesses powerful gravity that would make it hard to walk upon.
"Now, whether this planet really has an ocean at the surface or rock, we can not tell with current observations, but having water in the atmosphere is a good start". The researchers from UCL also believe that there may be nitrogen and methane, too. K2-18b is likely to be more hostile as it is exposed to more high-energy radiation.
The detection of water vapour around a potentially habitable planet is being hailed as a first step in what will become a wider understanding of exoplanets.
An interesting characteristic of super-Earths is that majority are likely to be water worlds-terrestrial planets covered entirely by a deep global ocean. It is the only exoplanet to fulfil three requirements for habitability: the right temperatures, an atmosphere and the presence of water. When stellar light moves through a planet's atmosphere, it becomes scattered by the presence of different atmospheric elements and compounds.
But sophisticated tools developed at UCL have been able to translate data from the Hubble Space Telescope to make sense of the unique molecule signatures of water vapor. Over 4000 exoplanets have been detected but we don't know much about their composition and nature.
Astronomers have for the first time found a planet outside the solar system that has properties making it potentially habitable for life.
Additional study will have the ability to ascertain the area of cloud coverage and the proportion of water from the air.
But it is not Earth 2.0.
What the scientists are saying with this measurement is that Earth and this other planet start in roughly the same place, as far as their respective equilibrium temperatures are concerned (Earth is 257K, this K2-18b is 265K +/- 5K).
The planet is rocky, but it's no Earth twin.
Björn Benneke (University of Montreal) led a team in applying for time on the Hubble Space Telescope, observing the planet as it crossed the face of its star. "It is highly unlikely that this world is habitable in any way that we understand based on life as we know it", the Space Telescope Science Institute's Hannah Wakeford told Nature.
The next generation of space telescopes, including the NASA/ESA/CSA James Webb Space Telescope and ESA's ARIEL mission, will be able to characterise atmospheres in more detail as they will carry more advanced instruments.