Within 10 years, new space telescopes might be able to determine whether K2-18b's atmosphere contains gases that could be produced by living organisms.
Details were published in the scientific journal Nature Astronomy.
The lead scientist, Prof Giovanna Tinetti of University College London (UCL) described the discovery as "mind blowing".
"This is the first time that we have detected water on a planet in the habitable zone around a star where the temperature is potentially compatible with the presence of life," she said.
The habitable zone is the region around a star where temperatures are sufficiently benign for water to exist in liquid form on the surface of a planet.
K2-18b is 111 light-years - about 650 million million miles - from Earth, too far to send a probe. So the only option is to wait for the next generation of space telescopes to be launched in the 2020s and to look for gasses in the planet's atmosphere that could only be produced by living organisms, according to UCL's Dr Ingo Waldmann.
"This is one of the biggest questions in science and we have always wondered if we are alone in the Universe," Dr Waldmann said. "Within the next 10 years, we will know whether there are chemicals that are due to life in those atmospheres."
What is an exoplanet?
- Planets beyond our Solar System are called exoplanets
- The first exoplanet was discovered in 1992, orbiting a pulsar (a neutron star that emits electromagnetic radiation)
- More than 4,000 have been detected to date using several techniques
- Many of these worlds are large planets believed to resemble Jupiter or Neptune
- Many giant planets have been found orbiting very close to their stars
The team behind the discovery looked through the planets discovered by the Hubble Space Telescope between 2016 and 2017. The researchers determined some of the chemicals in their atmosphere by studying the changes to the starlight as the planets orbited their suns. The light filtered through the planets' atmospheres was subtly altered by the composition of the atmosphere.
Only K2-18b revealed the molecular signature of water, which is a vital ingredient for life on Earth. Computer modelling of the data suggested that up to 50% of its atmosphere could be water.
The new planet is just over twice the size of Earth - a so called "super Earth" and has a temperature cool enough to have liquid water, between zero and 40C.
Dr Angelos Tsiaras, a member of the UCL team, said that finding water in the atmosphere of a "potentially habitable exoplanet was incredibly exciting".
He said: "It brings us closer to answering the fundamental question: Is the Earth unique?"
Other astronomers, however, dispute the claim that the planet is potentially habitable. One analysis indicates that planets that are 1.5 times the mass of the Earth are unlikely to have a rocky surface. There is also concern that K2-18b's size and gravity, some of the other gases in its atmosphere and the radiation from its star would make it difficult for the world to support life.
Other critics suggest that the planet should not be classified as a super-Earth, but as a mini Neptune instead. This is a class of planet that is typically more massive than a super-Earth (it includes worlds that are up to 10 times the mass of our own planet). Prof Tinetti, however, maintains her view that the new world is potentially habitable.
She said it had a mean density that's comparable to that of Mars (which is composed of silicates) and Jupiter's moon Europa, and much higher than Neptune's.
"K2-18b cannot be classified as a mini Neptune, it is more likely to be a planet with an interior of rock and ices. These types of planets, [are] sometimes called 'ocean planets'," she told BBC News.
"Now, whether this planet really has an ocean at the surface or rock, we cannot tell with current observations, but having water in the atmosphere is a good start."
Another issue is that astronomers can't agree what conditions are needed for habitability. We only have the Earth to go on but life may also be possible on other types of worlds
It is likely to require a survey of the chemical composition of, perhaps, hundreds of worlds and an understanding of how they are created and evolve, according to Prof Tinetti.
"The Earth really stands out in our own Solar System. It has oxygen, water and ozone. But if we find all that around a planet around a distant star we have to be cautious about saying that it supports life," she said.
"This is why we need to understand not just a handful of planets in the galaxy but hundreds of them. And what we hope is that the habitable planets will stand out, that we will see a big difference between the planets that are habitable and the ones that are not."
Dr Beth Biller at Edinburgh University's Institute of Astronomy said she believed that evidence of life on a planet around a distant star would eventually be discovered.
"That would be a paradigm shift for all of humanity," she told BBC News.
"It's not going to be ET phoning home, necessarily - more likely microbes or some other simple life. Even so (when it happens) it's going to be huge".
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.
Water has been detected on other planets but they have been either too big or too hot to support life. Cooler smaller planets are much harder to detect. The team at UCL was able to do this by developing algorithms capable of teasing out the chemical composition of the atmospheres of potentially habitable worlds
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. Nasa's Tess mission is expected to detect hundreds more in coming years.
(Image copyright: ESA/UCL: Caption -