The finding makes the world - which is called K2-18b - a plausible candidate in the search for alien life.
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."