UK scientists have detected water vapour on a distant ‘super-Earth’ in a “world first” discovery.
Scientists now say K2-18b is “the only planet outside our solar system that we know has the correct temperature, an atmosphere and water" to support life.
K2-18b is twice as big and eight times heavier than our Earth, and located about 110 light years away.
The exoplanet was first spotted by Nasa's Kepler spacecraft in 2015 but analysis of data has now revealed new details that have never been seen on a super-Earth.
Current equipment is only able to determine basic factors such as distance, mass and temperature but sophisticated tools developed at University College London allowed scientists to translate existing data from the Hubble Space Telescope and make sense of the unique molecule signatures of water vapour.
"It's the only planet outside our solar system that we know has the correct temperature, an atmosphere and water," said first author Dr Angelos Tsiaras.
"Of course, K2-18b is not a second Earth, because it is a planet that is much bigger, has a different atmospheric composition. It's orbiting a completely different star, so it doesn't look like Earth.
"The search for habitable planets, it's very exciting, but it's here to always remind us that this (Earth) is our only home and it's probably out of the question if we will be able to travel to other planets."
While K2-18b is too far away for astronomers to see, they can look at how starlight is filtered through the planet's atmosphere as it passes around its own sun, called K2-18.
It is a lot closer to its star than Earth is to the Sun, meaning it only takes around 33 days to transit.
Although the planet sits in the habitable zone, scientists say that instruments available at present are not able to determine any signs of life.
It will also require many more observations to be sure that it is a habitable planet.
Scientists, who have published their findings in the Nature Astronomy journal after more than a year of work, believe that the water content could be as low as 0.01 per cent or as high as 50 per cent.
"We don't know how much water there is; however, what these models clearly say is that there is an atmosphere and that there is water there," said co-author Dr Ingo Waldmann.
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Researchers do not think K2-18b is the only super-Earth like this and expect to find others with similar characteristics among more than 4,000 exoplanets that have already been discovered.
They hope that newer technology, such as the James Webb Space Telescope due to launch in March 2021, will be able to unlock more secrets beyond our solar system.
"We are looking forward to the next generation of telescopes and go even further," Dr Tsiaras added.
Co-author Professor Giovanna Tinetti said: "Our discovery makes K2-18b one of the most interesting targets for future study."