Water vapour was found in the atmosphere of K2-18b, one of the hundreds of "super-Earths", worlds ranging in size between Earth and Neptune, documented in a growing new field of astronomy devoted to the exploration of so-called exoplanets elsewhere in the Milky Way galaxy.
More than 4,000 exoplanets of all types and sizes have been detected overall.
The latest discovery was reported in research by a team of scientists at University College London (UCL) published in the peer-reviewed journal Nature Astronomy.
The findings were revealed from observations made with the Hubble Space Telescope, which analysed starlight filtered through K2-18b's atmosphere.
More precisely, it marks the first time scientists have found water in the atmosphere around a super-Earth, as opposed to a gas giant, orbiting a star within its "habitable zone," just the right distance for liquid water to potentially exist on the surface.
Alternative to Earth?
K2-18b orbits a dwarf star in the constellation Leo that lies 100 light-years from Earth.
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 it is impossible," Angelos Tsiaras, an astronomer at UCL, said.
"Given it's so far away we don't really have any other choice but stay on our own Earth, so it's important to make Earth great again rather than looking for an alternative to go to," Tsiaras said.
Dim prospects for life
Aside from the tremendous distance separating Earth from K2-18b, the exoplanet is likely exposed to far more radiation than Earth, diminishing the prospects for life evolving there.
However, the discovery brings astronomers closer to answering the fundamental question of how unique Earth is in the universe, the scientists said.