A team of astronomers from University College London have discovered water molecules in the atmosphere of a nearby exoplanet, a super-Earth twice the size of our planet and eight times its mass. This object is located in the habitable zone of its host star, making this the first detection of water in a world that could potentially host life.

The planet is known as K2-18b and is located 110 light-years away. It orbits its red dwarf star every 33 days and has a companion planet, a second super-Earth that is closer to the star and likely too hot to support life. As reported in Nature Astronomy, the team studied K2-18b's atmosphere and detected a clear water signature.

“Finding water in a potentially habitable world other than Earth is incredibly exciting,” said lead author Dr Angelos Tsiaras in a statement. “K2-18b is not ‘Earth 2.0’ as it is significantly heavier and has a different atmospheric composition. However, it brings us closer to answering the fundamental question: Is the Earth unique?”

The team tested several models for the planet’s atmosphere, with water content varying from 50 percent to as little as 0.01 percent. For reference, Earth’s values are between 0.01 and 4 percent. Some of these models had clouds and some didn't, but realistically it's probably not extremely cloudy as the atmosphere is transparent enough for water emissions to be visible.

There is also the question about the nature of this super-Earth. We don’t have the technology to image the planet directly so we can’t tell what the planet is like. Given that it has the density of Mars, it most likely has a solid surface. However, just because there’s water in its atmosphere, that doesn’t mean there is water on its surface. The team admits more data is needed to truly understand K2-18b.

“With so many new super-Earths expected to be found over the next couple of decades, it is likely that this is the first discovery of many potentially habitable planets. This is not only because super-Earths like K2-18b are the most common planets in our galaxy, but also because red dwarfs – stars smaller than our Sun – are the most common stars,” co-author Dr Ingo Waldmann added.

The planet was first discovered during the K2 mission using NASA’s Kepler telescope. Follow-up studies slowly revealed its nature, but there was plenty left to uncover. This team used data collected by the Hubble Space Telescope over eight transits of the planet across the disk of its star. The data is part of a larger spectroscopic survey on low-mass planets. The Principal Investigator of the data, Björn Benneke, Assistant Professor at Université de Montréal, and his team released a pre-print paper of their findings, submitted to The Astrophysical Journal, showing that water is indeed present in the atmosphere of K2-18 b and it could possibly rain down on the planet surface.

“Thanks to our observations and our climate model of this planet, we have shown that its water vapour can condense into liquid water. This is a first,” Benneke said in a statement.

Future telescopes will be able to go even further than this. Missions starting in the 2020s, like the James Webb Space Telescope, the European Ariel, and ground observatories like the Extremely-Large Telescope, will also help in furthering our study of exoplanets.