K2-18b’s orbit also takes it seven times closer to its star than Earth gets to the sun. But because it circles a type of dim red star known as an M dwarf, that orbit places it in the star's potentially life-friendly zone. Crude models predict that K2-18b’s effective temperature falls somewhere between -100 and 116 degrees Fahrenheit (-73 and 46 degrees centigrade), and if it is about as reflective as Earth, its equilibrium temperature would be roughly the same as our home planet’s.

The fact that researchers have detected water on this type of planet bolsters hope for finding habitable worlds beyond our solar system.

“This is the only planet right now that we know outside the solar system that has the correct temperature to support water, it has an atmosphere, and it has water in it—making this planet the best candidate for habitability that we know right now,” University College London astronomer Angelos Tsiaras, a coauthor of one of the two studies, said during a press conference.

Seeing the light

In the past two decades, astronomy has undergone a revolution. Since the first detection of exoplanets in 1992, scientists have cataloged thousands of alien worlds orbiting distant stars—some of which show signs of having atmospheres.

For a handful of these planets, astronomers have even spotted signs of atmospheric water vapour. But previously, worlds with confirmed water were uninhabitable for life as we know it. For instance, in 2018, NASA announced the discovery of water vapour in the atmosphere of WASP-39b, an enormous Saturn-size planet where the day side reaches a scorching 1,430 degrees Fahrenheit (776 degrees centigrade).

Researchers had been hoping to spot water vapour in the atmosphere of a more Earth-like planet, especially one in its star’s habitable zone, but these worlds are relatively small, making observations of their atmospheres extraordinarily difficult. The larger the planet, the easier the detection, so researchers focused their efforts on super-Earths: planets with masses up to 10 times that of our home planet, which means they might also sport rocky surfaces.

To put K2-18b to the test, University of Montréal astronomer Björn Benneke asked to use the Hubble Space Telescope to watch the super-Earth pass in front of its home star, which it does every 33 days.

As it makes each transit, the star’s light shines through K2-18b’s atmosphere. But not all that starlight makes it through, since chemical compounds in the atmosphere absorb light at tell-tale frequencies. Water, in particular, absorbs near-infrared light at specific wavelengths, creating a visible sign of water vapour.

Between 2015 and 2018, Benneke’s team used Hubble to monitor nine K2-18b transits. When they stacked up the transits’ data sets and cleaned them up, they found that telltale sign of water vapour. Benneke and his colleagues have posted a version of their study on the arXiv, a repository for scientific preprints.

Chance of rain

Independently, a team led by Tsiaras and Ingo Waldmann at University College London used the same Hubble data to do an analysis of their own, which appears today in Nature Astronomy. Like Benneke’s team, they also found evidence of water vapour in the atmosphere of K2-18b. In their paper, Tsiaras and Waldmann say that statistically, there’s only a one-in-3,000 chance that the results are a fluke.

In addition to bolstering the search for life-friendly exoplanets, the discovery may open the door to understanding alien weather. Benneke’s team points out that conditions in the atmosphere of K2-18b might allow for the formation of liquid water droplets—and maybe even rain. Water-vapour clouds have been found before in brown dwarfs, hulking objects that hover on the border between planet and star. If the results hold, K2-18b would be the first confirmed exoplanet with clouds of water vapour.

Both research teams say that the studies make K2-18b an ideal target for follow-up missions, including NASA’s upcoming James Webb Space Telescope and the European Space Agency’s future ARIEL space telescope. Unlike Hubble, these telescopes will be able to see other atmospheric gases, such as methane, ammonia, and carbon dioxide—and potentially even chemical markers for life.