In a first for astronomers studying worlds beyond our solar system, data from the Hubble Space Telescope have revealed water vapor in the atmosphere of an Earth-size planet. Although this exoplanet orbits a star that is smaller than our sun, it falls within what’s known as the star’s habitable zone, the range of orbital distances where it would be warm enough for liquid water to exist on a planet’s surface.

The discovery, announced this week in two independent studies, comes from years of observations of the exoplanet K2-18b, a super-Earth that’s about 111 light-years from our solar system. Discovered in 2015 by NASA’s Kepler spacecraft, K2-18b is very unlike our home world: It’s more than eight times the mass of Earth, which means it’s either an icy giant like Neptune or a rocky world with a thick, hydrogen-rich atmosphere.

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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, 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 vapor. But previously, worlds with confirmed water were uninhabitable for life as we know it. For instance, in 2018, NASA announced the discovery of water vapor in the atmosphere of WASP-39b, an enormous Saturn-size planet where the day side reaches a scorching 1,430 degrees Fahrenheit.

Researchers had been hoping to spot water vapor 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. (See some of the most alien landscapes on Earth.)

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 telltale frequencies. Water, in particular, absorbs near-infrared light at specific wavelengths, creating a visible sign of water vapor.

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 vapor. 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 vapor 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-vapor 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 vapor.