The exoplanet, which orbits a relatively benign red dwarf star, is “the best candidate for habitability that we know right now.”

As depicted in this artist’s concept, exoplanet K2-18 b orbits a red dwarf star and has an extended atmosphere containing at least some water vapour. Another exoplanet in the system is closer to the star, but it is within the star’s habitable zone.

Astronomers have discovered water vapour in the atmosphere of a super-Earth exoplanet orbiting within its star’s habitable zone. The discovery implies that liquid water could exist on the rocky world’s surface, possibly forming a global ocean.

The discovery, made with NASA’s Hubble Space Telescope, is the first detection of water vapour in such a planet’s atmosphere. Because the planet, dubbed K2-18 b, is likely to have a temperature similar to Earth, the newly discovered water vapour makes it one of the most promising candidates for follow-up studies with next-generation space telescopes.

“This is the only planet right now that we know outside the solar system that has the correct temperature to support water, an atmosphere, and water, making this planet the best candidate for habitability that we know right now,” said lead author Angelos Tsiaras, an astronomer at University College London, at a press conference.

K2-18 b: The fundamentals

K2-18 b is located 110 light-years away in the constellation Leo, and it orbits a small red dwarf star about one-third the mass of our Sun. Red dwarfs are notorious for being active stars that emit powerful flares, but this particular star appears to be surprisingly docile, according to the researchers.

This is good news for the water-bearing planet, whose 33-day orbit brings it roughly twice as close to its star as Mercury is to the Sun. “Because the star is much cooler than the Sun, the planet receives similar radiation to Earth,” Tsiaras explained. “And, according to calculations, the temperature of the planet is also similar to that of the Earth.”

According to the paper, K2-18 b has a temperature range of -100 °F (-73 °C) to 116 °F (47 °C). For reference, temperatures on Earth can range from -120 °F (-84 °C) in Antarctica to above 120 °F (49 °C) in Africa, Australia, and the Southwestern United States.

Although K2-18 b has some of the most Earth-like features ever seen in an exoplanet — water, habitable temperatures, and a rocky surface — the researchers emphasise that the world is still far from Earth. To begin with, K2-18 b is roughly twice the diameter of Earth, making it roughly eight times as massive. This places K2-18 b near the upper limit of what we call a super-Earth, which typically refers to planets with masses ranging from one to ten Earth masses.

However, the density of K2-18 b is what truly distinguishes it as a rocky planet. K2-18 b has a composition most similar to Mars or the Moon, with a density roughly twice that of Neptune. Because the planet is thought to have a solid surface and an extended atmosphere with at least some water vapour, researchers believe K2-18 b could be a water world with a global ocean covering its entire surface.

They cannot, however, be certain.

The uncertainty stems from Hubble’s inability to probe the atmospheres of distant exoplanets in great detail. For example, using a sophisticated algorithm, the researchers were able to extract the undeniable signal of water vapour in K2-18 b’s atmosphere, but they couldn’t tell how much water vapour was actually present. As a result, in their paper, they took a conservative approach and provided a broad-range estimate of water abundance — somewhere between 0.01 percent and 50 percent.

The researchers say we’ll have to wait for the next generation of advanced space telescopes to come online before we can determine exactly how much water is on K2-18 b. NASA’s James Webb Space Telescope, set to launch in 2021, and the European Space Agency’s Atmospheric Remote-sensing Infrared Exoplanet Large Survey (ARIEL), set to launch in the late 2020s, are particularly well-suited to the task.

The new study was published in Nature Astronomy on September 11th. The study’s preprint is available at arXiv.org.