This gas-giant exoplanet has water-rich clouds. Here’s why it thrills astronomers.3 min read

Three teams of astronomers have been fascinated by an alien world known as K2-18b. But what’s all the fuss about?

An artist’s interpretation of the water-rich atmosphere of K2-18b. (Image credit: Alex Boersma)

In September, two teams announced that they had found signs of liquid water in the planet’s atmosphere — a landmark discovery in the search for potentially habitable alien worlds. But the mere presence of water isn’t the only condition necessary for life. Other conditions, like temperature and pressure, can also affect a planet’s habitability. Now, a third team reports that the pressures of liquid water on the same world may be good for life to evolve — another intriguing development for scientists.

“We recognized pretty early on that this is a very unique target,” Björn Benneke, an astronomer at the University of Montreal, told Benneke led one of the teams that announced the atmospheric analysis in the September 2019 study, which was published in The Astrophysical Journal Letters in December. He presented the findings in January at a meeting of the American Astronomical Society.

In the Hubble and Spitzer observations, Benneke and his team found the signature of water in the exoplanet’s atmosphere — and not just any form of water. “Even more exciting, we discovered in the data that there’s a cloud deck on the planet,” Benneke said.

The clue was that starlight beaming through the atmosphere came to an abrupt stop at a certain altitude. Benneke and his colleagues used models to determine that the height was the perfect pressure and temperature for water to survive.

“The only plausible explanation is that these are liquid water clouds, very similar to what we have on Earth,” Benneke said.

As the clouds fill with water droplets, they most likely create rainfall. But that rain would never touch the ground. Instead, it would fall until temperatures and pressures caused it to evaporate once again, the researchers said. Benneke compared rainfall on K2-18b to terrestrial virga, which occurs when high temperatures and pressures cause rain to evaporate before it reaches the ground.

Finding a similar weather pattern on an exoplanet is an intriguing prospect to Benneke.

“[K2-18b] is the coldest planet with [atmospheric] detection, the smallest planet, the least-massive planet,” he said. “To me, it’s the most exciting one.”

A common mystery

Although K2-18b has the right conditions for Earth-like clouds to form, that doesn’t make the planet itself Earth-like. Instead, it is classified as a sub-Neptune, a gas giant without a surface. NASA’s Kepler spacecraft determined that sub-Neptunes are likely the most common type of exoplanets in the Milky Way, making up more than three-fourths of the planetary population.

Nonetheless, astronomers are having a difficult time understanding the relatively small gas giants, and that’s one reason the K2-18b findings are so exciting. “We don’t quite know what’s going on with these planets,” Benneke said. “It’s probing this regime of planets that we have a very poor understanding of right now.”

Sub-Neptunes have masses that fall somewhere between Earth and Neptune, and there’s no analogue in the solar system. Understanding worlds like K2-18b can help improve scientists’ knowledge of how planets grow and evolve. “If you want to understand planets as a whole, the diversity of planets, it’s very critical that you understand the most common ones,” Benneke said.

Featured Image: Alex Boersma

Sebastien Clarke
Sebastien Clarke

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