For the first time, two teams of astronomers have discovered water vapor around a small planet orbiting the habitable zone of a distant star. They even found traces of rain in the adjacent clouds. This confirmed the earlier assumptions of astronomers that water, considered an essential component of life, also occurs in the atmosphere of small exoplanets.

Astronomer Nikku Madhusudhan of the Astronomical Institute of Cambridge University says:

“It's very exciting. No one would expect such a discovery, even recently. ”

Water vapor has previously been found in the hot gaseous atmospheres of giant exoplanets, but its discovery around smaller exoplanets has been a challenge. Astronomers study the atmosphere by analyzing the light coming from the host star when the exoplanet is in front of it or passing it. If the planet has an atmosphere, certain wavelengths of light will be absorbed by atmospheric atoms or molecules, leaving characteristic lines in the stellar spectrum. This technique works best on large planets with a large atmosphere, which then passes through a larger amount of stellar light. Even so, only a few telescopes, such as the Hubble Space Telescope, are sensitive enough to detect faint lines. Astronomers used the Hubble Space Telescope to experiment and observe several smaller exoplanets the size of Neptune and Earth, which did not produce the desired result.

Planet K2-18b

Imagine the planet K2-18b. This nearby planet, orbiting a red dwarf about 110 light-years from Earth, was considered the main candidate for finding liquid water. Although its star is much colder than the Sun, its short orbit, lasting just 33 days, means it receives almost the same amount of heat as the Earth from the Sun. The presence of liquid water on the surface of the planet could be stable and its location is therefore in the habitable zone of its star. A team of astronomers from the United States and Canada have been allowed to study the K2-18b for several years using the Hubble telescope. Scientists have gathered data from eight orbits of the planet in front of its star.

"It needs to be confirmed, but our records also show traces of water vapor in the clouds," says team leader Björn Benneke of the University of Montreal, Canada. The team, which posted its results on arXiv yesterday and also forwarded it to The Astronomical Journal, also obtained data from NASA's Spitzer and Kepler space telescopes and used them all in the K2-18b climate model. The most likely interpretation of the model is that the planet has clouds of condensed liquid water.

"There is indeed rain on this planet as it does on Earth," Benneke says. "If you were flying in a hot air balloon and had some breathing equipment at your disposal, you probably wouldn't be hurt."

K2-18b - Scale Neptune

But this does not mean that K2-18b has a surface with land and oceans just like the Earth. K2-18b is roughly twice the diameter and eight times the volume of our planet. According to Benneke, it is more of a diminished Neptune with a thick opaque cover that probably hides a rocky or icy core. "It's not the other Earth," said Angelos Tsiaras, team leader at University College London (UCL), who today published his own analysis of publicly available Hubble telescope data in Nature Astronomy. Both teams agree on the presence of water vapor and possible clouds. "Clouds are supposed to be there," says Giovanna Tinetti of the UCL team.

Benneke states that on K2-18b there can be a water cycle with rainfall falling from the atmosphere even without the “earth surface”, vaporizing in a dense and hot lower gaseous layer to lift and re-condense into the clouds.

The result encourages astronomers to further explore. Madhusudhan says another handful of small water-containing exoplanets could be within reach of the Hubble telescope. At greater distances, scientists will have to wait for the successor to the Hubble Space Telescope, the James Webb Space Telescope (JWST) to be launched in 2021. "JWST will be magnificent," says Madhusudhan, and with his help the "clouds" of such planets will be discovered.