Two teams have detected signs that K2 18b has a damp atmosphere

Quote Originally Posted by Science News
Clouds of water droplets and even rain may exist in the soggy skies of a faraway exoplanet.

A combination of observations with space telescopes and simulations suggests that planet K2 18b has water vapor in its atmosphere, and might be the first planet orbiting a distant star found to support liquid water, thought to be an essential ingredient for life.

“Water vapor exists everywhere in the universe,” says astronomer Björn Benneke of the University of Montreal, who reported the potential discovery in a paper posted September 10 at “But it’s not so easy to make liquid water; you need the right pressure and the right temperature. That’s what makes this planet special.”

The exoplanet-hunting Kepler space telescope discovered K2 18b in 2015. The planet orbits a dim red dwarf star about 110 light-years away, and is bigger and heavier than Earth: about 2.5 times Earth’s radius and about eight times its mass.

“From the beginning, that makes it not an Earthlike planet,” astronomer Angelos Tsiaras of University College London, whose team independently detected water vapor in K2 18b’s atmosphere in a study published September 11 in Nature Astronomy, said in a Sept. 10 news teleconference. But tantalizingly, the planet’s distance from its star places it in the habitable zone, the region around a star where a planet could have temperatures conducive to liquid water (SN: 6/14/17).

In 2016 and 2017, a group led by Benneke used the Hubble Space Telescope to probe K2 18b for signs of an atmosphere as the planet passed in front of its star. Molecules in the planet’s atmosphere absorbed certain wavelengths of the star’s light, alerting astronomers to their presence.

Tsiaras and colleagues accessed that data from a public archive and used specially designed software to analyze it. The team found that the planet has an atmosphere, and that the atmosphere imprints the telltale signature of water vapor molecules on the filtered starlight. The atmosphere also contains hydrogen and helium, the team reports.

“Until now, the planets for which we had the atmosphere observed and found water were gas giants, planets more similar to Jupiter, Saturn or Neptune,” Tsiaras says. K2 18b’s location in the habitable zone, size and watery atmosphere mean that “this is the best candidate for habitability that we now have.”