Clouds of water droplets and even rain can be found in the humid sky on a distant exoplanet.
A combination of observations with space telescopes and simulations suggests that the planet K2 18b has water vapor in its atmosphere, and may be the first planet orbiting a distant star that has been found to support liquid water, which is considered an important ingredient for life.  "Water vapor is everywhere in the universe," says astronomer Björn Benneke of the University of Montreal, who reported on the potential discovery in a paper published September 10 on arXiv.org. “But it is not so easy to make liquid water; you need the right pressure and temperature. That is what makes this planet special. ”
The exoplanet-hunting Kepler space telescope discovered K2 1
"From the beginning, it does not make it an earth-like planet, "astronomer Angelos Tsiaras of University College London, whose team independently detected water vapor in the atmosphere of K2 18b in a study published September 11 Nature Astronomy said in a September 10 teleconference, but luring the planet's distance from its star it in the habitable zone, the region around a star where a planet can have temperatures that contribute to liquid water ( SN: 6/14/17 ).
In 2016 and 2017, a group used was led by the Benneke Hubble Space Telescope to examine 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. stars in the starlight and warned astronomers of their presence.
Tsiaras and colleagues accessed this data from a public archive and used specially designed software to analyze it. The team discovered that the planet has an atmosphere and that the atmosphere prints the legitimate 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 were observed and found water, gas giants, planets more like Jupiter, Saturn or Neptune," says Tsiaras. K2 18b's placement in the inhabited zone, its size and the watery atmosphere means that "this is the best candidate for habitation we currently have."
Benneke and colleagues took the work a step further and observed K2 18b with the Spitzer space telescope. The combination of the observations Hubble, Spitzer and Kepler suggest that clouds form at some level in the planet's atmosphere and absorb more starlight than at other levels, the team found.
When Benneke and colleagues simulated the planet's climate, they found that the region where the clouds condense may have the right pressure and temperature for the formation of liquid water. This means that liquid water droplets can condense from the clouds and rain down, says Benneke.
"It is quite likely that this planet has liquid rain on it," he says. "This is actually one of the most exciting results from this data."
Benneke believes that K2 18b's raindrops would never hit solid ground. Instead, they would reach a point in the planet's thick atmosphere where the pressure and temperature were so great that the droplets would evaporate. Then the water would rise into the atmosphere again, condense to clouds and rain down again. "There's a bit of a pedal cycle," he suggests.
Other exoplanet experts remain skeptical. "There is no definitive evidence" of raindrops, says astronomer Sara Seager of MIT. "It's a firm, yet speculative statement."
But liquid water, if present on K2 18b, does not mean that something lives – or can live – on the planet. Its size places the exoplanet somewhere between the Earth's orbit and Neptune, which means it's not clear if it has a rocky surface where life as we know it can develop. Most exoplanets in the Milky Way fall within this size range, but it is difficult to see if they are rocky super-Earths, gaseous mini-Neptunes or wet-water worlds ( SN: 6/19/17 ).
"It's one of those really mysterious planets that is the most common type of planet in our galaxy, as far as we can tell," says Seager. "We have no idea what they are." Future observations with NASA's planned James Webb space telescope may be able to determine how much water K2 18b contains, which would help to find out its composition, she says ( SN: 4/19/16 ).