Astronomers find it may be raining on planet warm enough for life

Astronomers using the Hubble Space Telescope have discovered a planet where it may be raining, just like on Earth. They detected clouds of water vapour in the alien world’s atmosphere, increasing chances that it could be home to life.

An artist’s impression of the exoplanet K2-18b. Image credit: Alex Boersma

The planet, dubbed K2-18b, lies 111 light-years away. It orbits in the habitable zone of its red dwarf star and so enjoys temperatures similar to those on our own world.

Björn Benneke, of the University of Montreal, who led the discovery team, said: “This represents the biggest step yet taken towards our ultimate goal of finding life on other planets, of proving that we are not alone.

“Thanks to our observations and our climate model of this planet, we have shown that its water vapour can condense into liquid water. This is a first.”

The planet where it may be raining, which lies in the constellation of Leo, was discovered by NASA’s Kepler Space Telescope in 2015. Early observations indicated that it was nine times the size of Earth. Since then, astronomers have been eager to learn more about it.

Professor Benneke’s team used Hubble data to monitor the planet as it orbits its star every 33 days. The planet passes in front of the star so that, for a brief moment, the star’s light shines
through the planet’s atmosphere.

Hubble made eight observations of these transits, each lasting 6.5 hours. Further observations were made with another NASA space telescope, Spitzer.

This allowed the astronomers to find signatures in the light that showed what the atmosphere is made of. It has previously been near impossible to probe the atmospheric conditions on alien planets, but Hubble’s powerful eye came up trumps.

The team’s results, published yesterday online, show that planet K2-18b’s similarities to Earth suggest that it may have a similar water cycle where water condenses into clouds allowing liquid water rain to fall.

The paper’s conclusions say: “The discovery of water absorption in the atmosphere of the habitable-zone exoplanet K2-18b represents a milestone in our search for habitable worlds outside the Solar System.

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“Given the relatively low irradiation by the star, K2-18b’s temperature is low enough that the detected water vapor can plausibly condense to form liquid droplets. It is therefore possible that liquid water rain precipitates in the mid-atmosphere of K2-18b.”

The star, K2-18, is of the type M3 and much cooler than our own Sun, but the planet lies closer to it, and so enjoys very similar warmth.

The planet where it may be raining will be a prime candidate for study by NASA’s next space telescope, the James Webb, which will be much more powerful than Hubble, and will be able to study its atmosphere in more detail.

Dr Josh Lothringer, of Johns Hopkins University, was part of Benneke’s discovery team. He told Skymania via Twitter: “Based on the planet’s known mass and radius, we know it has a low bulk density, which tells us it has an envelope of H and He. The only other gas we can detect is H2O, but JWST should tell us about CO2, CO, CH4, NH3, and maybe more!

“And when we say its atmosphere is too thick for life, we mean that it likely doesn’t have an observable surface and any surface pressure would be tens of thousands (or more) times the pressure at Earth’s sea level.”

One potential fly in the ointment is that the planet’s atmosphere is much denser than Earth’s, which probably rules out life as we know it from existing on the surface.

Other astronomers have cautioned that K2-18b is unlikely to be a rocky world, and is probably more like a mini-Neptune than a super-Earth.

Red dwarf stars are also prone to emitting violent flares which could bombard the planet with dangerous radiation.

A second study has also just been published using the same data, by scientists at University College London, confirming the discovery of water vapour in planet K2-18b’s atmosphere.

The paper is titled Water Vapor on the Habitable-Zone Exoplanet K2-18b.

Related: Hubble find water world that is new type of planet

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