Astronomers have seen signs of water rain in the atmosphere of a planet beyond the Solar System.
The discovery is a glimpse of water molecules around a distant world that is not much larger than Earth. Called K2-18 b, the planet is 34 parsecs (110 light years) from Earth in the constellation Leo. In particular, it is located in the "habitable zone" around its star, the distance at which liquid water could exist, making extraterrestrial life possible in its hydrogen-rich atmosphere.
"That's the exciting thing about this planet," says Björn Benneke, a planetary astronomer at the University of Montreal in Canada. He is the lead author of an article describing the discovery that was published on the arXiv prepress server on September 10.one.
A team of competing scientists reports their own badysis of the same planet on September 11 in Astronomy of naturetwo. The lead author of that article, planetary astronomer Angelos Tsairas, of University College London (UCL), says the finding is exciting because the planet is twice the diameter of the Earth and because little is known about the atmospheres of such small worlds .
Astronomers have previously found water in the atmospheres of giant gas exoplanets, but studying the atmosphere of a distant planet becomes more difficult as the planet gets smaller. Scientists have been pushing the limits to try to examine planets that are smaller than Neptune but larger than Earth, a category that turns out to be surprisingly common among the thousands of exoplanets found so far.
Benneke and his colleagues decided to watch K2-18b because it falls in that range. They used the Hubble Space Telescope to observe how the planet pbaded in front of its star, temporarily dimming its light, on eight different occasions.
The scientists badyzed how the color of the star's light changed as it filtered through the planet's atmosphere. They combined this with data from the Spitzer space telescope, which examines more wavelengths of light. The researchers concluded that they were seeing water vapor in the planet's atmosphere, as well as signs that that steam was condensing in liquid water.
It is the first time that astronomers see a water cycle of this type, which changes from gas to liquid and vice versa, in a small and distant world.
The UCL team that wrote the second article badyzed the Hubble data of the Benneke group. Observations were uploaded to a public access file immediately after being collected.
The UCL researchers proposed three possible explanations of what they were seeing, any of which is equally likely. In the first scenario, the planet has no clouds and 20–50% of its atmosphere is water. In the second and third scenario, involving different amounts of clouds and other molecules in the atmosphere, the planet's atmosphere contains between 0.01% and 12.5% water.
But the presence of water alone does not mean that a planet is a good place to seek life, a point illustrated by one of the closest neighbors on Earth, Venus. It is a planet the size of the Earth in the habitable zone of its star that once had water vapor in its atmosphere, but the sun's rays have eliminated much of that water, leaving its sterile surface.
K2-18 b could be equally promising. "It is very unlikely that this world is habitable in any way that we understand as a function of life as we know it," says Hannah Wakeford, a planetary astronomer at the Space Telescope Science Institute in Baltimore, Maryland.
Still, finding water in the planet's atmosphere is "extremely exciting," says Neale Gibson, an astrophysicist at Trinity College in Dublin, "and the fact that two teams find the same result is very encouraging." Future observations, such as the ones that the James Webb Space Telescope will pick up after its planned launch for 2021, should help pinpoint exactly what this distant world is like.