Water discovered for first time on planet that may be habitable

James Marshall
September 12, 2019

NASA's Kepler space telescope first spotted K2-18b in 2015.

Water vapour has been discovered on a "super-Earth" 110 light years away that is estimated to be twice the size of Earth and eight times its mass.

Astronomers made the discovery of atmospheric water vapour in K2-18 b's atmosphere using 2016 and 2017 data from the Hubble Space Telescope.

Even without an Earth-like surface, Benneke says K2-18b could have a water cycle, with rain falling through the atmosphere, evaporating in a dense and warm gaseous layer lower down, only to rise up again and recondense into clouds. But while the temperature range and presence of water are good signs, K2-18 b isn't necessarily habitable.

Much remains unknown about the exoplanet K2-18 b, the researchers stressed.

While water already has been identified in the atmospheres of hot gas giants circling other stars, the latest findings represent the first detection of water vapor in the atmosphere of another type of exoplanet, Tsiaras said.

The University College London scientists say it's the only exoplanet known so far to have both water and temperatures needed for life, making it a prime candidate for potential life.

Not only could this discovery help us to understand the atmospheres of habitable zone exoplanets in general, but those of habitable zone rocky exoplanets in close orbit around red dwarf stars.

[1] The observations were achieved from 9 transits of K2-18b with the NASA/ESA Hubble Space Telescope's Wide Field Camera 3 (WFC3), as part of the HST proposals 13665 and 14682 (PI: Björn Benneke).

This transit can also help us study a planet's atmosphere.

It could either be a rocky planet with an extended atmosphere or an icy planet with a high concentration of water in its interior.

"They're the ones that could be able to find water in the atmospheres of truly Earth-sized planets", Dr Horner said.

"The water signature is so strong that [with] even a tiny bit of water, you would immediately see the signature", Tinetti said. As the water condenses into liquid, the drops would fall toward the core, then revert to gas as the atmospheric pressure increases. The most likely interpretation of the model is that the planet has clouds of condensed liquid water.

For now, scientists know K2-18b takes 33 days to orbit its star, so one year there is one month here.

At the moment, the study of exoplanets atmospheres is limited, as this kind of measurement requires very high precision, which current instruments were not built to deliver. It is the only exoplanet to fulfil three requirements for habitability: the right temperatures, an atmosphere and the presence of water. Whether clouds form in the atmosphere where liquid water can exist is based on planetary models.

Whether or not life could exist on K2-18b is unknown, but the discovery of water vapor makes it a promising target for future observations-especially for the James Webb Telescope (JWST), which is now scheduled for launch in 2021 and will be able to detect "biosignatures"-and is possibly an indication that life is present". "It's very hard to observe an atmosphere with water through an atmosphere with water", Waldmann, who's also based at UCL, said about trying to observe K2-18 b through Earth's atmosphere.

'The search for habitable planets, it's very exciting, but it's here to always remind us that this (Earth) is our only home and it's probably out of the question if we will be able to travel to other planets'.

"It's a step in the right direction", Sara Seager, a professor of planetary science, physics and aerospace engineering at the Massachusetts Institute of Technology who was not involved with this study, told Space.com. "Is it a rocky planet with this giant envelope?" But it ticks one more box in the hunt for worlds where the conditions are ripe for life. "We hope to get better data on this planet ... with the upcoming James Webb Space Telescope, it could look at a much broader range of the spectrum and do much more detailed observations", she added.

These findings were published today (Sept. 11) in the journal Nature Astronomy.

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