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A distant planet has water vapor

Water seen for first time on an exoplanet of this size

by Sam Lemonick
September 12, 2019 | A version of this story appeared in Volume 97, Issue 36


Photograph of Hubble Space Telescope orbiting Earth.
Credit: NASA
Hubble Space Telescope data suggest an exoplanet a bit bigger than Earth has atmospheric water vapor.

For the first time, two groups of scientists have identified water vapor in the atmosphere of a distant planet not much bigger than Earth.

Scientists think the planet, known as K2-18b, is about nine times as massive as Earth and twice its diameter. It’s the right distance from its star to be habitable but probably looks more like gaseous Neptune than rocky Earth. Water vapor has been detected on exoplanets before, but only on larger, hotter gas giants.

Using the Hubble Space Telescope, a team led by Björn Benneke of the University of Montreal captured images of the planet as it passed in front of the red dwarf star it orbits 110 light-years from Earth. Independently, Benneke’s group and Angelos Tsiaras, Ingo P. Waldmann, and colleagues at University College London used computer programs to analyze light from the star as it passed through the planet’s atmosphere and deduce the atmospheric composition spectroscopically (arXiv 2019, arXiv: 1909.04642 and Nat. Astron. 2019, DOI: 10.1038/s41550-019-0878-9). Both groups agree that K2-18b’s atmosphere likely contains gaseous hydrogen, helium, and water. Benneke’s group further suggests that the planet’s temperature and pressure ranges mean liquid water may condense as clouds and precipitation.

Finding out more will have to wait for future telescopes such as the orbiting James Webb Space Telescope or the ground-based Extremely Large Telescope in Chile, which will be able to see signatures of other molecules, including CH4 and CO2, and tell scientists more about the chemical composition of K2-18b.



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