Hubble Telescope detects water vapour in the atmosphere of smallest exoplanet GJ 9827d

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Astronomers using the NASA/ESA Hubble Space Telescope have made a groundbreaking discovery in their search for exoplanets. For the first time, they have detected water vapour in the atmosphere of a small exoplanet called GJ 9827d. This is a significant step towards determining the prevalence and diversity of atmospheres on rocky planets.

GJ 9827d is only about twice the diameter of Earth, making it the smallest exoplanet where water vapour has been detected in its atmosphere. This discovery could be an example of potential planets with water-rich atmospheres elsewhere in our galaxy.

“This would be the first time that we can directly show through an atmospheric detection that these planets with water-rich atmospheres can actually exist around other stars,” said team member Björn Benneke of the Université de Montréal.

However, it remains too early to tell whether the planet’s atmosphere is mostly made of water, left behind after a primaeval hydrogen/helium atmosphere evaporated under stellar radiation, or if Hubble spectroscopically measured a small amount of water vapour in a puffy hydrogen-rich atmosphere.

“Our observing programme was designed specifically with the goal of not only detecting the molecules in the planet’s atmosphere, but of actually looking specifically for water vapour. Either result would be exciting, whether water vapour is dominant or just a tiny species in a hydrogen-dominant atmosphere,” said the science paper’s lead author, Pierre-Alexis Roy of the Université de Montréal.

The team used the Hubble Space Telescope to observe the planet during 11 transits over three years. During transits, starlight is filtered through the planet’s atmosphere and carries the spectral fingerprint of water molecules.

If the planet’s atmosphere were predominantly water vapour, it would be an inhospitable, steamy world, as it is as hot as Venus at roughly 425 degrees Celsius. At present, the team is left with two possibilities. The planet could be a mini-Neptune, still clinging to a hydrogen-rich envelope laced with water, or it could be a warmer version of Jupiter’s moon Europa, which has twice as much water as Earth beneath its crust.

“The planet GJ 9827d could be half water, half rock. And there would be a lot of water vapour on top of some smaller rocky body,” said Benneke.

The discovery of water vapour in GJ 9827d’s atmosphere could mean that the planet formed farther away from its host star, where the temperature is cold and water is available in the form of ice, than its present location. In this scenario, the planet would have then migrated closer to the star and received more radiation. The hydrogen was then heated and escaped, or is still in the process of escaping, the planet’s weak gravity.

The Hubble programme has opened the door to studying GJ 9827d in more detail. It is a good target for the NASA/ESA/CSA James Webb Space Telescope to do infrared spectroscopy to look for other atmospheric molecules.

GJ 9827d was discovered by NASA’s Kepler Space Telescope in 2017. It completes an orbit around a red dwarf star every 6.2 days. The star, GJ 9827, lies 97 light-years from Earth in the constellation Pisces.

Journal Reference

  1. Roy, P., Benneke, B., Piaulet, C., A., M., Crossfield, I. J., Morley, C. V., Kreidberg, L., Brande, J., Delisle, S., Greene, T. P., K., K., Barman, T., Christiansen, J. L., Dragomir, D., Fortney, J. J., Howard, A. W., Kosiarek, M. R., & Lothringer, J. D. (2023). Water absorption in the transmission spectrum of the water-world candidate GJ9827d. The Astrophysical Journal Letters, Volume 954, Number 2. DOI: 10.3847/2041-8213/acebf0

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