Discovery of Water Vapor in Small Exoplanet’s Atmosphere

With a diameter approximately twice that of Earth, GJ 9827 d could be an example of a planet with a water-rich atmosphere, according to researchers at UdeM.

This is an artist’s concept of the exoplanet GJ 9827d, the smallest exoplanet where water vapour has been detected in the atmosphere. The planet could be an example of potential planets with water-rich atmospheres elsewhere in our galaxy. With only about twice Earth’s diameter, the planet orbits the red dwarf star GJ 9827. Two inner planets in the system are on the left. The background stars are plotted as they would be seen to the unaided eye looking back toward our Sun. The Sun is too faint to be seen.
Credit: NASA, ESA, Leah Hustak (STScI), Ralf Crawford (STScI)

Astronomers using NASA’s Hubble Space Telescope observed the smallest exoplanet where water vapor has been detected in the atmosphere. At only approximately twice Earth’s diameter, the planet GJ 9827d could be an example of potential planets with water-rich atmospheres elsewhere in our galaxy.

The study, carried out by scientists from Centre for Research in Astrophysics of Quebec (CRAQ) based at the Université de Montréal, was published on September 12 in The Astrophysical Journal Letters.

“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 Trottier Institute for Research on Exoplanets at Université de Montréal. “This is an important step toward determining the prevalence and diversity of atmospheres on rocky planets.”

Two plausible scenarios

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

“Our observing program was designed specifically with the goal to not only detect the molecules in the planet’s atmosphere, but to actually look 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 CRAQ and the Trottier Institute for Research on Exoplanets at Université de Montréal.

In the first scenario, the planet would still be clinging to a hydrogen-rich atmosphere laced with water, making it a mini-Neptune.

In the second, it would rather resemble a larger and hotter version of Europa, Jupiter’s moon, which has twice as much water under its crust as is found on Earth. In this case, as Björn Benneke pointed out, “the planet GJ 9827d could be half water, half rock: there would be a lot of water vapour on top of some smaller rocky body”.

Smaller and smaller planets

“Until now, we had not been able to directly detect the atmosphere of such a small planet. And we’re slowly getting in this regime now,” added Benneke. “At some point, as we study smaller planets, there must be a transition where there’s no more hydrogen on these small worlds, and they have atmospheres more like Venus (which is dominated by carbon dioxide).”

Because the planet is as hot as Venus, around 430 degrees Celsius, it definitely would be an inhospitable, steamy world if the atmosphere were predominantly water vapour.

Two possible formation stories

If the planet has a residual water-rich atmosphere, then it must have 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 heated and escaped or is still in the process of escaping the planet’s weak gravity.

In the other case, the planet would have form close to the hot star and would have only trace of water in its atmosphere.

Space telescopes on the case

The Hubble program observed the planet during 11 transits—events in which the planet crossed in front of its star—that were spaced out over three years. During transits, starlight is filtered through the planet’s atmosphere and has the spectral fingerprint of water molecules. If there are clouds on the planet, they are low enough in the atmosphere so that they don’t completely hide Hubble’s view of the atmosphere, and Hubble is able to probe water vapour above the clouds.

This Hubble discovery opens the door to future study of these types of planets by the James Webb Space Telescope. JWST can see much more with additional infrared observations, including carbon-bearing molecules like carbon monoxide, carbon dioxide, and methane. Once we get a total inventory of a planet’s elements, we can compare those to the star it orbits and understand how it was formed.

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.

About this study

The paper «Water Absorption in the Transmission Spectrum of the Water World Candidate GJ 9827 d», was published in The Astrophysical Journal Letters on September 12, 2023. The lead author is Pierre-Alexis Roy, a Ph.D. student at CRAQ and the Trottier Institute for Research on Exoplanets (iREx), at Université de Montréal.

About the Centre for Research in Astrophysics of Quebec

The Centre for Research in Astrophysics of Quebec (CRAQ) brings together all the astrophysicists in Quebec. Nearly 150 people, including some fifty researchers and their students from Université de Montréal, McGill University, Université Laval, Bishop’s University, Cégep de Sherbrooke, Collège de Bois-de-Boulogne and a number of other collaborating institutions are part of the cluster. The CRAQ is under the direction of David Lafrenière of the Université de Montréal. The CRAQ is one of the strategic clusters funded by the Fonds de recherche du Québec – Nature and Technologies (FRQNT).

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