Hubble Detects Water Vapor in Atmosphere of Gliese 9827d

Using the Wide Field Camera 3 (WFC3) instrument onboard the NASA/ESA Hubble Space Telescope, astronomers have detected water vapor in the transmission spectrum of the sub-Neptune exoplanet Gliese 9827d.

Gliese 9827 is a bright K-type dwarf star some 97 light-years away from Earth in the constellation of Pisces.

Also known as GJ 9827, K2-135 or EPIC 246389858, the star hosts a trio of transiting massive exoplanet, discovered recently by NASA’s Kepler/K2 mission.

The outermost planet, Gliese 9827d (GJ 9827d), completes an orbit around its parent star every 6.2 days and has a radius of 1.96 Earth radii.

Hubble observed this 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.

“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 Dr. Björn Benneke, an astronomer with the Trottier Institute for Research on Exoplanets at the Université de Montréal.

“This is an important step toward determining the prevalence and diversity of atmospheres on rocky planets.”

“Water on a planet this small is a landmark discovery. It pushes closer than ever to characterizing truly Earth-like worlds,” said Dr. Laura Kreidberg, an astronomer at the Max Planck Institute for Astronomy.

However, it remains too early to tell whether Hubble spectroscopically measured a small amount of water vapor 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 vapor,” said Dr. Pierre-Alexis Roy, also from the Trottier Institute for Research on Exoplanets at the Université de Montréal.

“Either result would be exciting, whether water vapor is dominant or just a tiny species in a hydrogen-dominant atmosphere.”

“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,” Dr. Benneke said.

“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 Gliese 9827d is as hot as Venus, at 427 degrees Celsius (800 degrees Fahrenheit), it definitely would be an inhospitable, steamy world if the atmosphere were predominantly water vapor.

At present the team is left with two possibilities.

One scenario is that the planet is still clinging to a hydrogen-rich atmosphere laced with water, making it a mini-Neptune.

Alternatively, it could be a warmer version of Jupiter’s moon Europa, which has twice as much water as Earth beneath its crust.

“Gliese 9827d could be half water, half rock. And there would be a lot of water vapor on top of some smaller rocky body,” Dr. Benneke said.

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.

The alternative theory is that the planet formed close to the hot star, with a trace of water in its atmosphere.

“Observing water is a gateway to finding other things,” said Dr. Thomas Greene, an astrophysicist at NASA’s Ames Research Center.

“This Hubble discovery opens the door to future study of these types of planets by the NASA/ESA/CSA James Webb Space Telescope.”

“Webb 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.”

The results were published in the Astrophysical Journal Letters.

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Pierre-Alexis Roy et al. 2024. Water Absorption in the Transmission Spectrum of the Water World Candidate GJ 9827d. ApJL 954, L52; doi: 10.3847/2041-8213/acebf0