Hubble discovers water vapor in the atmosphere of a small exoplanet

Astronomers using the NASA/ESA Hubble Space Telescope have observed the smallest exoplanet to have water vapor detected in its atmosphere. At only about twice the diameter of Earth, planet GJ 9827d could be an example of potential planets with water-rich atmospheres elsewhere in our galaxy.

GJ 9827d was discovered by NASA’s Kepler space telescope in 2017. It orbits a red dwarf star every 6.2 days. The star GJ 9827 is 97 light years from Earth, in the constellation Pisces.

“This would be the first time that we could directly demonstrate, through atmospheric detection, that these planets with water-rich atmospheres can actually exist around other stars,” said Björn Benneke, a member of the University team. from Montreal. “This is an important step toward determining the prevalence and diversity of atmospheres on rocky planets.”

The study is published in Letters from the astrophysical journal.

However, it is too early to say whether Hubble spectroscopically measured a small amount of water vapor in a swollen, hydrogen-rich atmosphere, or whether the planet’s atmosphere is mostly water, left after evaporation of a primitive atmosphere of hydrogen and helium under stellar radiation. .

“Our observation program was designed specifically with the goal of not only detecting molecules in the planet’s atmosphere, but also looking specifically for water vapor. Either result would be exciting, whether water vapor is dominant or whether it is simply a tiny species in a hydrogen-dominant atmosphere,” said the paper’s lead author, Pierre-Alexis Roy of the University of Montreal.

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

Because the planet is as hot as Venus, at around 425°C, it would certainly be an inhospitable and humid world if the atmosphere were mostly water vapor.

Right now, the team is left with two possibilities. The planet still clings to an envelope rich in hydrogen and water, making it a mini-Neptune. Alternatively, it could be a hotter version of Jupiter’s moon Europa, which contains twice as much water as Earth under its crust. “Planet GJ 9827d could be half water, half rock. And there would be a lot of water vapor on top of a smaller rocky body,” Benneke said.

If the planet has a residual water-rich atmosphere, then it must have formed further from its host star, where the temperature is cold and water is available as ice, than its current location. In this scenario, the planet would then have migrated closer to the star and would have received more radiation. The hydrogen then heated and escaped, or is still escaping, from the planet’s low gravity. The alternative theory is that the planet formed close to the hot star, with a trace of water in its atmosphere.

The Hubble program observed the planet during 11 transits – events in which the planet crossed in front of its star – spaced three years apart. During transits, starlight is filtered through the planet’s atmosphere and bears the spectral imprint of water molecules. If there are clouds on the planet, they are low enough in the atmosphere that they do not completely obscure Hubble’s view of the atmosphere, and Hubble is able to probe water vapor above the clouds.

Hubble’s discovery opens the door to more detailed study of the planet. It is a good target for the NASA/ESA/CSA James Webb Space Telescope to perform infrared spectroscopy to search for other atmospheric molecules.