Scientists spot smallest exoplanet where water vapor detected in atmosphere

By Dean Murray via SWNS

Astronomers announced Thursday (January 25) they have observed the smallest exoplanet where water vapor has been detected in its 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.

However, because the planet is as hot as Venus at roughly 425 degrees Celcius, it would be an inhospitable, steamy world if the atmosphere were predominantly water vapor.

Björn Benneke of the Université de Montréal is part of the team using the NASA/ESA Hubble Space Telescope to examine the star, which lies 97 light-years from Earth in the constellation Pisces.

He says: "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.

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

The team says 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.

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

Björn Benneke says: "Until now, we had not been able to directly detect the atmosphere of such a small planet. And we’re slowly getting into this regime now.

"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)."

At present the team is left with two possibilities. The planet is still clinging to a hydrogen-rich envelope 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.

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

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 carries the spectral fingerprint of water molecules. If there are clouds on the planet, they are low enough in the atmosphere that they don’t completely hide Hubble’s view of the atmosphere, and Hubble is able to probe water vapor above the clouds.

ESA says Hubble’s discovery opens the door to studying the planet in more detail, adding: "It’s 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.