Astronomers use Webb to probe a ‘vapor world’ in the constellation Pisces

A Canadian-led international study has revealed new insights into the atmosphere of GJ 9827 d, an exoplanet orbiting the star GJ 9827 in the constellation Pisces, about 98 light-years from Earth, thanks to the James Webb Space Telescope (JWST).

The study found that the planet’s atmosphere contains a high concentration of heavier molecules, including a significant amount of water vapor, leading astronomers to think it could very well be a “world of steam”.

Published in Letters from the astrophysical journalThe study was led by Caroline Piaulet-Ghorayeb, a Ph.D. candidate at the Trottier Institute for Research on Exoplanets (IREx) at the University of Montreal, in collaboration with researchers from around the world.

Earlier this year, using data from the Hubble Space Telescope (HST), IREx researchers announced that they had detected water in the atmosphere of GJ 9827 d, making it, in fact, about twice the size of Earth, the smallest exoplanet with a confirmed atmosphere.

Together, these important discoveries open new avenues for the search for life beyond our solar system and improve our understanding of planetary formation and composition.

An elusive goal – until now

For years, scientists have focused on detecting the atmospheres of large gas giants and mini-Neptunes, planets much larger than Earth and whose atmospheres are dominated by hydrogen, such as Jupiter and Neptune in our solar system.

But until now, detecting atmospheres around smaller planets, closer to the size of Earth, remained an elusive goal.

So far, all the planets we have detected that have an atmosphere are giant planets, or at best mini-Neptunes,” said Piaulet-Ghorayeb, the lead author of the study.

“These planets have atmospheres composed primarily of hydrogen, making them more similar to the gas giants of the Solar System than to terrestrial planets like Earth, whose atmospheres are dominated by heavier elements.”

Rich in heavier molecules

What sets GJ 9827 d apart is the composition of its atmosphere.

By combining JWST/NIRISS and HST data, Piaulet-Ghorayeb showed that, unlike the hydrogen-dominated atmospheres of larger planets, that of GJ 9827 d is rich in heavier molecules, with a good amount of hydrogen vapor. water.

This discovery marks the first robust detection of an exoplanet atmosphere where hydrogen is not the dominant component, instead suggesting a heavier, water-rich atmosphere.

“Its molecular weight is closer to the carbon dioxide or nitrogen-rich atmospheres we currently look for on smaller rocky planets, where we would eventually look for life,” Piaulet-Ghorayeb said.

She and her team carried out their observations via JWST’s Canadian instrument, the Near-Infrared Imager and Slitless Spectrograph (NIRISS). Using transmission spectroscopy, they analyzed starlight passing through the planet’s atmosphere as it transited (passed in front of) its host star, GJ 9827.

They then combined the new JWST observations with previous HST observations to confidently show that the observed spectral features are caused by the planet’s atmosphere and not contamination from the system’s star.

Thanks to data from JWST/NIRISS SOSS, scientists can finally distinguish two types of atmospheres for the planet: one is cloudy with few heavier elements, mainly hydrogen with water present only in quantities tiny; the other has a high density with many heavier elements and lots of water.

Since GJ 9827 d is close to its star, its atmosphere is likely a mixture of gas and a dense, superheated state, rather than having distinct layers or clouds.

Not considered habitable

Although GJ 9827 d itself is not considered habitable due to its proximity to its host star and the resulting high surface temperatures (around 350°C), this discovery represents a major step forward in research. habitable environments.

The presence of a heavy, water-rich atmosphere on a small planet like GJ 9827 d provides proof of concept that such atmospheres exist and can be studied with the precision of JWST.

And this makes the prospect of finding habitable planets, similar to Earth, more plausible in the long term.

“This is a big step toward the goal of finding atmospheres around smaller, Earth-like planets,” Piaulet-Ghorayeb said.

“GJ 9827 d is the first planet to detect an atmosphere rich in heavy molecules, just like the terrestrial planets of the solar system, and the first long-confirmed example of a ‘steam world’ put forward by the scientific community. “

Such “vapor worlds” are thought to have thick, water-rich atmospheres, without surface ice or liquid water, instead retaining vapor atmospheres due to their proximity to their host stars. They are similar to the icy moons Europa and Ganymede, but close enough to their stars that water appears as vapor in the atmosphere, not under a layer of ice.

Further planned JWST observations of GJ 9827 d in the coming months could shed more light on the components of its vapor atmosphere, astronomers hope.