Astronomers probe ‘vapor world’

A Canadian-led international study has revealed new information about the atmosphere of the exoplanet GJ 9827 d – orbiting the star GJ 9827 in the constellation Pisces, about 98 light-years from Earth – using the space telescope James-Webb (JWST).

Astronomers have shown that the planet’s atmosphere contains a high concentration of heavier molecules, including a substantial buildup of water vapor, leading them to believe it could very well be a “world of steam”.

Published in The Astrophysical Journal Lettersthe study was led by Caroline Piaulet-Ghorayeb, doctoral candidate at the Trottier Institute for Research on Exoplanets (iREx) at the University of Montreal, in collaboration with researchers from several countries.

Earlier this year, thanks to data from the space telescope Hubble (HST), members of the iREx team announced that they had detected water in the atmosphere of the exoplanet GJ 9827 d, which in fact, due to its size approximately twice that of of Earth, the smallest exoplanet with a confirmed atmosphere.

These important discoveries open new avenues for the search for life beyond the solar system and improve our understanding of planet formation and composition.

An unattainable goal until now

For years, scientists have focused their efforts on detecting atmospheres around gas giant planets and mini-Neptune (planets much larger than Earth with atmospheres dominated by hydrogen, such as Jupiter and Neptune, in the solar system).

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

“So far, all the planets we have detected that have an atmosphere are giant planets or, at best, mini-Neptunes,” says Caroline Piaulet-Ghorayeb, lead author of the study. These planets have atmospheres composed mainly of hydrogen, which makes them more similar to the gas giants of the solar system than to rocky planets like Earth, whose atmospheres are dominated by heavier elements.

Rich in heavier molecules

The particularity of the exoplanet GJ 9827 d lies in the composition of its atmosphere.

By combining data from telescopes James-Webb et Hubble, Caroline Piaulet-Ghorayeb shows that, unlike the atmospheres of large planets dominated by hydrogen, that of the exoplanet GJ 9827 has a high concentration of heavier molecules and a significant quantity of water vapor.

This discovery marks the first robust detection of an exoplanet atmosphere where hydrogen is not the main component, instead indicating a heavier, water-laden atmosphere.

“The molecular weight of this atmosphere is closer to that of atmospheres rich in carbon dioxide or nitrogen that we are currently looking for on small rocky planets, where we could possibly look for life,” adds Caroline Piaulet-Ghorayeb.

These observations were made using the Canadian JWST instrument, the Near-Infrared Imager and Slitless Spectrograph (NIRISS). Using transmission spectroscopy, the research team analyzed light from the star passing through the planet’s atmosphere as it passed in front of its host star, GJ 9827.

The team then combined the new JWST findings with previous HST observations to show confidently that the spectral features studied are due to the planet’s atmosphere and not contamination by the system’s star.

With data taken from NIRISS in SOSS mode, astronomers can finally distinguish two types of atmospheres for the planet: one is cloudy and contains few heavy elements, mainly hydrogen and some traces of water; the other is very dense and contains several heavy elements, including a lot of water.

As the exoplanet GJ 9827 d is close to its star, its atmosphere is likely a mixture of gases and a superheated, dense state, rather than distinct layers or clouds.

Probably not habitable

Although the exoplanet GJ 9827 d is not thought to be habitable due to its proximity to its host star and the resulting high surface temperatures (around 350 °C), this discovery represents an advance for the search for habitable environments.

The presence of a heavy, water-filled atmosphere on a small planet like GJ 9827 d proves that such atmospheres exist and can be studied thanks to the finesse of JWST. This makes the prospect of finding habitable planets, comparable to Earth, more plausible.

“This is a big step towards the search for atmospheres around small planets similar to Earth,” explains Caroline Piaulet-Ghorayeb. Exoplanet GJ 9827 d is the first planet where we detect an atmosphere rich in heavy molecules, much like the terrestrial planets of the Solar System, and the first long-confirmed example of a “vapor world” put forward by the scientific community. »

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

Astronomers hope that further observations of the exoplanet GJ 9827 d through JWST, planned in the coming months, will reveal more about the components of its vapor atmosphere.