Astronomers at IREx Dive Deeper into “Steam-World” Exoplanet GJ 9827 d

A recent paper in Astrophysical Journal Letters, led by Caroline Piaulet-Ghorayeb, a Ph.D. candidate at the University of Montréal’s Trottier Institute for Research on Exoplanets (IREx), in collaboration with researchers worldwide, has revealed new insights into the atmosphere of exoplanet GJ 9827 d using 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.

GJ 9827 d, a planet orbiting the star GJ 9827 in the constellation Pisces, is located about 98 light-years from Earth. Earlier this year, IREx researchers, using Hubble Space Telescope (HST) data, announced the detection of water in the atmosphere of GJ 9827 d, making it, at roughly two times the size of the Earth, the smallest exoplanet to date with a confirmed atmosphere. Together, these significant findings open new avenues for the search for life beyond our Solar System and enhance our understanding of planetary formation and composition.

Understanding Atmospheres Around Mini-Neptunes and Super-Earths

Caroline Piaulet-Ghorayeb, PhD candidate at the Trottier Institute for Research on Exoplanets and Université de Montréal, is the lead author of the scientific paper (Credit: C. Piaulet-Ghorayeb).

For years, scientists have focused on the detection of atmospheres on large gas giants and mini-Neptunes — planets much larger than Earth and with atmospheres dominated by hydrogen, like Jupiter and Neptune in our Solar System. However, detecting atmospheres around smaller planets, closer to Earth-sized, has remained an elusive goal. GJ 9827 d, approximately twice the size of Earth, represents a big step forward in this quest.

Lead author, Caroline Piaulet-Ghorayeb, expressed the significance of this investigation: “For now, all the planets for which we have atmosphere detections are giant planets, or at best mini-Neptunes. These planets have atmospheres made up mostly of hydrogen, making them more similar to gas giants in the Solar System than to terrestrial planets like Earth, which have atmospheres dominated by heavier elements.”

The Unique Atmosphere of GJ 9827 d

What sets GJ 9827 d apart is its atmosphere’s composition. Unlike the hydrogen-dominated atmospheres of larger planets, Piaulet-Ghorayeb’s work, combining JWST/NIRISS and HST data, reveals that GJ 9827 d’s atmosphere is rich in heavier molecules, with a significant presence of water vapour. This contrasts with the hydrogen-dominated atmospheres observed on larger exoplanets like gas giants and mini-Neptunes. This discovery marks the first robust detection of an exoplanet atmosphere where hydrogen is not the dominant component, suggesting instead a heavier, water-rich atmosphere. Piaulet-Ghorayeb elaborated on this discovery: “For GJ 9827 d, we have determined the presence of water in an atmosphere where the dominant component is not hydrogen. This means the planet has a heavy atmosphere closer in molecular weight to the carbon dioxide or nitrogen-rich atmospheres that we are currently looking for on smaller rocky planets, where we would eventually look for life.”

Cutting-Edge Observations with JWST’s NIRISS

The findings detailed in this recent article were made possible through observations with the Canadian instrument on JWST, the Near-Infrared Imager and Slitless Spectrograph (NIRISS). Using transmission spectroscopy, the team analysed the star light passing through the planet’s atmosphere as it transited (passed in front of) its host star, GJ 9827. The team 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 by contamination from the system’s star. With the data from JWST/NIRISS SOSS, scientists can finally distinguish between two types of atmospheres for the planet: one that is cloudy with few heavier elements, and where water is only present in trace amounts with hydrogen as the main component and another that has a high density with heavier elements such as water being abundant… Piaulet-Ghorayeb and her team found that the atmosphere of GJ 9827 d is more dense, likely rich in heavier elements, and contains a lot of water vapour. Because the planet is close to its star, the atmosphere is likely a mix of gas and a superheated, dense state, rather than having distinct layers or clouds.

Implications for the Search for Life

While GJ 9827 d itself is not thought to be habitable due to its proximity to its host star and resulting high surface temperatures (around 350 degrees Celsius), the discovery is a major leap forward in the search for habitable environments. The presence of a heavy, water-rich atmosphere on a small planet like GJ 9827 d provides a proof of concept that such atmospheres exist and can be studied with the JWST’s precision, making the prospect of finding habitable, Earth-like planets more plausible. Piaulet-Ghorayeb emphasised the broader impact of the discovery: “This is a huge step towards the goal of searching for atmospheres around smaller, terrestrial-like planets. GJ 9827 d is the first planet where we detect an atmosphere rich in heavy molecules like the terrestrial planets of the solar system, and the first confirmed example of a ‘steam world’ that has been proposed to exist by the scientific community for a long time.”

A Step Toward Understanding “Steam Worlds”

GJ 9827 d’s atmosphere, rich in water vapour and heavier elements, provides a unique example of a potential “steam world.” These are planets hypothesised to have thick, water-rich atmospheres without surface ice or liquid water, instead maintaining steam atmospheres due to their proximity to their host stars. This discovery further supports the existence of such planets, which can be thought of as bigger analogs of the icy moons Europa and Ganymede, only close enough to the star that water is in the steam form in the atmosphere, rather than under an ice layer.

Further planned JWST observations of GJ 9827 d in the coming months which could shed more light on the components of its steam atmosphere. The detection of GJ 9827 d’s unique atmosphere opens new possibilities for studying other small planets and their potential to host life.

With advanced telescopes like JWST, Canadian astronomers are poised to make even more groundbreaking discoveries in the coming years.

The article “JWST/NIRISS Reveals the Water-rich ‘Steam World’ Atmosphere of GJ 9827 d” was published on October 4, 2024 in Astrophysical Journal Letters. The first author is Caroline Piaulet-Ghorayeb, a PhD candidate at the Trottier Institute for Exoplanet Research at the Université de Montréal. Other iREx researchers who contributed to this article are Björn Benneke (UdeM), Michael Radica (UdeM), Louis-Philippe Coulomb (UdeM), Pierre-Alexis Roy (UdeM), Romain Allart (UdeM), Charles Cadieux (UdeM), and David Lafrenière (UdeM). Other contributors are from the University of Michigan, Max Planck Institute for Astronomy, Space Research Institute, University of Colorado, Leiden Observatory, University of Washington, and SRON Netherlands Institute for Space Research.

For more information

Scientific article on Astrophysical Journal Letters
UdeM Nouvelles press release

Scientific Contacts

Caroline Piaulet-Ghorayeb

Ph.D. Candidate
Trottier Institute for Research on Exoplanets
Université de Montréal

caroline.piaulet@umontreal.ca

Prof. Björn Benneke

Professor
Trottier Institute for Research on Exoplanets
Université de Montréal

bjorn.benneke@umontreal.ca

Media Contact

Dr. Heidi White

JWST Outreach Scientist
Trottier Institute for Research on Exoplanets
Université de Montréal