Webb Space Telescope Discovers Planet with Unusual Atmosphere

The James Webb Space Telescope has revealed a giant planet around a red dwarf with an atmosphere poor in heavy elements, a finding that challenges theories about planet formation.

Astronomers using the James Webb Space Telescope have taken a close look at TOI-5205 b, a strange exoplanet often called "forbidden," and found something unexpected. Its atmosphere appears to be unusually poor in heavy elements compared to its star. This contradicts long-held ideas about how giant exoplanets form and evolve.

TOI-5205 b is similar in size to Jupiter but orbits a much smaller star, about four times the size of Jupiter and about 40% the mass of the Sun. When a planet passes in front of its star in an event called a “transit,” it blocks about 6% of the star’s light. Scientists analyze these transits using spectrographs, which separate light into different colors, to identify the chemical composition of the planet’s atmosphere and better understand its origin.

A challenge to theories of planet formation

Planets form inside rotating disks of gas and dust that orbit young stars. Astronomers generally agree that giant planets form in these disks, but systems like TOI-5205 b are difficult to explain. A large planet orbiting a small, cool star so closely doesn't fit well with existing models.

To study these unusual systems, the team is leading the largest exoplanet program in Webb's second cycle, "Red Dwarfs and the Seven Giants." The project focuses on rare worlds like TOI-5205 b, which are typically grouped as "giant exoplanets around M-type dwarf stars."

Shobham Kanodia first confirmed TOI-5205 b in 2023 using follow-up observations of data from the TESS space telescope, which identified it as a candidate. He is now among the leaders of the team that has carried out the first detailed study of its atmosphere.

Data from three observed transits reveal an unexpected pattern. The planet's atmosphere contains fewer heavy elements relative to hydrogen than Jupiter and even less than its star. This is unusual because giant planets are typically richer in heavy elements compared to their stars.

The observations also revealed methane (CH₄) and hydrogen sulfide (H₂S) in the atmosphere, adding further detail to the chemical profile.

Incompatibility between the atmosphere and the interior

To interpret these results, researchers Simon Müller and Ruit Hald at the University of Zurich used planetary structure models. Their analysis suggests that the planet as a whole is about 100 times richer in metals than its atmosphere, based on measurements of the transits.

"The metallicity, from the observations, is much lower than what our models predicted for the overall chemical composition, which is calculated from measurements of a planet's mass and radius. This suggests that its heavy elements moved inward during formation and now its atmosphere and interior are not mixing," explained Canodia. "In essence, these results indicate that the planet's atmosphere is very rich in carbon and poor in oxygen."

The team also took into account the effects of starspots on the host star, which can distort observational data. These dark areas altered the measured light by amplifying certain wavelengths and obscuring possible atmospheric signals.

for the scientific article DOI: 10.3847/1538-3881/ae4976

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