Astronomers used the James Webb Space Telescope (JWST) to conduct more in-depth observations of TOI-5205 b, a giant exoplanet known as the "forbidden planet", and found that the content of heavy elements in its atmosphere is unusually low, even lower than that of the parent star it orbits. This discovery directly challenges the traditional theory of planet formation and evolution.
Relevant research was published in The Astronomical Journal, led by Caleb Cañas of NASA's Goddard Space Flight Center, and participated by Shubham Kanodia of the Carnegie Institution for Science and a multinational research team.
TOI-5205 b is similar in size to Jupiter, but its parent star is a much smaller red dwarf: its stellar radius is about four times that of Jupiter and its mass is about 40% that of the Sun. When a planet passes in front of a star from Earth's line of sight, it blocks about 6% of the star's luminosity. Scientists use spectrometers to analyze the changes in light of different wavelengths during the transit, thereby inverting the chemical composition of the planet's atmosphere and inferring its formation history.
According to the mainstream theory, planets are born in rotating disks of gas and dust around young stars, and giant planets generally evolve in material-rich disks. However, near a smaller and cooler star like TOI-5205, there is a huge, close-orbiting planet. Such a planet-star ratio and orbital configuration are difficult to reasonably explain with existing models, so it is called a "forbidden planet."
To systematically study such "anomalous" planets, Kanodia, Cañas and Jessica Libby-Roberts of the University of Tampa are leading the implementation of one of the largest exoplanet observation projects in the second cycle of JWST - the "Red Dwarfs and the Seven Giants" project, specifically focusing on this type of giant planets orbiting M-type dwarfs, collectively known as GEMS (giant exoplanet and M dwarf systems).
TOI-5205 b was originally discovered by NASA's Transiting Exoplanet Survey Satellite (TESS) and marked as a candidate. Kanodia confirmed the planet's existence through follow-up observations in 2023, and is currently one of the core members of the team that used JWST to produce the first detailed characterization of its atmosphere.
Through data analysis of three transit events, the team found that the abundance of heavy elements relative to hydrogen in the atmosphere of TOI-5205 b is not only lower than that of Jupiter, but even lower than that of its parent star. This is contrary to what people generally expect from giant planets - usually giant planets are enriched with more heavy elements during their formation, so their overall "metallicity" tends to be higher than that of their host stars. Observations also detected the presence of methane (CH₄) and hydrogen sulfide (H₂S) in the planet's atmosphere, providing more clues to its chemical structure.
To understand the observed "metal-poor" phenomenon in the atmosphere, Simon Muller and Ravit Helled of the University of Zurich used models of the planet's internal structure to deduce the overall composition of TOI-5205 b. The results indicate that the planet's overall "metallicity" is likely about 100 times higher than the composition of the atmosphere measured by transit methods. In short, the interior of the planet is rich in heavy elements, but these heavy elements are not effectively mixed into the atmosphere.
Kanodia, a co-author of the paper, explained that there is a clear gap between the overall metal content expected by the model and the observed atmospheric metal content, which suggests that during the formation process of the planet, heavy elements tend to migrate inward and are "locked" in the deep interior, and the material mixing efficiency with the outer atmosphere is low. Based on various evidences, the team believes that TOI-5205 b has an abnormal atmospheric environment that is "rich in carbon and poor in oxygen."
During the data processing, the researchers also specifically included the influence of the parent star's sunspots. The darker regions on the surface of these stars will change the observed spectral characteristics in subtle ways: enhancing the relative intensity of certain bands while masking potential atmospheric signals. If not corrected, it is easy to bias the determination of atmospheric composition. Wallack and Kanodia are currently further verifying this correction method in a new JWST observation, hoping to provide a more reliable observation and analysis framework for future studies of the atmospheres of planets around highly active stars.
This research is part of the GEMS survey program. The goal is to systematically observe transiting giant planets orbiting M-type dwarf stars and clarify their formation processes, internal structures, and atmospheric properties. The participating team also includes Carnegie Institution for Science astronomers Peter Gao, Johanna Teske and Nicole Wallack, as well as Anjali Piette, now a faculty member and a former Carnegie postdoctoral fellow.