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Astronomers studying a distant world known as TOI-5205 b have found new details about its atmosphere
Using observations from the James Webb Space Telescope, researchers found that TOI-5205 b contains fewer heavy elements in its atmosphere than was previously suggested, especially when compared to its own host star.
The findings, published in The Astronomical Journal, suggest that existing models of planet formation may need to be revisited.
A giant planet around a small star
TOI-5205 b stands out because of its unlikely pairing. The planet is about the same size as Jupiter but orbits a relatively small, cool red dwarf star. Its host star is only about 40 per cent as massive as the Sun, making the presence of such a large planet difficult to explain within current theories.
When the planet passes in front of its star, TOI-5205 b blocks about six per cent of the star’s light. By analysing this light using spectrographs, astronomers can determine the chemical composition of the planet’s atmosphere.
Observations of three transits revealed an unusual atmospheric signature. The planet’s atmosphere contains a lower proportion of heavy elements than both its host star and similar planets in our solar system.
This is surprising because giant planets are generally expected to have atmospheres enriched with heavier elements. Instead, TOI-5205 b appears to have an atmosphere dominated by lighter elements, such as hydrogen.
The data also confirmed the presence of methane and hydrogen sulfide, providing further insight into the planet’s chemical makeup.
A hidden interior
To better understand the discrepancy, researchers used advanced models to study the planet’s internal structure. These models suggest that while the atmosphere appears low in heavy elements, the planet’s interior may be significantly richer in them.
This points towards a scenario in which heavier materials sank toward the planet’s core during its formation, leaving the outer atmosphere relatively depleted. The findings indicate that the planet’s interior and atmosphere may not be mixing as expected.
Rethinking planet formation
This discovery is part of an effort to study “forbidden” planets, giant worlds orbiting small stars that defy traditional formation models. These planets are being investigated through a dedicated research initiative aimed at understanding their origins, structures, and atmospheres.
TOI-5205 b was first identified as a candidate by NASA’s Transiting Exoplanet Survey Satellite before being confirmed through follow-up observations. The latest study represents the first detailed look at its atmospheric composition.
Beyond its individual characteristics, TOI-5205 b offers a new opportunity to test and refine theories of planetary formation. Its unexpected properties suggest that the processes shaping planets around smaller stars may be more complex than previously thought.
As astronomers continue to study similar systems, discoveries like this could reshape our understanding of how planets form and evolve across the galaxy.