An exoplanet, TOI-5205 b, which is almost as large as Jupiter, orbits a small red star. By many estimates, this red star should not have been able to form it due to the star’s mass.
The fact that TOI-5205 b exists as it does at all raises intriguing questions about how it came to be. In addition, the James Webb Space Telescope has confirmed that this planet may be even stranger than previously thought. Specifically, the atmosphere of TOI-5205 b is deficient in heavy elements relative to the hosting star. This discrepancy suggests that the outer layers of the planet and its deep interior have not developed in the same way through the formation process.
The findings from the study of TOI-5205 b were recently published in The Astronomical Journal. The research was led by Caleb Cañas, a NASA Goddard Space Flight Center scientist, along with an international team of researchers. This team included Shubham Kanodia, a Carnegie Science astronomer who contributed to the confirmation of the stellar companion in 2023, and who was also a co-leader of the new study.
TOI-5205 b belongs to a rare group of giant exoplanets orbiting M dwarf stars, sometimes referred to as GEMS. These planets are gas giants and orbit very small, cool stars. The presence of such large planets cannot be explained cleanly by current planet formation theories. They cannot have formed in typical disks around cooler stars, which contain less solid material.
In the case of TOI-5205 b, the difference in heavy elements between the star and the planet is clear. TOI-5205 b is roughly 1.08 times the mass of Jupiter and 0.94 times the radius of Jupiter. Its host star has a mass of only 39% the mass of the Sun. This indicates that something else occurred during the planet’s formation. TOI-5205 b orbits very close to its stellar companion, with an orbital period of approximately 1.63 days.
Three transits allowed astronomers to track the planet crossing in front of its star. With JWST’s NIRSpec, astronomers divided that light into different wavelengths for analysis. They looked for chemical fingerprints within the atmosphere.
Methane and hydrogen sulfide were identified.
Another explanation is more difficult to come by. When comparing both Jupiter and TOI-5205 b, the atmosphere of TOI-5205 b is much lower in metallicity. This means there is less than one would expect of oxygen, nitrogen, sulfur, and other elements. In comparison, these elements are more abundant in Jupiter’s atmosphere and the host star, TOI-5205, is metal-rich.
According to Kanodia, “the observations provide much lower metal abundances than our modeled predictions for the bulk composition of the planet were calculated via planet mass and radius.” Therefore, this means that during the planet’s formation, the heavy elements within TOI-5205 b migrated toward the core and no longer mixed with the atmosphere. As a result, the conclusion is that the atmosphere will likely be carbon-rich and unable to sustain the presence of significant amounts of oxygen (i.e., the atmosphere will be a “carbon-rich, oxygen-poor planet”).
Using retrieval techniques, the team found a subsolar metallicity and a carbon-to-oxygen ratio greater than solar. Essentially, this means that the TOI-5205 b atmosphere appears to have fewer oxygen-rich molecules and more carbon-rich chemistries. Methane was identified at very high levels, while no water was detected in the atmosphere of TOI-5205 b. This suggests potential significance. Finding water was anticipated to be simpler in this type of atmosphere, given that this gas giant has a temperature higher than average for gas giants.
The star played a crucial role in limiting the capability to identify water within TOI-5205 b. Moreover, there is a possibility that the star has caused additional issues. The researchers clearly note from observations that the surface of the host star has variable intensity because of starspots. These starspots create unwanted variations in the incoming light from the star. This variability can affect the spectrum taken during each transit.
Therefore, the observations show that the spectrum could have been distorted by the host star’s surface. The effects of contamination can confuse the separation of the atmosphere of TOI-5205 b from contamination from the star’s spectrum.
In conclusion, they find that any upward slope in the continuum of the spectrum at bluer wavelengths is not just a result of clouds or haze. It is also due to contributions coming from stellar contamination.
This complicates the interpretation of any water-based measurements obtained from the atmosphere of TOI-5205 b. Methane and hydrogen sulfide, however, did not appear to be as affected by either clouds or haze. They did yield some degree of agreement with the detection of water.
The researchers reliably identified a strong detection of methane and a weaker but still significant hydrogen sulfide signal. Oxygen-bearing molecules have not been reported with significant reliability.
The atmosphere of TOI-5205 b is different from its interior. The atmosphere hints that TOI-5205 b is metal-poor, but when considering the planet in its entirety, the planet is not. The team used thermal evolution models to estimate the bulk metallicity of TOI-5205 b to be 0.17 ± 0.07. This corresponds to an equivalent heavy element mass of 57 ± 25 Earth masses. This would indicate that TOI-5205 b contains a significant population of heavy elements that the atmosphere alone does not show.
For comparison, the interior of Jupiter is assumed to contain a bulk metallicity of approximately 20 to 30 Earth masses in heavy elements.
Because of this difference between the atmosphere and the interior, the researchers conclude that TOI-5205 b may not have formed through in situ accumulation of materials from its surrounding protoplanetary disk, as seen with many other planets. Instead, it may have formed when most of the heavy elements were added to the planet at a very early time. Additionally, there could have been significant carbon-to-oxygen accumulation early in the planet’s formation before any clear signals of what contributed to its formation emerged.
The researchers also pointed out the broader question of how giant planets can form around low-mass stars. If planets similar to TOI-5205 b are common, models for planet formation around low-mass stars may have to be modified.
The study raises practical implications for research, as it represents a strong test case for existing planet formation models. The results indicate that the visible atmosphere of a giant planet does not necessarily reflect the full contents of the planet. This is especially true around very active M dwarf stars.
The effects of the host star’s contamination are strong enough to influence the results of observations carried out using the James Webb Space Telescope. Therefore, future work will remain important when assessing the accuracy of planet formation models and determining whether TOI-5205 b and similar planets formed in this manner around red dwarfs.
Research findings are available online in the journal The Astronomical Journal.
The original story “Giant planet that shouldn’t exist is forcing astronomers to rethink planetary science” is published in The Brighter Side of News.
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