Astronomers have discovered a novel ‘forbidden’ giant planet that just isn’t much smaller than its star.
The planet, TOI-5205 b, has been studied using the James Webb Space Telescope (JWST).
Researchers say the findings could reshape understanding of how such worlds develop within the adolescence of a star.
The study, published this week in The Astronomical Journal, was led by Caleb Cañas of NASA Goddard Space Flight Center and included Shubham Kanodia of Carnegie Science.
TOI-5205 b is roughly the dimensions of Jupiter but orbits a relatively small, cool star.
Such systems are sometimes described as ‘forbidden’ because current theories struggle to elucidate how such large planets can form so near low-mass stars.
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When the planet passes in front of its host star – an event often called a transit – it blocks about 6% of the star’s light. By analysing this light with spectrographs, astronomers can determine the chemical composition of the planet’s atmosphere.
Observations of three transits revealed a surprising result.
The team found that the planet’s atmosphere comprises a lower concentration of heavy elements – often called ‘metallicity’ in astronomy – than each Jupiter and its own host star.
This makes it unlike any giant planet studied to this point.
Methane and hydrogen sulphide were also detected within the atmosphere, though these findings were less unexpected.
To raised understand the outcomes, researchers used computer models to estimate the planet’s internal composition.
These suggested the planet as an entire could also be far richer in heavy elements than its outer atmosphere indicates.
‘We observed much lower metallicity than our models predicted for the planet’s bulk composition, which is calculated from measurements of a planet’s mass and radius.
‘This means that its heavy elements migrated inward during formation and now its interior and atmosphere should not mixing,” Kanodia explained.
‘In summary, these results suggest a really carbon-rich, oxygen-poor planetary atmosphere.’
Planets form from discs of gas and dirt that surround young stars. While giant planets are thought to emerge from these discs, systems like TOI-5205 b challenge existing models.
The research forms a part of the ‘Red Dwarfs and the Seven Giants’ programme, which is using JWST to review similar systems — sometimes known as GEMS, or giant exoplanets around M-dwarf stars.
Further observations of TOI-5205 b and similar systems are expected to assist make clear how these so-called “forbidden” worlds come into being.
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