Giant 'forbidden planet' orbits small star

Scientists have discovered that a highly unusual giant planet - sometimes called 'forbidden' - could have an atmosphere with fewer heavier elements that its host star.

University of Birmingham astrophysicist Dr Anjali Piette worked in an international research team to analyse James Webb Space Telescope data from the exoplanet TOI-5205 b. This is a Jupiter-sized planet orbiting a star itself about four times the size of Jupiter and about 40 percent the mass of the Sun.

Publishing its findings in The Astronomical Journal, the team, led by NASA Goddard Space Flight Center and Carnegie Science, reveal the planet’s atmosphere has a lower concentration of heavy elements, relative to hydrogen, than a gas giant planet in our own Solar System like Jupiter.

The exoplanet also has a lower metallicity - the abundance of elements heavier than hydrogen and helium present in an astronomical object - than its own host star.

Dr Anjali Piette said: "These findings have implications for our understanding of the giant planet formation process that occurs early in a star’s lifespan. The planet having a lower metallicity than its own host star makes it stand out among all the giant planets that have been studied to date.”

When TOI-5205 b passes in front of its host star - a phenomenon astronomers call a ‘transit’ the planet blocks about six percent of its light. By observing this transit with telescope instruments called spectrographs that split the light into its constituent colours, astronomers can try to decipher the planet’s atmospheric makeup and learn more about its history and relationship with its host star.

Planets are born from the rotating disc of gas and dust that surrounds a star in its youth. While it is commonly accepted that giant planets form in these cloudy disks that result from the birth of the host star, the existence of massive planets like TOI-5205b orbiting cool stars at close distances raises many questions about this process.

Planet and atmosphere

Observations of three transits of TOI-5205 b revealed the nature of the planet’s atmosphere, including the presence of methane (CH₄) and hydrogen sulphide (H₂S).

The team used sophisticated models of planetary interiors to predict that the entirety of TOI-5205 b composition is about 100 times more metal-rich than its atmosphere, as measured by the transits.

Dr Shubham Kanodia, from Carnegie Science, said: “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 suggests that its heavy elements migrated inward during formation and now its interior and atmosphere are not mixing. These results suggest a very carbon-rich, oxygen-poor planet atmosphere.”

The research is part of the GEMS Survey, a programme dedicated to studying transiting giant planets around M-dwarf stars to understand their formation, structure, and atmospheres.

The researchers worked together to correct for the effects that starspots on TOI-5205 b’s host star had on their data. Because the star is heavily spotted, it left an imprint on the data - brightening some wavelengths and masking potential signatures in the atmosphere.

Researchers are now validating this method in a more-recent JWST project in the same planetary system, which will prove useful for future investigations of this and other planets around active stars.