A ‘forbidden planet’ the size of Jupiter has astronomers stumped

There’s a “forbidden” oddball planet about 282 light-years away from Earth. The Jupiter-esque planet known as TOI-5205 b is the first exoplanet of its kind with an atmosphere containing far fewer heavy elements than similarly sized objects, as well as its own red dwarf star. According to an international team writing in The Astronomical Journal, this forbidden planet challenges common assumptions about cosmic evolution. 

TOI-5205 b was discovered in 2023 using NASA’s Transiting Exoplanet Survey Satellite (TESS). Since then, astronomers around the world have been studying the planet’s composition through a range of techniques including spectrography. Whenever a planet travels in front of its host star—a process known as transit—it blocks around six percent of overall light. With space telescope spectrography, researchers can break down the remaining wavelengths into specific color bands, each corresponding with the elements in its atmosphere.

After three transits, astronomers determined TOI-5205 b possesses an atmosphere with compounds like methane and hydrogen sulphide. However, they were more surprised by the planet’s interior. Using advanced computer modeling systems taking into account the planet’s mass and radius, the team now believes TOI-5205 b is around 100 times more metal-rich than its atmosphere. Regardless, its metallicity is far lower than its star.

“We observed much lower metallicity than our models predicted for the planet’s bulk composition,” study co-author and Carnegie Science astronomer Shubham Kanodia explained in a statement. “This suggests that its heavy elements migrated inward during formation, and now its interior and atmosphere are not mixing.”

The result is a metallic planet with a carbon-rich, yet oxygen-poor atmosphere. TOI-5205 b orbits a star that is about four times larger than Jupiter, but also contains only 40 percent the mass of the sun. Planets typically evolve from the rotating dust and gas disc encompassing a young star. However, TOI-5205 b’s size and close distance to its comparatively cool star creates more questions than it answers. Much more analysis is needed, but it is very clear that the “forbidden” planet is complicating once-accepted notions of planetary lifecycles.

“These findings have implications for our understanding of the giant planet formation process that occurs early in a star’s lifespan,” said study co-author and University of Birmingham astronomer Anjali Piette. “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.”

 

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