The discovery could challenge our theories of how gas giants like Jupiter form.
An unusual planetary system with a Jupiter-sized planet circling a tiny star that is only four times the size of the gas giant in our solar system has been found by astronomers. This “forbidden” arrangement of a large planet orbiting a small star may cast doubt on ideas regarding the formation of gas giant planets.
The “exoplanet,” also known as an extrasolar planet, revolves around the red dwarf star TOI 5205, which is much colder and smaller than the sun. These M-dwarf stars, the most prevalent type of stellar body in the Milky Way, are smaller than the sun and have temperatures that are comparatively cool.
Though on average this class of stars hosts more planets around them than other star types, it was previously believed that their formation makes them unlikely to be orbited by gas giants. The discovery of this exoplanet — designated TOI 5205b — by astronomers using N.A.S.A.’s Transiting Exoplanet Survey Satellite (TESS) telescope challenges that concept. The planet was confirmed and characterized by the team using various ground-based telescopes and instruments.
It is quite amazing that the host star, TOI-5205, which is roughly four times the size of Jupiter, has managed to create a planet the size of Jupiter. Shubham Kanodia, the team’s leader and an astronomer with Carnegie Science, stated in a release. (new tab appears)
Although gas giants have been found circling M dwarf stars in the past, none of them have been found orbiting a TOI-5205 that is so low mass.
Protoplanetary discs, rotating discs of gas and dust that encircle young stars, are where planets are born. Its center star was created when the same material collapsed, leaving behind this material. Planet cores are created when dense regions collapse due to their own gravity, at which point they begin to gather more material.
According to current planet formation theories, it would require material 10 times the mass of Earth to give rise to a gas giant. This first creates a rocky core, and from this core, a giant planet eventually develops by accumulating gas. But this procedure must move quickly.
“In the beginning, if there isn’t enough rocky material in the disk to form the initial core, then one cannot form a gas giant planet. And at the end, if the disk evaporates away before the massive core is formed, then one cannot form a gas giant planet. And yet TOI-5205b formed despite these guardrails,” Kanodia explained in the statement. “Based on our nominal current understanding of planet formation, TOI-5205b should not exist; it is a ‘forbidden’ planet.”
To picture how unbalanced this system is to planetary systems that astronomers expect, imagine our star the sun squashed down to the size of a grapefruit. That size reduction would mean the largest gas giant in our solar system, Jupiter, would be about the size of a garden pea.
The TOI-5205 system is more like a pea orbiting a lemon.
The size disparity in the size is so great that when TESS used the drop in light caused by a planet as it passes in front of its star, known as the transit method, that dip in light was 7% of the star’s total light output.
That makes the dimming of TOI-5205 by this Jupiter-sized exoplanet the largest known drop in light caused by an exoplanet transit.
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This extreme dip in light or technically, “large transit depth,” could make the system ideal for follow-up investigations with the James Webb Space Telescope (JWST).
Observations with the JWST could help determine the composition of TOI-5205 b’s atmosphere and may shed light on the processes that birthed this “forbidden” planet.
The team’s research is published in The Astronomical Journal(opens in new tab).
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