A “forbidden” planet that should not be able to exist according to our current understanding of how gas giants form has been found by astronomers. The large gas giant — dubbed “TOI-5205b” — was first spotted by NASA’s Transiting Exoplanet Survey Satellite (TESS) around TOI-5205. This is a red dwarf star that lies some 282 light years from the Earth.
Paper author and astrophysicist Dr Shubham Kanodia of the Carnegie Earth & Planets Lab in Washington DC specialises in the study of dwarf stars.
So-called M dwarfs like TOI-5205 are the most common stars in the Milky Way — making up some three-quarters of the stars in our galaxy — but cannot be seen from Earth with the naked eye.
Because of their relatively small size, they tend to be about half as hot as the Sun and much redder, having low luminosities but very long lifespans.
And while they tend to host more planets, on average, than their more massive counterparts, the way such star systems form makes M dwarfs extremely unlikely to end up hosting gas giants.
Dr Kanodia said: “The host star, TOI-5205, is just about four times the size of Jupiter, yet it has somehow managed to form a Jupiter-sized planet, which is quite surprising.”
Although a small number of gas giants have previously been found orbiting older M dwarf stars until now no such planets had even been found around a low-mass M dwarf.
Planets — from rocky bodies like Earth to gas giants like Saturn, Jupiter and TOI-5205b — all form out of the rotating disks of dust and gas that surround young stars.
According to the most prominent theory of planetary formation, gas giants are born when around ten Earth’s worth of rocky material accumulates within a circumstellar disk.
It is this material that forms the central core of the nascent gas giant,
This then sweeps up gas from the neighbouring regions of the disk to build up the planet’s atmosphere.
Dr Kanodia said: “TOI-5205b’s existence stretches what we know about the disks in which these planets are born.
“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. Based on our nominal current understanding of planet formation, TOI-5205b should not exist. It is a ‘forbidden’ planet!”
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The team added: “To grasp the size comparison here, a Jupiter-like planet orbiting a Sun-like star could be compared to a pea going around a grapefruit.
“For TOI-5205b, because the host star is so much smaller, it is more like a pea going around a lemon.
“In fact, when the Jupiter-mass TOI-5205b crosses in front of its host, it blocks about seven percent of its light — one of the largest known exoplanet transits.”
Fortunately, Dr Kanodia and her team have determined that TOI-5205b’s large transit depth makes it ideal for future observations with NASA’s new James Webb Space Telescope, which could help shed light on how the gas giant formed and its atmospheric composition.
The full findings of the study were published in The Astronomical Journal.
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