Scientists Discovered a Massive 'Forbidden' Planet That Shouldn't Exist

TOI-5205b is so massive that it suggests our best theories about how planets form are falling short.
Scientists Discovered a Massive 'Forbidden' Planet That Shouldn't Exist
Image by Katherine Cain, courtesy of the Carnegie Institution for Science. 
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Scientists have discovered a huge Jupiter-sized planet orbiting a very small star, a combination described as “forbidden” by one researcher because it challenges theories about how planets form, reports a new study.  

The newly discovered exoplanet belongs to an exceedingly rare population of forbidden worlds that raise questions about how the smallest and coolest stars in the universe, known as M-dwarfs, manage to host gas giants. 


M-dwarfs, also known as red dwarfs, are typically less than half the mass of the Sun, but what they lack in heft they more than make up for in abundance and longevity. These cool and faint dwarfs are by far the most common type of star in the Milky Way, and they are expected to burn for trillions of years, orders of magnitude longer than the projected 10-billion lifespan of the Sun. As a result, scientists have long been fascinated with the properties of M-dwarfs, including their potential capacity to host life.

Now, researchers led by Shubham Kanodia, a postdoctoral fellow who studies planetary science at the Carnegie Institute for Science, have used NASA’s Transiting Exoplanet Survey Satellite (TESS) to spot a Jupiter-sized gas giant called TOI-5205b, which is a full quarter of the size of its star, marking the first time that such a large world has been found around such a small star. 

Indeed, Kanodia and his colleagues noted that TOI-5205b, which is about 280 light years from Earth, “has one of the highest mass ratios for M-dwarf planets,” which “stretches conventional theories of planet formation…that cannot easily recreate the conditions required to form such planets,” according to a recent study published in The Astronomical Journal.


“Our group specializes in studying M-dwarfs and the planets that orbit them, having built some of the pioneering instruments to perform these measurements,” Kanodia said in an email to Motherboard. “We're particularly interested in the giant planets that orbit these low-mass stars because their discovery is a relatively recent development. Up until quite recently we didn't think that they could form, and should be quite rare.” 

“And while that still might be the case,” he added, “we are now starting to build up a sample of enough of these planets that we can move from stamp-collecting to understanding the sample of these planets.”

Indeed, in a blog post about the discovery, Kanodia noted that a handful of these “forbidden” gas giants have been found orbiting M-dwarfs, though TOI-5205b is the first to be found circling a star that is 40 percent of the Sun’s mass. This gas giant is so big that it blocks out a full seven percent of its star’s light as it “transits”—the term for a planets’ passage in front of a star from our perspective on Earth—distinguishing the system as “one of the deepest transits of a confirmed exoplanet orbiting a main-sequence star,” reports the study.

“Only after the first observation of the transits of this planet from the ground using the 3.5 meter Apache Point Observatory (APO) telescope did we realize how large the transit depth (and hence planet) was,” Kanodia said. “Observing TOI-5205 that night was a last minute pivot since the planned object we were hoping to observe using APO was unavailable due to cloudy conditions, and therefore we pivoted to this transit, which panned out quite well!” 


This odd star-planet couple shouldn’t exist, according to planetary formation theories. Planets typically form in disks of gas and dust that surround young stars; huge worlds like Jupiter accumulate a lot of rock from these disks in order to capture the gassy outer layers that make them so distinct. It’s not yet clear how small systems, like TOI-5205, are able to produce enough material to seed gas giants within the narrow lifespan of an M-dwarf disk.

“[W]hat makes it interesting is not just the fact that the mass ratio is so large, but that the planet formation challenge is further compounded by the low host-star mass,” Kanodia explained. “The smaller (lower mass) host star implies that it should take longer to form the rocky core for such a planet, which makes things even more difficult.”

“These objects are unlikely or forbidden and expected to be very rare due to the reasons outlined above,” he added. “There just isn't enough material in the primordial disks for such M-dwarfs to form them in a timely manner (or at least that's what we used to think).”

However, Kanodia and his colleagues hope that the bizarre properties of TOI-5205b could help solve this riddle. Because the gas giant blocks out so much of its star’s light, it’s possible to closely examine the properties of its atmosphere as it transits.

“The large transit depth makes TOI-5205b a compelling target to probe its atmospheric properties, as a means of tracing the potential formation pathways,” the team noted in the study, adding that “it is also an object easily accessible with James Webb Space Telescope (JWST) observations,” referring to the most powerful space observatory ever launched.

To that end, the discovery of TOI-5205b may help to explain the enigmatic processes that create these big planetary fish in small stellar ponds. For now, it’s tantalizing to imagine the strange properties of this alien Jupiter that exists in such a nearby system, regardless of its backstory.

Update: This article has been updated to include comments from lead author Shubham Kanodia.