Exoplanet TOI-5205b is a gas giant about the same size and mass as Jupiter orbiting the red dwarf TOI-5205. But this is not unusual; planets revolve around stars all the time.
What is unusual is the size of TOI-5205 compared to a Jupiter-sized exoplanet. The star is not four times the radius of its planet, but just under 40 percent of the radius and mass of the Sun.
In addition, TOI-5205b’s orbit is uncomfortably close, orbiting the red dwarf every 1.6 days.
This is one of the first times such a large exoplanet has been discovered orbiting such a tiny red dwarf, and astronomers have never figured out how such a strange pair came about. This goes against our current understanding of planetary formation.
“The host star, TOI-5205, is about four times the size of Jupiter,” says astronomer Shabham Kanodia of the Carnegie Institution of Science, “however, she somehow managed to form a planet the size of Jupiter, which is quite amazing.”
Red dwarfs are the smallest main sequence stars we know of; a little less, and we begin to swerve into brown dwarf territory, not quite stars.
They have low mass, luminosity and heat, and burn their hydrogen reserves so slowly that their estimated lifespan could be in the trillions of years, much longer than the current age of the universe, which is 13.8 billion years.
Because they are so small and dim, red dwarfs cannot be seen with the naked eye; nevertheless, they are the most numerous stars in the Milky Way.
However, of the 5,250 exoplanets confirmed at the time of writing, only 240 orbit red dwarfs; only about a dozen of them are Jupiter-sized or larger.
What we know about planet formation suggests that such star-exoplanet pairs are unlikely. Stars form from clumps in dense clouds of gas and dust.
As they grow, the material around them coalesces into a disk that feeds the small star, a bit like water running down a sewer.
Once a star becomes massive enough, it emits powerful winds that blow away nearby material, interrupting the star’s growth.
What is left in this disk then forms objects that orbit the star, sticking together to gradually grow into planets.
Our current modeling suggests that creating a gas giant requires about 10 Earth masses of rocky disk material to create a planetary core, which then stores gas to create a giant, extended atmosphere.
This process also needs to happen relatively quickly before the young star blows up the remnants of the disk.
Our models also suggest that a tiny red dwarf in the disk should not have enough material for this process. must occur within the required time period.
“The existence of TOI-5205b expands our knowledge of the disks in which these planets are born,” says Kanodia.
“In the beginning, if there is not enough rocky material in the disk to form the initial core, then it is impossible to form a gas giant planet.
And in the end, if the disk evaporates before a massive core is formed, it cannot form a gas giant planet. And yet TOI-5205b formed despite these barriers.
Based on our nominal current understanding of planetary formation, TOI-5205b should not exist; it’s a “forbidden” planet.”
The exoplanet was discovered on the basis of transits, that is, when a rotating world passes between us and its host star.
This passage dims the light of the star; if the star’s own brightness is known, astronomers can calculate the exoplanet’s size.
The gravitational interaction between the star and the exoplanet also causes the star to “wobble” slightly in place, and measuring this wobble allows astronomers to calculate the exoplanet’s mass.
Based on these observations, Kanodia and his team determined that the exoplanet is 1.08 times the mass and 1.03 times the radius of Jupiter.
This is 27.2 percent of the radius of the star, which is 39.4 and 39.2 percent of the mass and radius of the Sun, respectively.
It seems extreme, but what’s even more exciting is that TOI-5205b is not alone.
A paper recently uploaded to the arXiv preprint server and accepted for publication details the discovery of TOI-3235b, an exoplanet with 0.665 times the mass and 1.07 times the radius of Jupiter orbiting a red dwarf with 0.394 times the mass and with a radius 0.37 times that of Jupiter. Sun with a period of 2.6 days.
And in 2021, astronomers announced the discovery of TOI-519b, a gas giant 14 times the mass and 1.07 times the radius of Jupiter, orbiting a red dwarf of 0.37 the mass and radius of the Sun, with a period of 1.27 days.
Thus, these exoplanets may exist, albeit rarely. It will be up to future work to figure out how to do this. Fortunately, the transits of these exoplanets can help.
Light from a star changes as it passes through an exoplanet’s atmosphere; with a powerful infrared telescope like the JWST, these changes can be studied in great detail to determine what these atmospheres are made of.
Perhaps they will also contain some clues leading to the solution of this fascinating mystery.
Contact us: [email protected]