An ‘impossible’ planet discovered: Where is TOI-5205b and why shouldn’t it exist?

Astronomers have been an exciting new discovery that could spark a rethink in our understanding of how planets are formed.

The report, led by Shubham Kanodia of the Carnegie Institution for Science, identified a small red dwarf star with a unique characteristic. Despite its size, TOI-5205b is at the centre of a planetary system that includes a huge gas giant roughly the size of Jupiter.

How does TOI-5205b host a gas giant?

TOI-5205b is considered a red dwarf – also known as an M dwarf – planet. They are typically smaller than our sun and are the most common stars in our galaxy. Due to their smaller size they tend to be roughly half as hot as the sun too, meaning that they appear less bright and often taken on a redder colour. From the earth there are no red dwarfs visible to the naked eye.

Due to the lower luminosity they tend to have far long lifespans. Their longer lifespan allows them to host more planets, because they accumulate over time, but the way in which they are formed means they rarely host gas giants.

TOI-5205b is a rare outlier that is able to support a huge gas giant, something that could offer new insight into the formation of planetary systems.

“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,” Kanodia said, upon announcing the new findings.

There have been a few other examples found with gas giants orbiting older red dwarf stars, but this is the first instance of a gas giant being present within a planetary system that centres on a low-mass red dwarf star.

What does TOI-5205b tell us about planet formation?

TOI-5205b is still considerably larger than the gas giant that orbits around it but the size discrepancy is less pronounced than is the case in most planetary systems. In our own solar system, the size ratio between the gas giant Jupiter and the sun is similar to that between a pea and a grapefruit.

However in the case of TOI-5205b, the size comparison is more like a pea orbiting around a lemon. This difference may seem fairly unremarkable but it sheds new light on the process of planetary formation.

Planets are formed in a disk of spinning gas and dust that is drawn together by young stars. It is typically believed that it takes the mass of around 10 earths to accumulate together and form a rocky core at the centre of a galaxy, and many millions of years for a gas giant to then form.

“TOI-5205b’s existence stretches what we know about the disks in which these planets are born,” Kanodia explained. “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.”