A ‘forbidden planet’ orbiting a small star is challenging gas giant formation theories according to US researchers
Based on our current understanding of planet formation, TOI-5205b should not exist; it is a ‘forbidden planet’, according to a team of astronomers led by the Carnegie Institution for Science’s Shubham Kanodia.
The team has discovered an unusual planetary system in which a large gas giant planet orbits a small red dwarf star called TOI-5205. Their findings challenge long-held ideas about planet formation.
Smaller and cooler than our sun, M dwarfs are the most common stars in our Milky Way galaxy. Due to their small size, these stars tend to be about half as hot as the sun and much redder.
They have very low luminosities, but extremely long lifespans. Although red dwarfs host more planets, on average, than other more massive types of stars, their formation histories make them unlikely candidates to host gas giants.
The newly discovered planet – TOI 5205b – was first identified as a potential candidate by NASA’s Transiting Exoplanet Survey Satellite (TESS). Kanodia’s team, which included Carnegie’s Anjali Piette, Alan Boss, Johanna Teske, and John Chambers, then confirmed its planetary nature and characterised it using a variety of ground-based instruments and facilities.
Kanodia, who specialises in studying these stars, which comprise nearly three-quarters of our galaxy, yet can’t be seen with the naked eye, 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!”
giant planet
A small number of gas giants have been discovered orbiting older M dwarf stars. But until now, no gas giant has been found in a planetary system around a low-mass M dwarf like TOI-5205.
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 7% of its light – one of the largest known exoplanet transits.
Planets are born in the rotating disk of gas and dust that surrounds young stars. The most commonly used theory of gas planet formation requires about ten Earth masses of this rocky material to accumulate and form a massive rocky core, after which it rapidly sweeps up large amounts of gas from the neighbouring regions of the disk to form the giant planet we see today.
The time frame in which this happens is crucial.
Kandola explained: “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.”
The team demonstrated that the planet’s very large transit depth makes it extremely conducive for future observations with the recently launched JWST, which could shed some light on its atmosphere and offer some additional clues about the mystery of its formation.
The findings are published in The Astronomical Journal.
Image 1: Artist’s conception of a large gas giant planet orbiting a small red dwarf star called TOI-5205. © Image by Katherine Cain, courtesy of the Carnegie Institution for Science.
Image 2: “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,” said lead author Shubham Kanodia. © Artwork by Katherine Cain, courtesy of the Carnegie Institution for Science.
