Astronomers led by Shubham Kanodia of Carnegie have identified a peculiar planetary system in which a huge gas giant planet revolves around a dim red dwarf star known as TOI-5205. Their findings which are published in The Astronomical Journal, challenge long-held ideas about planet formation.
The most prevalent stars in our Milky Way galaxy are M dwarfs, which are smaller and cooler than our Sun. These stars are typically much redder and half as hot as the Sun because to their small size. 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.
NASA’s Transiting Exoplanet Survey Satellite (TESS) first identified the newly discovered planet TOI 5205b as a potential candidate. Kanodia’s team, which included Carnegie’s Anjali Piette, Alan Boss, Johanna Teske, and John Chambers, then confirmed its planetary nature and characterized it using a variety of ground-based instruments and facilities.
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.
Shubham Kanodia
“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!” exclaimed Kanodia, who specializes in studying these stars, which comprise nearly three-quarters of our galaxy yet can’t be seen with the naked eye.
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.
A Jupiter-like planet orbiting a Sun-like star may be likened to a pea circling a grapefruit; however, because the host star of TOI-5205b is so much smaller, it is more like a pea circling 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.
Planets are born in the rotating disk of gas and dust that surrounds young stars. According to the most popular idea, this rocky material must first build up to the size of around 10 Earth masses to form a gigantic rocky core, which then quickly scavenges large quantities of gas from nearby portions of the disk to create the giant planet we see today.
The time frame in which this happens is crucial.
“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.”
The scientists showed that the planet’s very wide transit depth makes it incredibly favorable for subsequent studies with the recently launched JWST, which could clarify its atmosphere and provide some additional hints regarding the enigma of its genesis.
The TESS follow-up research was conducted using the Habitable-zone Planet Finder (HPF; Texas, US) and Low Resolution Spectrograph (LRS2; Texas, US) on the 10-m Hobby Eberly Telescope, the ARCTIC camera on the 3.5-m Apache Point Observatory (APO; New Mexico, US), the NN-Explore Exoplanet Stellar Speckle Imager (NESSI, Arizona, US) at the 3.5-m WIYN telescope, the 0.6-m Red Buttes Observatory (RBO, Wyoming, US), and the 0.3 m Three Hundred Millimeter Telescope (TMMT, Chile).
