Circumbinary planetary systems contain planets that orbit around two stars in the center instead of just one, like in our Solar System.
An artist’s rendition of a multiplanetary circumbinary system. Image credit: NASA’s Ames Research Center / JPL-Caltech / T. Pyle.
Circumbinary planets are planets that orbit both stars of a central binary system.
They were once confined to science fiction (Tatooine), but the discovery of Kepler-16b paved the way for the detection of 14 transiting planets in 12 binary systems by NASA’s Kepler/K2 and TESS missions.
“Of the 12 transiting circumbinary planet systems discovered so far, only one hosts multiple circumbinary planets: Kepler-47,” said Ohio State University astronomer David Martin and his colleagues.
“Kepler-47b, d and c have orbital periods of 49.5, 187.4 and 303.2 days respectively, with Kepler-47c’s orbit placing it within the system’s habitable zone.”
“Planet-forming disks around binaries are harsh environments for planet formation to take place in, and circumbinary planet discoveries provide insights into the formation and migration mechanisms at play in these unique environments.”
The newly-discovered planet orbits TOI-1338, a binary system some 1,317 light-years away in the constellation of Pictor.
Also known as TIC 260128333, TYC 8533-950-1 and 2MASS J06083197-5932280, the system consists of two eclipsing stars that orbit each other with a period of 14.6 days.
One star, TOI-1338A, is about 10% more massive than our Sun, while the other, TOI-1338B, is cooler, dimmer and only one-third the Sun’s mass.
In 2020, TOI-1338b, a circumbinary planet 6.9 times the size of Earth and an orbital period of 95 days, was discovered in this system using TESS data.
The new world, TOI-1338c, was discovered in the radial-velocity data obtained by the HARPS and ESPRESSO spectrographs, as part of the Binaries Escorted By Orbiting Planets (BEBOP) project.
Nicknamed BEBOP-1c, the planet has a mass of 65 Earth masses and an orbital period of 215 days.
“With an orbital period around 6.5 times that of the binary, BEBOP-1c is almost as close as it can be to the two stars — any closer and it would be kicked out of the system by the binary’s strong gravitational field,” said Monash University astronomer Rosemary Mardling.
“And with a period only twice that of BEBOP-1c, TOI-1338b is also perilously close to being thrown out of the nest. This tells us much about the conditions in which such planets form.”
“Planets are born in a disk of matter surrounding a young star, where mass progressively gathers into planets,” added University of Birmingham astronomer Lalitha Sairam.
“In the case of circumbinary geometries, the disk surrounds both stars. As both stars orbit one another, they act like a giant paddle that disturbs the disk close to them and prevents planet formation except for in regions that are quiet and far away from the binary.”
“It is easier to pinpoint the location and conditions of planet formation in circumbinary systems compared to single stars like the Sun.”
A paper describing the discovery was published in the journal Nature Astronomy.
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M.R. Standing et al. Radial-velocity discovery of a second planet in the TOI-1338/BEBOP-1 circumbinary system. Nat Astron, published online June 12, 2023; doi: 10.1038/s41550-023-01948-4
