In our solar system, the planets orbit around a star — the Sun. That’s all fine and neat, but sometimes, planets orbit around two stars. This is called a circumbinary system, and it’s pretty rare. Until now, astronomers only discovered one such system. Now, an international team of astronomers has made the second.

The planetary blues

Circumbinary systems were popularized by the Star Wars franchise. In Star Wars, Tatooine, the homeworld of Anakin and Luke Skywalker, orbits two stars.

Astronomers named the planet BEBOP-1c, for the Binaries Escorted By Orbiting Planets (BEBOP) initiative. There could be another cultural reference hidden here, as Cowboy Bebop is one of the most famous sci-fi anime productions.

Culture aside, the system where BEBOP-1c resides is called TOI-1338. It marks only the second circumbinary system we’ve discovered.

In 2020, using data from NASA’s TESS space telescope, scientists found the circumbinary planet TOI-1338b in the same system. This planet was identified using the transit method. When a planet crosses in front of the stars, it causes a slight dim in the star’s luminosity. But there are limitations to the transit method.

“The transit method permitted us to measure the size of TOI-1338b, but not its mass which is the planet’s most fundamental parameter,” said lead author Matthew Standing, a researcher at The Open University.

The BEBOP team was already monitoring this system using another detection method at the time, called the Doppler method.

Doppler and mass

This method, also called the wobble method, or radial-velocity method, relies on accurately measuring the velocity of stars. Planets orbiting the star can influence this velocity. By measuring small tweaks, researchers can detect planets and estimate the mass of these planets. This is the same method that led to the discovery of the first exoplanet, for which Michel Mayor and Didier Queloz received the Nobel Prize for Physics in 2019.

Equipped with state-of-the-art instruments on telescopes located in Chile’s Atacama Desert, the team aimed to determine the planet’s mass found by TESS. Despite years of effort, their endeavor fell short. However, they made an unexpected discovery—the second planet, BEBOP-1c, and successfully measured its mass.

“BEBOP-1c has an orbital period of 215 days, and a mass 65 times larger than Earth, which is about five times less than Jupiter’s mass,” Standing said. “This was a difficult system to confirm, and our observations were interrupted by the COVID pandemic when telescopes in Chile closed for six months during a critical part of the planet’s orbit. This part of the orbit only became observable again last year, when we finalized the detection.”

More Tattooines awaiting discovery

Currently, astronomers discovered only two planets around the TOI-1338/BEBOP-1 circumbinary system. However, the team believes there may be more awaiting discovery through similar observations.

Circumbinary planets, though rare, play a vital role in understanding the formation of planets.

“Planets are born in a disc of matter surrounding a young star, where mass progressively gathers into planets,” said Lalitha Sairam, a researcher at the University of Birmingham and second author of the study. “In the case of circumbinary geometries, the disc surrounds both stars. As both stars orbit one another, they act like a giant paddle that disturbs the disc 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”.

While the size of BEBOP-1c remains unknown, its mass has been determined. The researchers now plan to employ the transit method to measure its size.

The study was published in the journal Nature Astronomy.