New Tatooine-like multi-planetary system identified

English version: Diana Taborda

An international team, including astrophysicists Alexandre Correia, of the Faculty of Sciences and Technology, University of Coimbra (FCTUC) and João Faria, of the Institute of Astrophysics and Space Sciences Portugal (IA), has identified the second exoplanet orbiting binary star BEBOP-1 (or TOI-1338). The finding was published today in the journal Nature Astronomy.

The first circumbinary planet orbiting BEBOP-1, called TOI-1338b or BEBOP-1b, was discovered in 2020, in the same system, using data from NASA's TESS space telescope. This exoplanet, with a similar diameter to Saturn, orbits the two stars in 95 days.

As planetary transits are only able to measure the planet's diameter, the BEBOP project (an acronym for Binaries Escorted By Orbiting Planets) seeks to identify circumbinary planets by observing variations in the radial velocities of stars.

João Faria (IA and Dept. of Physics and Astronomy, Faculty of Sciences, University of Porto) explains that “This result demonstrates how the radial velocity method can be used to find exoplanets orbiting binary stars, allowing us to determine not only the orbit but also the mass of the planet,”. By knowing the mass, together with the diameter determined by transits, it is possible to calculate the density and determine the composition of the planet.

The BEBOP project used the ESPRESSO and HARPS spectrographs, both on telescopes of the European Southern Observatory (ESO), to try to observe BEBOP-1b. Despite their efforts, the team failed to find the planet, but this apparent failure allowed them to set a maximum limit of the planet's mass to around 22 Earth masses.

However, the search for the known exoplanet led the team to the discovery of BEBOP-1c, a planet with a mass about four times larger than Neptune and an orbital period of 215 days. With two planets identified, it now becomes the second-ever discovery of a multi-planetary circumbinary system (a system with planets that orbit around a binary star - similar to the Tatooine planet from “Star Wars”).

"We studied the global dynamics of the system at the University of Coimbra, and we were able to confirm that the system is stable when the planets' orbits are almost circular and are roughly in the same orbital plane as the two stars," explains Alexandre Correia, researcher at the Physics Centre of the University of Coimbra (CFisUC) and professor at the Physics Department of FCTUC.

"This system was difficult to confirm, and our observations were interrupted by the COVID pandemic, when the telescopes in Chile closed for 6 months during a critical part of this planet's orbit. This part of the orbit didn't become observable again until last year, when we were finally able to finalise the detection.", explains the lead author, Matthew Standing - currently working at The Open University - who started the project whilst at the University of Birmingham.

Planets form from the protoplanetary disks of materials accreting to newborn stars. In the case of circumbinary systems, the disc orbits both stars, while the stars orbit each other. Gravity and motion of a binary system create an instability zone closer to the stars, and so planets only form further apart.

João Faria explains that "The detection of BEBOP-1c around a Binary star system allows us to study the conditions under which these planets form, which are different from those that existed during the formation of the Solar System. The data analysis presented several challenges due to the dominating effect that the binary system has on radial velocities, which we had to model before finding the planet's signal".