Circumbinary planets that orbit around both stars of a central binary star system challenge our understanding of planet formation. With only 12 binary systems known to host circumbinary planets, identifying more of these planets and their physical properties could help discern some of the physical processes that govern planet formation.
An international team of astronomers, led by researchers at the University of Birmingham, recently announced the second-ever discovery of a multi-planetary circumbinary system. By analyzing radial-velocity data obtained by the HARPS and ESPRESSO spectrographs, astronomers detected a planet called BEBOP-1c.
The planet got its name from the project that collected the data. BEBOP stands for Binaries Escorted By Orbiting Planets. The BEBOP-1 system is also known as TOI-1338.
The Birmingham team also donated data from NASA’s TESS space telescope that were used to find a circumbinary planet in the same system in 2020. The transit method was used to find that planet, observed as it repeatedly crossed in front of the brighter of the two stars.
Using data from NASA’s TESS space observatory, to which the Birmingham team contributed, a circumbinary planet named TOI-1338b was found in the same system in 2020. That planet was found using the transit method, and it was observed because it repeatedly passed in front of the brighter of the two stars. The transit method allowed scientists to measure the size of TOI-1338b but not its mass which is the planet’s most fundamental parameter.
The scientists tried to estimate the planet’s mass discovered by TESS using cutting-edge equipment mounted on two telescopes in Chile’s Atacama Desert. Despite their best efforts and years of work, the crew could not accomplish that; instead, BEBOP-1c, a second planet, was found, and its mass was determined.
BEBOP-1c has a 215-day orbital period and a mass that is 65 times that of the Earth but only around five times that of Jupiter. This was a challenging system to confirm, and the COVID outbreak caused telescopes in Chile to close for six months during a crucial period of the planet’s orbit, interfering with views. When they completed the detection last year, this portion of the orbit only recently became observable once more.
Only two planets are now known to exist in the TOI-1338/BEBOP-1 circumbinary system, but more may be found in the future with similar discoveries to those made by the researchers.
Although rare, circumbinary planets are crucial in furthering our understanding of how planets are formed.
The team does not yet know the size of BEBOP-1c, only its mass. However, researchers will now attempt to use the transit method to measure the size of BEBOP-1c.
The team could set rigid upper bounds on the mass of TOI-1338b despite not finding it. Now that its density is found to be less than that of a Victoria sponge cake, the planet is ideal for further research using the James Webb satellite telescope. If these observations materialize, they might shed light on the chemistry of the environment that gave rise to this unusual circumbinary planet.
Journal Reference:
- Standing, M.R., Sairam, L., Martin, D.V. et al. Radial-velocity discovery of a second planet in the TOI-1338/BEBOP-1 circumbinary system. Nature Astronomy (2023). DOI: 10.1038/s41550-023-01948-4