Radial-velocity discovery of a second planet in the TOI-1338/BEBOP-1 circumbinary system

Circumbinary planets, those 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, along with their physical properties, could help to discern some of the physical processes that govern planet formation. Here we analyse radial-velocity data obtained by the HARPS and ESPRESSO spectrographs and report the detection of BEBOP-1 c, a gas giant planet with a mass of 65.2 ± 11.8 Earth masses ( M ⊕ ) orbiting around both stars of an eclipsing binary star system with a period of 215.5 ± 3.3 days. The system TOI-1338, hereafter referred to as BEBOP-1, which also hosts the smaller and inner transiting planet TOI-1338 b, is only the second confirmed multiplanetary circumbinary system. We do not detect TOI-1338 b with radial-velocity data alone, and we can place an upper limit on its mass of 21.8 M ⊕ with 99% confidence. TOI-1338 b is amenable to atmospheric characterization using JWST, so the BEBOP-1 system has the potential to act as a benchmark for circumbinary exo-atmospheric studies. The radial-velocity technique could detect a small gas giant orbiting a binary star and determine its mass: 65.2 ± 11.8 Earth masses. The system also hosts a smaller inner planet, making it one of the few known multiplanetary circumbinary systems.