A Comprehensive Photodynamical Solution for the Multi-Planet Circumbinary System TOI-1338

TOI-1338 (EBLM J0608-59, BEBOP-1) was identified as an eclipsing binary in 2009 as a part of the Eclipsing Binary Low Mass (EBLM, Triaud et al.) project and later became the first transiting circumbinary planet discovered with the TESS mission. The binary consists of a 1.1 solar mass star paired with a 0.3 solar mass companion on a 14.6 day orbit. The transiting planet has a radius of 7.3 Earth Radii and an orbital period of around 95 days. Using more than 120 high-precision radial velocity measurements of the primary star with the ESPRESSO instrument, Standing et al. (2023) found a second planet in the system that has a period of around 215 days and a mass of about 65 Earth masses. We utilize all of the TESS data (through Sector 69), ground-based data from ASTEP, and all of the available radial velocity data to construct a comprehensive photodynamical model for the system. We show that the second planet’s mass is between 68.2 and 79.3 Earth masses at 95% confidence. The transiting planet has a mass of between 1.1 and 9.1 Earth masses and a density of less than 0.12 grams/cc, both at 95% confidence. Both planets have roughly circular and coplanar orbits, with eccentricities less than 0.04 and 0.08 for TOI-1338b and TOI-1338c, respectively. In terms of its mass, radius, and low density, TOI-1338b is similar to the outer planet in the only other known multi-planet circumbinary system, namely Kepler-47.