The Curious Case of TOI-2267: Three Earth-sized Exoplanets in a Cool Star Binary System

Temperate small exoplanets around cool stars only recently moved onto the scientific centre stage but swiftly entered the spotlight. We yet have to assemble various puzzle pieces to understand what makes a true "Earth twin" in these systems (if it even exists). Open questions include the formation of these ubiquitous targets, probing evolution hypotheses of photo-evaporation and core-powered mass loss, and constraining habitability definitions.

We report the discovery and validation of three temperate Earth-sized exoplanets, TOI-2267.03 and .01, likely orbiting the primary and TOI-2267.02 likely orbiting the secondary in a binary system of cool stars (primary: M5V, 3030 K; secondary: M6V, 2890 K, separated by 0.4''). The exoplanet pair likely hosted by the primary star orbits close to a 3:2 mean motion resonance, with periods of 2.28 and 3.49 days. These findings are supported by multi-band ground-based photometry that allowed us to confirm the planetary nature of TOI-2267.01 and .03. However, TOI-2267.02 with a period of 2.03 days remains a candidate due to its shallower transit.

The configuration with the three planets orbiting the same stellar component suggests a highly dynamically unstable system, implying the possibility of planets orbiting both stars. To date, only one other system is hypothesized to harbor planets orbiting the two stellar components of a binary: three super-Earths orbiting the two stars of K2-132, two F stars separated by 1''. However, the extremely shallow transits of these candidates orbiting K2-132 prevent any confirmation, maintaining the actual architecture of the system unknown. Hence, TOI-2267 stands out as the only system today where the existence of this unique configuration can be confirmed.

We present the results of our global analysis, the validation of the planet candidates, the planet formation scenario, and the evidence supporting the most likely architecture of the system. We discuss future plans to verify the transit of the TOI-2267.02 candidate, disentangle the host star for each planet, and confirm this odd double planetary system. These results could open a new venue to search for and characterize planetary systems in binaries of low-mass stars, profoundly impacting our understanding of how protoplanetary disks evolve to form such double planetary systems, where TOI-2267 will be a benchmark system.