Planet Hunters TESS V: a planetary system around a binary star, including a mini-Neptune in the habitable zone

TL;DR

This paper reports the discovery of a potentially habitable mini-Neptune orbiting a bright G dwarf star, in a complex binary system, using TESS data, citizen science, and follow-up observations, providing insights into planet formation in binaries.

Contribution

It presents the first validation of a long-period mini-Neptune in a binary system with a stellar companion, combining TESS data, citizen science, and archival observations.

Findings

Discovered a mini-Neptune with a 272-day orbit in the habitable zone.

Detected a non-transiting planet candidate with a 34-day period.

Identified a stellar companion with a 230-year orbit and high eccentricity.

Abstract

We report on the discovery and validation of a transiting long-period mini-Neptune orbiting a bright (V = 9.0 mag) G dwarf (TOI 4633; R = 1.05 RSun, M = 1.10 MSun). The planet was identified in data from the Transiting Exoplanet Survey Satellite by citizen scientists taking part in the Planet Hunters TESS project. Modeling of the transit events yields an orbital period of 271.9445 +/- 0.0040 days and radius of 3.2 +/- 0.20 REarth. The Earth-like orbital period and an incident flux of 1.56 +/- 0.2 places it in the optimistic habitable zone around the star. Doppler spectroscopy of the system allowed us to place an upper mass limit on the transiting planet and revealed a non-transiting planet candidate in the system with a period of 34.15 +/- 0.15 days. Furthermore, the combination of archival data dating back to 1905 with new high angular resolution imaging revealed a stellar companion orbiting the primary star with an orbital period of around 230 years and an eccentricity of about 0.9. The long period of the transiting planet, combined with the high eccentricity and close approach of the companion star makes this a valuable system for testing the formation and stability of planets in binary systems.