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An ultra-short-period super-Earth and a sub-Neptune orbiting the K dwarf TOI-4311
Authors: Eschen, YNE; Wilson, TG; Cameron, AC; ...; Correia, ACM
Ref.: Mon. Not. R. Astron. Soc. 549(3), stag952 (2026)
Abstract: We report the discovery and characterization of the multiplanetary system around TOI-4311, a K dwarf kinematically between the Galactic thick disc and Hercules stream. TOI-4311 hosts an ultra-short-period super-Earth (P similar to 0.99 d, 1 . 376(-0.080)(+0 . 077) R-circle plus) and a longer period sub-Neptune (P similar to 15 d, 2.47(-0 . 11)(+0.12) R-circle plus) that was first detected in the Transiting Exoplanet Survey Satellite photometry. Using follow-up observations with CHaracterising ExOPlanet Satellite and High Accuracy Radial Velocity Planet Searcher (HARPS), we refine the planetary radius of both planets, derive the mass of planet b ( 4 . 5(-1.4)(+1 . 5) M-circle plus), and confirm the planetary nature of planet c. Intriguingly, a third periodic signal is clearly detected in our HARPS Radial Velocities (RVs) that we cannot link to stellar activity. This signal could be attributed to a third planet (P similar to 38 d, Msin(i) = 26.4(-6.8)(+6 . 3) M-circle plus) in the system; however, with the current photometric data set we do not find a transit. Our dynamical analysis highlights that this potential outer planet would remain stable. Using the precise radius and mass for TOI-4311 b, we model its interior structure and find that it is very dense given the host star's galactic kinematics and chemistry. Hence, this system could challenge current formation theories and provide insights into planet formation across the galaxy.
