An Ultra-Short Period Super-Earth and a Sub-Neptune Orbiting the K dwarf TOI-4311

TL;DR

This paper reports the discovery and detailed characterization of a multi-planet system around the K dwarf TOI-4311, including an ultra-short-period super-Earth and a longer period sub-Neptune, with potential evidence for a third planet.

Contribution

It provides new observational data and analysis of the TOI-4311 system, including possible detection of an additional non-transiting planet, and insights into planetary formation in the galaxy.

Findings

The super-Earth has a radius of about 1.38 R⊕ and a mass of approximately 4.5 M⊕.

The sub-Neptune has a radius of about 2.47 R⊕ and a period of 15 days.

A third potential planet with a 38-day period and 26.4 M⊕ is suggested by RV data but not confirmed by transits.

Abstract

We report the discovery and characterisation of the multi-planetary system around TOI-4311, a K dwarf kinematically between the Galactic thick disk and Hercules stream. TOI-4311 hosts an ultra-short-period super-Earth (P$\sim$0.99 d, $1.376\substack{+0.077\\-0.080}$ R$_\oplus$) and a longer period sub-Neptune (P$\sim$15 d, $2.47\substack{+0.12\\-0.11}$ R$_\oplus$) that was first detected in the TESS photometry. Using follow-up observations with CHEOPS and HARPS, we refine the planetary radius of both planets, derive the mass of planet b ($4.5\substack{+1.5\\-1.4}$ M$_\oplus$) and confirm the planetary nature of planet c. Intriguingly, a third periodic signal is clearly detected in our HARPS RVs that we cannot link to stellar activity. This signal could be attributed to a third planet (P$\sim$38 d, Msin(i)=$26.4\substack{+6.3\\-6.8}$ M$_\oplus$) in the system, however with the current photometric dataset 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.