TESS discovery of a sub-Neptune orbiting a mid-M dwarf TOI-2136

Abstract

We present the discovery of TOI-2136b, a sub-Neptune planet transiting every 7.85 days a nearby M4.5V-type star, identified through photometric measurements from the TESS mission. The host star is located $33$ pc away with a radius of $R_{\ast }=0.34\pm 0.02R_{\odot }$, a mass of $0.34\pm 0.02M_{\odot }$ and an effective temperature of $3342\pm 100\mathrm{K}$. We estimate its stellar rotation period to be $75\pm 5$ days based on archival long-term photometry. We confirm and characterize the planet based on a series of ground-based multi-wavelength photometry, high-angular-resolution imaging observations, and precise radial velocities from CFHT/SPIRou. Our joint analysis reveals that the planet has a radius of $2.19\pm 0.17R_{\oplus }$, and a mass measurement of $6.4\pm 2.4M_{\oplus }$. The mass and radius of TOI2136b is consistent with a broad range of compositions, from water-ice to gas-dominated worlds. TOI-2136b falls close to the radius valley for low-mass stars predicted by the thermally driven atmospheric mass loss models, making it an interesting target for future studies of its interior structure and atmospheric properties.

19 pages, 15 figures, submitted to MNRAS

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