TOI-1696: a nearby M4 dwarf with a $3R_\oplus$ planet in the Neptunian desert

TL;DR

This paper reports the discovery and validation of a temperate sub-Neptune exoplanet around a nearby M4 dwarf, using combined TESS and ground-based observations, highlighting its potential for atmospheric studies.

Contribution

The study presents the first confirmed sub-Neptune in the Neptunian desert around a nearby M dwarf, with detailed characterization and validation using multi-instrument data.

Findings

Discovered a 3.09 R⊕ planet with a 2.5-day orbit.

Confirmed the planetary nature with high-resolution imaging and spectroscopy.

Identified the planet as a promising target for JWST atmospheric studies.

Abstract

We present the discovery and validation of a temperate sub-Neptune around the nearby mid-M dwarf TIC 470381900 (TOI-1696), with a radius of $3.09 \pm 0.11 \,R_\oplus$ and an orbital period of $2.5 \,\rm{days}$, using a combination of TESS and follow-up observations using ground-based telescopes. Joint analysis of multi-band photometry from TESS, MuSCAT, MuSCAT3, Sinistro, and KeplerCam confirmed the transit signal to be achromatic as well as refined the orbital ephemeris. High-resolution imaging with Gemini/'Alopeke and high-resolution spectroscopy with the Subaru/IRD confirmed that there are no stellar companions or background sources to the star. The spectroscopic observations with IRD and IRTF/SpeX were used to determine the stellar parameters, and found the host star is an M4 dwarf with an effective temperature of $T_{eff} = 3185 \pm 76\,\rm{K}$ and a metallicity of [Fe/H] $=0.336 \pm 0.060 \,\rm{dex}$. The radial velocities measured from IRD set a $2$-$\sigma$ upper limit on the planetary mass to be $48.8 \,M_\oplus$. The large radius ratio ($R_p/R_\star \sim 0.1$) and the relatively bright NIR magnitude ($J=12.2 \,\rm{mag}$) make this planet an attractive target for further followup observations. TOI-1696b is one of the planets belonging to the Neptunian desert with the highest transmission spectroscopy metric discovered to date, making it an interesting candidate for atmospheric characterizations with JWST.