TOI-532b: The Habitable-zone Planet Finder confirms a Large Super Neptune in the Neptune Desert orbiting a metal-rich M dwarf host

TL;DR

This paper confirms TOI-532b as a large super Neptune orbiting a metal-rich M dwarf, providing key data on its mass, radius, and position near the Neptune desert, and enhancing understanding of planet formation around M dwarfs.

Contribution

It presents the first precise mass and radius measurements of TOI-532b, a super Neptune around an M dwarf, and explores its implications for the Neptune desert and planetary composition.

Findings

TOI-532b has a radius of 5.82 R⊕ and a mass of 61.5 M⊕.

It is the largest and most massive super Neptune around an M dwarf with measured parameters.

The planet's position near the Neptune desert constrains planetary formation and evolution models.

Abstract

We confirm the planetary nature of TOI-532b, using a combination of precise near-infrared radial velocities with the Habitable-zone Planet Finder, TESS light curves, ground based photometric follow-up, and high-contrast imaging. TOI-532 is a faint (J$\sim 11.5$) metal-rich M dwarf with Teff = $3957\pm69$ K and [Fe/H] = $0.38\pm0.04$; it hosts a transiting gaseous planet with a period of $\sim 2.3$ days. Joint fitting of the radial velocities with the TESS and ground-based transits reveal a planet with radius of $5.82\pm0.19$ R$_{\oplus}$, and a mass of $61.5_{-9.3}^{+9.7}$ M$_{\oplus}$. TOI-532b is the largest and most massive super Neptune detected around an M dwarf with both mass and radius measurements, and it bridges the gap between the Neptune-sized planets and the heavier Jovian planets known to orbit M dwarfs. It also follows the previously noted trend between gas giants and host star metallicity for M dwarf planets. In addition, it is situated at the edge of the Neptune desert in the Radius--Insolation plane, helping place constraints on the mechanisms responsible for sculpting this region of planetary parameter space.