# A Low-Mass Exoplanet Candidate Detected By ${\it K2}$ Transiting the Praesepe M Dwarf JS 183

## TL;DR

This paper reports the discovery of a Neptune-sized transiting exoplanet around a low-mass M dwarf in the Praesepe cluster, providing valuable data to test stellar models at the fully convective boundary.

## Contribution

First detection of a Neptune-sized transiting planet around a low-mass M dwarf in a young open cluster, with implications for stellar structure models.

## Key findings

- The planet has a 10.13-day orbit and a radius of 0.32 R_J.
- The host star's density is the lowest known at the fully convective boundary.
- The star's low density challenges current stellar models near the fully convective boundary.

## Abstract

We report the discovery of a repeating photometric signal from a low-mass member of the Praesepe open cluster that we interpret as a Neptune-sized transiting planet. The star is JS 183 (HSHJ 163, EPIC 211916756) with $T_{\rm eff} = 3325\pm100$ K, $M_{*} = 0.44\pm0.04$ $M_{\odot}$, $R_{*} = 0.44\pm0.03$ $R_{\odot}$, and $\log{g_*} = 4.82\pm0.06$. The planet has an orbital period of 10.134588 days and a radius of $R_{P}= 0.32\pm0.02$ $R_J$. Since the star is faint at $V=16.5$ and $J=13.3$, we are unable to obtain a measured radial-velocity orbit, but we can constrain the companion mass to below about 1.7 $M_J$, and thus well below the planetary boundary. JS 183b (since designated as K2-95b) is the second transiting planet found with ${\it K2}$ that resides in a several hundred Myr open cluster; both planets orbit mid-M dwarf stars and are approximately Neptune-sized. With a well-determined stellar density from the planetary transit, and with an independently known metallicity from its cluster membership, JS 183 provides a particularly valuable test of stellar models at the fully convective boundary. We find that JS 183 is the lowest-density transit host known at the fully convective boundary, and that its very low density is consistent with current models of stars just above the fully convective boundary but in tension with the models just below the fully convective boundary.