TOI 540 b: A Planet Smaller than Earth Orbiting a Nearby Rapidly Rotating Low-mass Star

Abstract

We present the discovery of TOI 540 b, a hot planet slightly smaller than Earth orbiting the low-mass star 2MASS J05051443-4756154. The planet has an orbital period of $P=1.239149$ days ($\pm$ 170 ms) and a radius of $r=0.903\pm 0.052R_{\mathrm{Earth}}$, and is likely terrestrial based on the observed mass-radius distribution of small exoplanets at similar insolations. The star is 14.008 pc away and we estimate its mass and radius to be $M=0.159\pm 0.014M_{\mathrm{Sun}}$ and $R=0.1895\pm 0.0079R_{\mathrm{Sun}}$, respectively. The star is distinctive in its very short rotational period of $P_{\mathrm{rot}}=17.4264+/-0.0094$ hours and correspondingly small Rossby number of 0.007 as well as its high X-ray-to-bolometric luminosity ratio of $L_X/L_{\mathrm{bol}}=0.0028$ based on a serendipitous XMM-Newton detection during a slew operation. This is consistent with the X-ray emission being observed at a maximum value of $L_X/L_{\mathrm{bol}}\simeq 10^{-3}$ as predicted for the most rapidly rotating M dwarfs. TOI 540 b may be an alluring target to study atmospheric erosion due to the strong stellar X-ray emission. It is also among the most accessible targets for transmission and emission spectroscopy and eclipse photometry with JWST, and may permit Doppler tomography with high-resolution spectroscopy during transit. This discovery is based on precise photometric data from TESS and ground-based follow-up observations by the MEarth team.

18 pages, 7 figures. Accepted for publication in The Astronomical Journal

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