# The Transiting Multi-planet System HD15337: Two Nearly Equal-mass Planets Straddling the Radius Gap

## TL;DR

This paper reports the discovery and characterization of two nearly equal-mass planets around HD 15337, one rocky super-Earth and one likely atmospheric sub-Neptune, providing insights into planet formation and atmospheric evolution.

## Contribution

It presents the first detailed mass and radius measurements of a super-Earth and sub-Neptune pair near the radius gap, and models the star's high-energy emission history.

## Key findings

- HD 15337b is a rocky super-Earth with 7.51 M_Earth and 1.64 R_Earth.
- HD 15337c is a sub-Neptune with 8.11 M_Earth and 2.39 R_Earth.
- The star's high-energy emission was significantly higher 150 Myr ago.

## Abstract

We report the discovery of a super-Earth and a sub-Neptune transiting the star HD 15337 (TOI-402, TIC 120896927), a bright (V=9) K1 dwarf observed by the Transiting Exoplanet Survey Satellite (TESS) in Sectors 3 and 4. We combine the TESS photometry with archival HARPS spectra to confirm the planetary nature of the transit signals and derive the masses of the two transiting planets. With an orbital period of 4.8 days, a mass of 7.51(+1.09)(-1.01) M_Earth, and a radius of 1.64+/-0.06 R_Earth, HD 15337b joins the growing group of short-period super-Earths known to have a rocky terrestrial composition. The sub-Neptune HD 15337c has an orbital period of 17.2 days, a mass of 8.11(+1.82)(-1.69) M_Earth, and a radius of 2.39+/-0.12 R_Earth, suggesting that the planet might be surrounded by a thick atmospheric envelope. The two planets have similar masses and lie on opposite sides of the radius gap, and are thus an excellent testbed for planet formation and evolution theories. Assuming that HD 15337c hosts a hydrogen-dominated envelope, we employ a recently developed planet atmospheric evolution algorithm in a Bayesian framework to estimate the history of the high-energy (extreme ultraviolet and X-ray) emission of the host star. We find that at an age of 150 Myr, the star possessed on average between 3.7 and 127 times the high-energy luminosity of the current Sun.