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

Kavli Affiliate: Robert Goeke

| First 5 Authors: Davide Gandolfi, Luca Fossati, John H. Livingston, Keivan G. Stassun, Sascha Grziwa

| Summary:

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.

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