Two young planetary systems around field stars with ages between 20-320 Myr from TESS

Kavli Affiliate: Andrew M. Vanderburg

| First 5 Authors: George Zhou, Samuel N. Quinn, Jonathan Irwin, Chelsea X. Huang, Karen A. Collins

| Summary:

Planets around young stars trace the early evolution of planetary systems. We
report the discovery and validation of two planetary systems with ages
$lesssim 300$ Myr from observations by the Transiting Exoplanet Survey
Satellite. TOI-251 is a 40-320 Myr old G star hosting a 2.74 +0.18/-0.18 REarth
mini-Neptune with a 4.94 day period. TOI-942 is a 20-160 Myr old K star hosting
a system of inflated Neptune-sized planets, with TOI-942b orbiting with a
period of 4.32 days, with a radius of 4.81 +0.20/-0.20 REarth, and TOI-942c
orbiting in a period of 10.16 days with a radius of 5.79 +0.19/-0.18 REarth.
Though we cannot place either host star into a known stellar association or
cluster, we can estimate their ages via their photometric and spectroscopic
properties. Both stars exhibit significant photometric variability due to spot
modulation, with measured rotation periods of $sim 3.5$ days. These stars also
exhibit significant chromospheric activity, with age estimates from the
chromospheric calcium emission lines and X-ray fluxes matching that estimated
from gyrochronology. Both stars also exhibit significant lithium absorption,
similar in equivalent width to well-characterized young cluster members. TESS
has the potential to deliver a population of young planet-bearing field stars,
contributing significantly to tracing the properties of planets as a function
of their age.

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