Two Young Planetary Systems around Field Stars with Ages between 20 and 320 Myr from TESS

Abstract

Zhou et al. Planets around young stars trace the early evolution of planetary systems. We report the discovery and validation of two planetary systems with ages 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 R ⊕ 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 R ⊕ , and TOI-942c orbiting in a period of 10.16 days with a radius of 5.79 +0.19 −0.18 R ⊕. 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 ∼ 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.

TOI-2076 and TOI-1807: Two Young, Comoving Planetary Systems within 50 pc Identified by TESS that are Ideal Candidates for Further Follow Up

“…Similarly, we also follow Zhou et al (2021) and measured the levels of core emission in the Ca II infrared triplet lines, finding equivalent widths of 0.36±0.01 Åfor TOI-1807 and 0.33±0.01 Åfor TOI-2076. Using the qualitative relationships provided in Žerjal et al (2017), these core emissions are consistent with stars with ages between 100 and 1000 Myr of age.…”

“…All of the transiting planets detected around pre-main-sequence stars appear to have anomalously large sizes when compared to exoplanets around field stars, while planets with ages of 0.5-1Gyr appear to have sizes that are more consistent with those of the field population (see, e.g., Mann et al 2017;David et al 2019;Livingston et al 2019;Bouma et al 2020;Tofflemire et al 2021, and references therein). It remains to be seen whether this size-age correlation is astrophysical or due to lower detection efficiencies for young stars (e.g., Zhou et al 2021). The TOI-1807 and TOI-2076 systems exist at an intermediate age (0.1-0.5 Gyr) when the most dramatic effects of photoevaporation are expected to be complete (Jackson et al 2012), though mass loss may proceed further over gigayear timescales for some planets through either core cooling (Gupta & Schlichting 2020) or photoevaporation (Rogers & Owen 2021).…”

“…Using the calibrations provided in Zhou et al (2021), we measured equivalent widths for the core emission in the Ca II H&K lines and convert them to the Mount Wilson Observatory HK Project (Vaughan et al 1978;Wilson 1978;Duncan et al 1991;Baliunas et al 1995 , respectively. Like X-ray, the level of Ca II core emission is related to the rotation, and therefore age, of the target stars.…”

“…Both TOI-1807 and TOI-2076 exhibit significant lithium absorption features. We measured Li 6708 Åequivalent widths for these target stars using the high-resolution observations from the TRES facility, as per the techniques described in Zhou et al (2021), with equivalent widths of 0.0841±0.0070Åand 0.0703± 0.0071Åfor TOI-1807 and 2076, respectively.…”

“…For example, the THYME survey has identified several planets in known young associations spanning a diversity of Galactic environments, such as the Tucana-Horologium and Ursa Major moving groups (Newton et al 2019;Mann et al 2020), the Scorpius-Centaurus OB association (Rizzuto et al 2020), the Pisces-Eridanus stream (Newton et al 2021), and even a previously unknown association (Tofflemire et al 2021). Other searches of TESS data have revealed planets orbiting young stars in the IC 2602 cluster (Bouma et al 2020) and in the field (Zhou et al 2021).…”