The TESS-Keck Survey. XII. A Dense 1.8 R _⊕ Ultra-short-period Planet Possibly Clinging to a High-mean-molecular-weight Atmosphere after the First Gigayear

Abstract

The extreme environments of ultra-short-period planets (USPs) make excellent laboratories to study how exoplanets obtain, lose, retain, and/or regain gaseous atmospheres. We present the confirmation and characterization of the USP TOI-1347 b, a 1.8 ± 0.1 R ⊕ planet on a 0.85 day orbit that was detected with photometry from the TESS mission. We measured radial velocities of the TOI-1347 system using Keck/HIRES and HARPS-N and found the USP to be unusually massive at 11.1 ± 1.2 M ⊕. The measured mass and radius of TOI-1347 b imply an Earth-like bulk composition. A thin H/He envelope (>0.01% by mass) can be ruled out at high confidence. The system is between 1 and 1.8 Gyr old; therefore, intensive photoevaporation should have concluded. We detected a tentative phase-curve variation (3σ) and a secondary eclipse (2σ) in TESS photometry, which, if confirmed, could indicate the presence of a high-mean-molecular-weight atmosphere. We recommend additional optical and infrared observations to confirm the presence of an atmosphere and investigate its composition.

Peas-in-a-pod across the Radius Valley: Rocky Systems Are Less Uniform in Mass but More Uniform in Size and Spacing

“…Weiss & Marcy (2014) posited that these two regimes of the M-R parameter space may be separated by a threshold radius R cut ≈ 1.5 R ⊕ , while Rogers (2015) determined via a hierarchical Bayesian framework that this boundary may instead lie closer to R cut ≈ 1.6 R ⊕ , a value that has since been corroborated by recent empirical fitting of expanded planetary catalogs (Lozovsky et al 2018;Baron et al 2023). Furthermore, it has been shown that extant precise (δM p /M p  0.3) mass measurements for rocky worlds with R cut 1.6 R ⊕ yield exceptionally close agreement (Rodríguez Martínez et al 2023;Rubenzahl et al 2024) with the semi-empirical M-R relation for Earth-composition (30% Fe, 70% MgSiO 3 ) planets presented by Zeng et al (2016). As such, we adopt hereafter this R cut = 1.6 R ⊕ threshold as the basis for classification of rocky and volatile-rich worlds throughout this analysis.…”

“…constituent planets, indicating that these CKS samples lack the requisite associated planetary mass data for a population-level uniformity analysis. However, this dearth may be alleviated in part by the substantial efficacy with which the M-R bivariate behavior of small, rocky (R p 1.6 R ⊕ ) worlds may be modeled (see Figure 5 of Rodríguez Martínez et al 2023; Figure 8 of Rubenzahl et al 2024) by the semi-empirical M-R relation for Earth-like planets presented by Zeng et al (2016). We therefore utilize this Zeng et al (2016) Earth-composition model to robustly infer planetary masses M p for the 118 objects (across 48 systems) within our CKS rocky sample (Figure 3, gold curve and points).…”

“…Despite the substantial efficacy of the Zeng et al (2016) Earth-like (30% Fe, 70% MgSiO 3 ) model in describing the M-R behavior of small, rocky worlds (Rodríguez Martínez et al 2023;Rubenzahl et al 2024), we wish to ascertain the effects of alternative compositional assumptions on primary statistical results. We repeat our mass uniformity test (Section 5.3) with two additional semi-empirical M-R prescriptions from Zeng et al (2016), a more iron-rich (50% Fe, 50% MgSiO 3 ) compositional model and a "pure rock" model that are respectively consistent with slightly higher and lower bulk densities than the Earth-like curve (see Figure 3).…”