TOI-1075 b: A Dense, Massive, Ultra-short-period Hot Super-Earth Straddling the Radius Gap

Abstract

Populating the exoplanet mass–radius diagram in order to identify the underlying relationship that governs planet composition is driving an interdisciplinary effort within the exoplanet community. The discovery of hot super-Earths—a high-temperature, short-period subset of the super-Earth planet population—has presented many unresolved questions concerning the formation, evolution, and composition of rocky planets. We report the discovery of a transiting, ultra-short-period hot super-Earth orbiting TOI-1075 (TIC351601843), a nearby (d = 61.4 pc) late-K/early-M-dwarf star, using data from the Transiting Exoplanet Survey Satellite. The newly discovered planet has a radius of 1.791 − 0.081 + 0.116 R ⊕ and an orbital period of 0.605 day (14.5 hr). We precisely measure the planet mass to be 9.95 − 1.30 + 1.36 M ⊕ using radial velocity measurements obtained with the Planet Finder Spectrograph mounted on the Magellan II telescope. Our radial velocity data also show a long-term trend, suggesting an additional planet in the system. While TOI-1075 b is expected to have a substantial H/He atmosphere given its size relative to the radius gap, its high density ( 9.32 − 1.85 + 2.05 g cm−3) is likely inconsistent with this possibility. We explore TOI-1075 b’s location relative to the M-dwarf radius valley, evaluate the planet’s prospects for atmospheric characterization, and discuss potential planet formation mechanisms. Studying the TOI-1075 system in the broader context of ultra-short-period planetary systems is necessary for testing planet formation and evolution theories and density-enhancing mechanisms and for future atmospheric and surface characterization studies via emission spectroscopy with the JWST.

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

“…With a core mass larger than 10 M ⊕ , TOI-1347 b, along with the USPs TOI-1075 b (Essack et al 2023) and HD 20329 b (Murgas et al 2022), are close to the theoretical limit for runaway accretion (see, e.g., Rafikov 2006). How did these planets evade runaway accretion and not become gas giants?…”

A super-Earth and a mini-Neptune near the 2:1 MMR straddling the radius valley around the nearby mid-M dwarf TOI-2096

“…For example, combining Doppler surveys with photometry observations provided by ground-and space-based missions has revealed a diversity of compositions (Hatzes & Rauer 2015;Luque & Pallé 2022). Some are highdensity "super-Earths", massive rocky planets (ρ ≳ 5.0 g cm −3 , e.g., Kreidberg et al 2019;Jindal et al 2020;Essack et al 2023). Others named "mini-Neptunes" have densities low enough to be consistent with the presence of extended H/He envelopes around a solid core (ρ ≲ 4 g cm −3 , e.g., Rogers et al 2011;Kite et al 2020).…”

A super-Earth and a mini-Neptune near the 2:1 MMR straddling the radius valley around the nearby mid-M dwarf TOI-2096

“…For example, combining Doppler surveys with photometry observations provided by ground-and space-based missions has revealed a diversity of compositions (Hatzes & Rauer 2015;Luque & Pallé 2022). Some are high-density "super-Earths", massive rocky planets (ρ 5.0 g/cm 3 , e.g., Kreidberg et al 2019;Jindal et al 2020;Essack et al 2023). Others named "mini-Neptunes" have densities low enough to be consistent with the presence of extended H/He envelopes around a solid core (ρ 4 g/cm 3 , e.g., Rogers et al 2011;Kite et al 2020).…”