[Paper Review] TOI-1634 b: An Ultra-short-period Keystone Planet Sitting inside the M-dwarf Radius Valley

Ryan Cloutier, David Charbonneau+64 more|arXiv (Cornell University)|Aug 1, 2021

Stellar, planetary, and galactic studiesPhysics and Astronomy131 references33 citations

TL;DR

This paper presents the discovery and characterization of TOI-1634 b, an ultra-short-period (0.989-day) exoplanet orbiting an M2 dwarf, which lies within the M-dwarf radius valley. With a radius of 1.790+0.080−0.081 R⊕ and a mass of 4.91+0.68−0.70 M⊕, its bulk composition is inconsistent with Earth-like rocky composition at 5.9σ, indicating a volatile-rich layer or non-terrestrial rocky composition, supporting the gas-depleted formation scenario over thermally-driven mass loss for M-dwarf planets.

ABSTRACT

Studies of close-in planets orbiting M dwarfs have suggested that the M dwarf radius valley may be well-explained by distinct formation timescales between enveloped terrestrials, and rocky planets that form at late times in a gas-depleted environment. This scenario is at odds with the picture that close-in rocky planets form with a primordial gaseous envelope that is subsequently stripped away by some thermally-driven mass loss process. These two physical scenarios make unique predictions of the rocky/enveloped transition's dependence on orbital separation such that studying the compositions of planets within the M dwarf radius valley may be able to establish the dominant physics. Here, we present the discovery of one such keystone planet: the ultra-short period planet TOI-1634 b ($P=0.989$ days, $F=121 F_{\oplus}$, $r_p = 1.790^{+0.080}_{-0.081} R_{\oplus}$) orbiting a nearby M2 dwarf ($K_s=8.7$, $R_s=0.45 R_{\odot}$, $M_s=0.50 M_{\odot}$) and whose size and orbital period sit within the M dwarf radius valley. We confirm the TESS-discovered planet candidate using extensive ground-based follow-up campaigns, including a set of 32 precise radial velocity measurements from HARPS-N. We measure a planetary mass of $4.91^{+0.68}_{-0.70} M_{\oplus}$, which makes TOI-1634 b inconsistent with an Earth-like composition at $5.9\sigma$ and thus requires either an extended gaseous envelope, a large volatile-rich layer, or a rocky portion that is not dominated by iron and silicates to explain its mass and radius. The discovery that the bulk composition of TOI-1634 b is inconsistent with that of the Earth favors the gas-depleted formation mechanism to explain the emergence of the radius valley around M dwarfs with $M_s\lesssim 0.5 M_{\odot}$.

Motivation & Objective

• To determine the bulk composition of a keystone planet within the M-dwarf radius valley to test competing formation models.
• To validate the planetary nature of TESS-discovered candidate TOI-1634 b using multi-wavelength follow-up observations.
• To assess whether the planet's mass and radius are consistent with thermally-driven mass loss or gas-depleted formation scenarios.
• To evaluate the implications of the planet's composition for the emergence of the radius valley around low-mass stars.
• To guide future atmospheric characterization with JWST by identifying planets resistant to hydrodynamic escape.

Proposed method

• Used TESS photometry to detect transit signals and validate the planetary nature of TOI-1634 b through extensive ground-based follow-up.
• Conducted 32 precise radial velocity measurements using HARPS-N to determine the planet's mass.
• Combined photometric and radial velocity data in a global Bayesian analysis to derive the planet's radius, mass, and orbital parameters.
• Applied stellar characterization techniques using Gaia, 2MASS, and ground-based photometry to determine host star properties.
• Used atmospheric modeling and eclipse observation simulations to predict JWST detectability of TOI-1634 b's atmosphere.
• Evaluated radial velocity sensitivity to rule out additional planets in the system, particularly in the habitable zone.

Experimental results

Research questions

• RQ1 Is the bulk composition of TOI-1634 b consistent with an Earth-like rocky planet, or does it require a gaseous envelope or volatile-rich layer?
• RQ2 Does the planet's location within the M-dwarf radius valley support the gas-depleted formation model over thermally-driven mass loss?
• RQ3 Can the planet's radius and mass be explained by core-powered mass loss or photoevaporation?
• RQ4 What atmospheric properties make TOI-1634 b resistant to hydrodynamic escape?
• RQ5 What is the detectability of atmospheric features in TOI-1634 b with future JWST observations?

Key findings

• TOI-1634 b has an orbital period of 0.989 days, radius of 1.790+0.080−0.081 R⊕, and mass of 4.91+0.68−0.70 M⊕, placing it within the M-dwarf radius valley.
• The planet's mass and radius are inconsistent with an Earth-like composition at 5.9σ significance, ruling out a silicate-iron rocky composition.
• The planet's composition is best explained by a volatile-rich layer with high mean molecular weight or a Ca/Al-enriched, under-dense rocky composition.
• The observed properties of TOI-1634 b are inconsistent with models of thermally-driven mass loss, such as photoevaporation or core-powered mass loss.
• The data support the gas-depleted formation scenario as the dominant mechanism shaping the M-dwarf radius valley for stars with masses ≲0.5 M⊙.
• JWST observations with MIRI or NIRSpec could distinguish between H2O-dominated, CO2-dominated, and solar-composition atmospheres with 2–5 eclipse observations.