Refining the transit timing and photometric analysis of TRAPPIST-1: Masses, radii, densities, dynamics, and ephemerides

TL;DR

This study refines the masses, densities, and orbital dynamics of the TRAPPIST-1 system using four years of transit data, revealing a mostly rocky composition and extremely stable, coplanar orbits, with implications for planetary formation.

Contribution

It provides the most precise planetary mass and density measurements to date for TRAPPIST-1, and offers new insights into its composition and orbital stability using comprehensive transit and dynamical analysis.

Findings

All seven planets have densities consistent with a rocky composition depleted in iron.

Planet masses are measured with 3-5% precision, surpassing current radial velocity capabilities.

The system's orbits are extremely coplanar and stable over 10 million years.

Abstract

We have collected transit times for the TRAPPIST-1 system with the Spitzer Space Telescope over four years. We add to these ground-based, HST and K2 transit time measurements, and revisit an N-body dynamical analysis of the seven-planet system using our complete set of times from which we refine the mass ratios of the planets to the star. We next carry out a photodynamical analysis of the Spitzer light curves to derive the density of the host star and the planet densities. We find that all seven planets' densities may be described with a single rocky mass-radius relation which is depleted in iron relative to Earth, with Fe 21 wt% versus 32 wt% for Earth, and otherwise Earth-like in composition. Alternatively, the planets may have an Earth-like composition, but enhanced in light elements, such as a surface water layer or a core-free structure with oxidized iron in the mantle. We measure planet masses to a precision of 3-5%, equivalent to a radial-velocity (RV) precision of 2.5 cm/sec, or two orders of magnitude more precise than current RV capabilities. We find the eccentricities of the planets are very small; the orbits are extremely coplanar; and the system is stable on 10 Myr timescales. We find evidence of infrequent timing outliers which we cannot explain with an eighth planet; we instead account for the outliers using a robust likelihood function. We forecast JWST timing observations, and speculate on possible implications of the planet densities for the formation, migration and evolution of the planet system.