# A seven-planet resonant chain in TRAPPIST-1

## TL;DR

The paper reports the discovery of a seven-planet resonant chain in the TRAPPIST-1 system, with new orbital data, resonance relations, and planetary characteristics, enhancing understanding of its complex orbital dynamics.

## Contribution

It provides the first measurement of TRAPPIST-1h's orbital period and confirms a resonant chain with three-body resonances linking all seven planets.

## Key findings

- TRAPPIST-1h has an orbital period of 18.77 days.
- The planetary system forms a complex resonant chain.
- TRAPPIST-1's star has a rotational period of 3.3 days.

## Abstract

The TRAPPIST-1 system is the first transiting planet system found orbiting an ultra-cool dwarf star. At least seven planets similar to Earth in radius and in mass were previously found to transit this host star. Subsequently, TRAPPIST-1 was observed as part of the K2 mission and, with these new data, we report the measurement of an 18.77 d orbital period for the outermost planet, TRAPPIST-1h, which was unconstrained until now. This value matches our theoretical expectations based on Laplace relations and places TRAPPIST-1h as the seventh member of a complex chain, with three-body resonances linking every member. We find that TRAPPIST-1h has a radius of 0.727 Earth radii and an equilibrium temperature of 173 K. We have also measured the rotational period of the star at 3.3 d and detected a number of flares consistent with a low-activity, middle-aged, late M dwarf.