Spin-orbital Tidal Dynamics and Tidal Heating in the TRAPPIST-1
Multiplanet System
Abstract
We perform numerical simulations of the TRAPPIST-1 system of seven
exoplanets orbiting a nearby M dwarf, starting with a previously
suggested stable configuration. The long-term stability of this
configuration is confirmed, but the motion of planets is found to be
chaotic. The eccentricity values are found to vary within finite ranges.
The rates of tidal dissipation and tidal evolution of orbits are
estimated, assuming an Earth-like rheology for the planets. We find that
under this assumption, the planets b, d, and e were captured in the 3:2
or higher spin–orbit resonances during the initial spin-down, but
slipped further down into the 1:1 resonance. Depending on its rheology,
the innermost planet b may be captured in a stable pseudosynchronous
rotation. Nonsynchronous rotation ensures higher levels of tidal
dissipation and internal heating. The positive feedback between the
viscosity and the dissipation rate—and the ensuing runaway
heating—are terminated by a few self-regulation processes. When the
temperature is high and the viscosity is low enough, the planet
spontaneously leaves the 3:2 resonance. Further heating is stopped
either by passing the peak dissipation or by the emergence of partial
melt in the mantle. In the post-solidus state, the tidal dissipation is
limited to the levels supported by the heat transfer efficiency. The
tides on the host star are unlikely to have had a significant dynamical
impact. The tides on the synchronized inner planets tend to reduce these
planets’ orbital eccentricity, possibly contributing thereby to the
system’s stability.