New study explores whether TRAPPIST-1’s seven planets could host moons

At a distance of approximately 40 light-years, the TRAPPIST-1 system consists of seven terrestrial-mass planets orbiting an ultracool M-dwarf star in one of the most compact planetary architectures currently known. Since its identification in 2017, the system has been of particular interest because three of its planets reside within the circumstellar habitable zone, where surface liquid water is theoretically possible. A long-standing unresolved issue, however, has been whether the strong gravitational interactions within this tightly packed system would allow natural satellites to remain dynamically stable.

A recent study by Shubham Dey and Sean Raymond, posted on the arXiv preprint server, addresses this question using extensive numerical modeling. Their results indicate that satellite stability is dynamically permitted, provided that any moons remain small in mass and orbit sufficiently close to their host planets.

The authors performed thousands of N-body simulations in which hypothetical moons were placed around each TRAPPIST-1 planet. For every planet, 100 test satellites were initialized on circular orbits, distributed from the Roche limit—inside of which tidal disruption would occur—outward toward the planet’s gravitational sphere of influence.

In simulations where each planet was treated in isolation, satellite orbits remained stable from the Roche limit out to approximately 50% of the Hill radius, in agreement with standard analytical expectations for satellite stability.

However, the TRAPPIST-1 planets are not dynamically isolated. They form a near-resonant orbital chain, leading to persistent mutual gravitational perturbations. When the simulations incorporated all seven planets simultaneously, the stable satellite region around each planet was reduced. In particular, the inner planet TRAPPIST-1 b and TRAPPIST-1 e, which lies within the habitable zone, experienced the strongest reduction in satellite stability.

Under full system interactions, the outer boundary of stable satellite orbits contracted to roughly 40–45% of each planet’s Hill radius. While the perturbation from any single neighboring planet was relatively weak, the combined resonant forcing of the entire system produced a cumulative narrowing of the stable region.

Despite this contraction, the simulations demonstrate that stable satellite orbits remain viable, provided that the moons orbit sufficiently close to their parent planets.

The study also examined long-term tidal evolution. Tidal dissipation within the planet–moon system causes orbital decay, leading sufficiently massive satellites to spiral inward and eventually collide with their host planets over gigayear timescales. The authors find that, over the estimated lifetime of TRAPPIST-1, only satellites with masses below approximately 10⁻⁷ Earth masses could survive tidal decay. The outer planets may permit marginally larger moons due to weaker tidal effects.

The existence of such moons remains observationally unconstrained, as current detection techniques lack the sensitivity required to identify satellites of this size in the TRAPPIST-1 system. Nevertheless, the results demonstrate that the system’s compact, resonant configuration does not dynamically forbid moons. Instead, it imposes stringent constraints on satellite mass and orbital distance, favoring small, tightly bound moons.

Con información de arXiv