An upper limit on late accretion and water delivery in the TRAPPIST-1 exoplanet system

• Published in: Nature astronomy Vol. 6; no. 1; pp. 80 - 88
• Main Authors: Raymond, Sean N., Izidoro, Andre, Bolmont, Emeline, Dorn, Caroline, Selsis, Franck, Turbet, Martin, Agol, Eric, Barth, Patrick, Carone, Ludmila, Dasgupta, Rajdeep, Gillon, Michael, Grimm, Simon L.
• Format: Journal Article Web Resource
• Language: English
• Published: London Nature Publishing Group UK 01-01-2022; Nature Publishing Group; Springer Nature
• Subjects: 639/33/34/862; 639/33/445/328; 639/33/445/862; Accretion; Astronomy; Astrophysics; Astrophysics and Cosmology; Aérospatiale, astronomie & astrophysique; Earth; Earth and Planetary Astrophysics; Letter; Moon; Physical, chemical, mathematical & earth Sciences; Physics; Physics and Astronomy; Physique, chimie, mathématiques & sciences de la terre; Planets; Space science, astronomy & astrophysics; Water delivery
• ISSN: 2397-3366; 2397-3366
• DOI: 10.1038/s41550-021-01518-6

The TRAPPIST-1 system contains seven roughly Earth-sized planets locked in a multiresonant orbital configuration
1
,
2
, which has enabled precise measurements of the planets’ masses and constrained their compositions
3
. Here we use the system’s fragile orbital structure to place robust upper limits on the planets’ bombardment histories. We use
N
-body simulations to show how perturbations from additional objects can break the multiresonant configuration by either triggering dynamical instability or simply removing the planets from resonance. The planets cannot have interacted with more than ~5% of one Earth mass (
M
⊕
) in planetesimals—or a single rogue planet more massive than Earth’s Moon—without disrupting their resonant orbital structure. This implies an upper limit of 10
−4
M
⊕
to 10
−2

M
⊕
of late accretion on each planet since the dispersal of the system’s gaseous disk. This is comparable to (or less than) the late accretion on Earth after the Moon-forming impact
4
,
5
, and demonstrates that the growth of the TRAPPIST-1 planets was complete in just a few million years, roughly an order of magnitude faster than that of the Earth
6
,
7
. Our results imply that any large water reservoirs on the TRAPPIST-1 planets must have been incorporated during their formation in the gaseous disk.
The resonant chain of the TRAPPIST-1 planets is dynamically fragile, as small perturbations during its lifetime would have disrupted it.
N
-body simulations show that the system could not have interacted with more than 0.05 Earth masses of material after its formation. Thus, any water in the planets must come from the planets’ original accretion.