TRAPPIST-1, an ultracool dwarf star located 38.8 light-years away in the constellation of Aquarius, hosts seven planets. The period ratios of neighboring pairs are close to the 8:5, 5:3, 3:2, 3:2, 4:3 and 3:2 ratios in increasing distance from the star. This compact, resonant configuration is a manifest sign of disk-driven migration; however, the preferred outcome of such evolution is the establishment of first-order resonances, not the high-order resonances observed in the inner system. Caltech astronomer Gabriele Pichierri and colleagues explain the orbital configuration of the TRAPPIST-1 system with a model that is largely independent of the specific disk migration and orbital circularization efficiencies. Together with migration, the two key elements of the team’s model are that the inner border of the protoplanetary disk receded with time and that the TRAPPIST-1 system was initially separated into two subsystems.
This artist’s impression displays TRAPPIST-1 and its planets reflected in a surface. Image credit: NASA / R. Hurt / T. Pyle.
“When we had only our own Solar System to analyze, one could just assume that the planets formed in the places where we find them today,” Dr. Pichierri said.
“However, when we discovered even the first exoplanet in 1995, we had to reconsider this assumption.”
“We are developing better models for how planets are formed and how they come to be in the orientations we find them in.”
Most exoplanets form out of the disk of gas and dust around newly formed stars and are then expected to migrate inward approaching the inner boundary of this disk.
This assembles planetary systems that are much closer to the host star than is the case in our Solar System.
In the absence of other factors, planets will tend to space themselves apart from one another at characteristic distances based on their masses and gravitational forces between the planets and their host star.
“This is the standard migration process,” Dr. Pichierri said.
“The positions of the planets form resonances between their orbital periods. If you take the orbital period of one planet and then you divide it by the orbital period of its neighboring planet, you get a ratio of simple integers, such as 3:2.”
So, for example, if one planet takes two days to orbit around its star, the next planet, farther out, will take three days.
If that second planet and a third one farther out are also in a 3:2 resonance, then the third planet’s orbital period will be 4.5 days.
“The outer planets behave properly, so to speak, with the simpler expected resonances,” Dr. Pichierri said.
“But the inner ones have resonances that are a bit spicier. The ratio between planet b and c’s orbits is 8:5, for example, and that between c and d is 5:3.”
“This narrow discrepancy in the outcome of TRAPPIST-1’s assembly is puzzling and represents a wonderful opportunity to figure out in detail what other processes were at play in its assembly.”
“In addition, most planetary systems are thought to have started in these resonant states but have encountered significant instabilities in their lifespan before we observe them today.”
“Most planets go unstable or collide with one another, and everything gets shuffled. Our own Solar System, for example, was affected by such an instability.”
“But we know of a few systems that have remained stable, that are more or less pristine specimens.”
“They, in effect, exhibit a record of their entire dynamical history that we can then attempt to reconstruct. TRAPPIST-1 is one of these.”
The challenge then was to develop a model that could explain the orbits of the TRAPPIST-1 planets and how they reached their current configuration.
The resulting model suggests that the inner four planets initially evolved alone in the expected 3:2 resonance chain.
It was only as the disk’s inner boundary expanded outward that their orbits relaxed out of the tighter 3:2 chain into the configuration we observe today.
The fourth planet, which originally sat on the inner boundary of the disk, moving farther out along with it, was later pushed back inward when three additional outer planets joined the planetary system at a later stage.
“By looking at TRAPPIST-1, we have been able to test exciting new hypotheses for the evolution of planetary systems,” Dr. Pichierri said.
“TRAPPIST-1 is very interesting because it is so intricate; it’s a long planetary chain. And it’s a great exemplar for testing alternative theories about planetary system formation.”
The findings appear in the journal Nature Astronomy.
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G. Pichierri et al. The formation of the TRAPPIST-1 system in two steps during the recession of the disk inner edge. Nat Astron, published online August 20, 2024; doi: 10.1038/s41550-024-02342-4
