TRAPPIST-1 is an ultracool dwarf star located 38.8 light-years away in the constellation Aquarius that hosts seven planets. The period ratios of the neighboring planets are closer as they move away from the star: 8:5, 5:3, 3:2, 3:2, 4:3, 3:2. This compact resonant configuration is a clear indication of disk-driven migration, but the desired outcome of such an evolution would be the establishment of a first-order resonance, rather than the higher-order resonances observed in the inner system. Astronomer Gabriele Pichierri of the California Institute of Technology and his 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 efficiency. Two key elements of the team's model are that, along with the migration, the inner boundary of the protoplanetary disk retreated over time, and that the TRAPPIST-1 system initially separated into two subsystems.
This artist's rendering shows TRAPPIST-1 and its planets as seen on the surface. Image courtesy of NASA / R. Hurt / T. Pyle.
“When all we had to analyse was the solar system, we could simply assume that planets formed where we see them today,” Dr Pichieri said.
“But when the first exoplanet was discovered in 1995, we had to rethink this assumption.”
“We're developing better models of how planets form and how they come to be oriented in the way that we found them.”
Most exoplanets are thought to form from a disk of gas and dust around a newly formed star, and then migrate inwards, approaching the inner boundary of this disk.
This results in a planetary system assembled much closer to the host star than is the case in our solar system.
In the absence of other factors, planets tend to move away from each other at characteristic distances based on their mass and the gravitational force between them and their host star.
“This is the standard transition process,” Dr. Pichieri said.
“The positions of the planets form resonances between their respective orbital periods. If you divide the orbital period of one planet by the orbital period of its neighbor, you get a simple integer ratio like 3:2.”
For example, if one planet takes two days to orbit a star, the next planet further away takes three days.
If the second planet and a more distant third planet were also in 3:2 resonance, the third planet's orbital period would be 4.5 days.
“The exoplanets behave nicely in simpler predicted resonances, so to speak,” Dr Pichieri said.
“But the inner ones have slightly more exciting resonances. For example, the orbital ratio of planets b and c is 8:5, and the ratio of c and d is 5:3.”
“This subtle difference in the outcome of TRAPPIST-1 assembly is puzzling and represents a unique opportunity to tease out in detail what other processes were at work in its assembly.”
“Moreover, most planetary systems are thought to have begun in such resonances, but have experienced significant instabilities during their lifetimes before we observe them today.”
“Most planets would become unstable or collide with each other, and everything would be in chaos. For example, our solar system was affected by such instability.”
“But we know there are some systems that are more or less pristine specimens that have remained stable.”
“They effectively represent a record of its entire dynamical history, and we can try to reconstruct it. TRAPPIST-1 is one of them.”
The challenge then was to develop a model that could explain the orbits of the TRAPPIST-1 planets and how they got to their current configuration.
The resulting model suggests that the inner four planets evolved alone within the originally predicted 3:2 resonant chain.
As the disk's inner boundary expanded outward, the orbits loosened from the tighter 3:2 linkage into the configuration observed today.
The fourth planet was originally located on the inner boundary of the disk and moved outward with the disk, but was pushed back inward at a later stage when three more outer planets joined the planetary system.
“By observing TRAPPIST-1, we were able to test an exciting new hypothesis about the evolution of planetary systems,” said Dr Pichieri.
“TRAPPIST-1 is very interesting because it's a very complex, long chain of planets, and it's a great example for testing alternative theories about the formation of planetary systems.”
of Survey results Published in a journal Natural Astronomy.
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G. Pichieri othersDuring the recession of the inner edge of the disc, the TRAPPIST-1 system forms in two steps. Nat AstronPublished online August 20, 2024, doi: 10.1038/s41550-024-02342-4
Source: www.sci.news
