An article published in the journal “Nature Astronomy” reports a possible reconstruction of the system of the ultra-cool dwarf star TRAPPIST-1 that led to the current configuration of its seven rocky planets (Image NASA/JPL-Caltech). A team of researchers examined their orbits and in particular their orbital resonances, concluding that the planets formed in two steps in a protoplanetary disk divided into two parts. Initially, this led to the formation of two planetary subsystems and only later did planetary migrations occur with influences between various planets that led to the current situation.
Since the confirmation of the presence of seven rocky planets, which occurred in February 2017, the TRAPPIST-1 system has become a sort of cosmic laboratory used to conduct different types of astronomical research. Today, over 5,000 exoplanets are known and over the years, astronomers have developed different models to explain their formation and evolution.
One topic of astronomical research concerns planetary migration, caused by the influence that different planets can exert on each other. This topic is connected to that of orbital resonance since gravitational influences tend to bring planets into positions with certain distances from each other. However, sometimes, the situation in a planetary system is more complex than usual, and that’s also true for the TRAPPIST-1 system.
In a system composed of rocky planets, one would expect a simple ratio between the orbital periods of the various planets. In simple terms, if the innermost planet takes two days to orbit its star, one would expect the next planet to take three days and so on. Instead of this 3:2 ratio, the ratio between the two innermost planets is 8:5 and that between the second and third planets is 5:3.
According to the authors of this study, this anomaly in the orbital resonance of the inner planets of TRAPPIST-1 is due to the formation and evolution of that planetary system. In this reconstruction, the four inner planets initially evolved separately with orbital resonances of 3:2 between neighbors. The fourth planet moved outwards but was later pushed back towards its star when the three outer planets formed, at a later stage. This would have generated further gravitational influences on the inner planets leading to the orbital resonances that we measure now.
The orbital resonance achieved, with ratios that today are 8:5, 5:3, 3:2, 3:2, 4:3, and 3:2 starting from the inner pair of planets, can provide long-term stability. These interactions are not only a scientific curiosity but also have implications for the climate of the various planets. That’s because they are tidally locked with their star, and this means that they always show the same face to it but gravitational interactions can cause a chaotic spin that sometimes changes that status, as explained in an article published in August 2019 in the journal “Monthly Notices of the Royal Astronomical Society”. Astronomers are still studying the consequences of the situation of this very compact planetary system.
