The Trojan asteroids share the same orbit as Jupiter and will soon be flown by the Lucy probe. At the moment they have never been observed directly, but are the subject of various studies trying to understand their origin. Researchers believe they have pierced one of the mysteries surrounding them.
At the L4 and L5 Lagrange points, the Trojan asteroids are in the same orbit as the planet Jupiter. There are two large clusters, each containing more than 10,000 asteroids. Among the unanswered questions about their formation, a study recently published in Astronomy & Astrophysics might well be answered: Why does one of the two clusters (L4) contain many more asteroids than the other?
The observation was made several decades ago, but no explanation seems satisfactory. Several values of L4/L5 have been estimated, ranging from 1.3 to 1.8. In the end, a population ratio of 1.6 was chosen based on the latest readings from the Sloan Digital Sky Survey, which is therefore the least likely to retain observational bias. The study explains that despite this difference, the two swarms share similar characteristics, suggesting a common origin.
Several scenarios to explain the L4/L5 ratio
Various scenarios have been proposed to achieve this value of L4/L5. One of them proposes that an icy giant planet encountered Jupiter at the beginning of the solar system during an instability of the outer solar system. The meeting would have caused the giant to shift toward the L5 cluster, attracting some of its population along the way. However, calculations have shown that in this scenario the L4/L5 ratio would be 1.3.
Another conjures up an immigration of proto-Jupiter, which would then have caused an excess in the L4 swarm. But to reach 1.6, the gas giant would have to move more than 3.5 AU (one astronomical unit = one Earth-Sun distance). However, such a value would “pose problems for the inner solar system, such as the amount of mass implanted in the main belt, which may be very large and incompatible with the currently low mass of the main belt of asteroids,” the study describes.
The team proposed another scenario this time: a migration from Jupiter, but outwards. The idea is not new and originally comes from the “Nice Scenario” set up in 2005 at the Observatory of the Côte d’Azur. He posits that another icy giant planet originally lay in the fledgling solar system. The Solar System’s other giant planets, for their part, were in a more compact configuration than they are today, and they would have migrated well after the gases from the protoplanetary disk dissipated when the additional icy giant planet was ejected toward the interstellar medium of Jupiter.
“We propose that a rapid outward migration — in terms of distance from the Sun — of Jupiter could distort the configuration of Jupiter’s Trojans, resulting in more stable orbits in the L4 swarm than in the L5 swarm, Jian Li explains in a publication first author of the study. This mechanism, which has temporarily induced distinct evolutionary paths for the two groups of asteroids that share Jupiter’s orbit, provides a new and natural explanation for the unbiased observation that L4 asteroids outnumber the L5 asteroids by about 1.6 times -Swarms.
An outward migration from Jupiter
To ensure the plausibility of such a scenario, the team modeled the evolution of Jupiter at the very beginning of the solar system. Or rather, they created a simplified solar system, including the Sun, Jupiter, and part of the Trojan asteroids. They then simulated Jupiter’s migration at different angles and speeds to reproduce the L4/L5 ratio of 1.6.
Their models managed to reproduce the population ratio between the two clusters, assuming Jupiter’s migration is rapid! The researchers complete their results with the explanation that as the rate of migration increases, so does the asymmetry. To the point that if the speed exceeds a critical threshold, L4’s population can disappear!
Answers from 2027 with the Lucy mission
If these results provide a reliable basis for explaining the relationship between L4 and L5, the researchers insist on the many uncertainties that remain in their study. Specifically, at the level of Jupiter’s ejection of the giant planet Ice: “According to the theory of angular momentum exchange, once Jupiter has ejected a planet, it is bound to jump inward,” the study explains.
In addition, the planets Saturn, Uranus and Neptune were not included in the model used. However, Saturn could well have a decisive influence in the proposed scenario. The team therefore emphasizes the need to develop new, more precise models. NASA’s Lucy mission will also help learn more about the Trojan asteroids. The probe was launched in November 2021 and should arrive at its destination in 2027. She will fly over the Trojan asteroids and detail their characteristics.