An article published in the journal “Nature Astronomy” describes a research on the migration of the solar system’s gaseous planets. A team of scientists studied two asteroids called Patroclus and Menoetius that are out of the ordinary as they form a pair of Jupiter trojans, which means that they orbit at a point of equilibrium of the Sun-Jupiter system. Their conclusion is that their existence is evidence of the shifting of the gaseous planets’ orbits.

The asteroid 617 Patroclus was discovered in 1906 but only in 2001 they discovered Menoetius, with which it forms the first binary discovered among Jupiter trojans. Over the years various measurements of the two asteroids have been carried out, which are inevitably approximate for reasons connected to the instruments used but also to their irregular shape. Patroclus dimensions are more or less 127 km x 117 km x 98 km while Menoetius dimensions are about 117 km x 108 km x 90 km.

In the vicinity of the Patroclus-Menoetius pair there are other trojan asteroids that follow Jupiter at the equilibrium point called L5 while another group of trojans precedes Jupiter at the equilibrium point called L4. These asteroids were captured during a period when the planets’ orbits weren’t yet stable and were influenced by their neighbors’ gravity.

Planets migrations are a subject of study that for years concerned the solar system but recently the continuous discovery of exoplanets in other solar systems is expanding these studies. In particular, hot Jupiters, gaseous planets very close to their stars, are taken into consideration. According to current models, they formed farther away and then got close to their star. In the case of the solar system planets, there’s the advantage of having much more information about them and also of knowing a lot of asteroids that may have been involved in past gravitational interactions.

According to Dr. David Nesvorny of the Southwest Research Institute (SwRI), who led this research, those interactions pushed Neptune outwards, where it met a large population of small primordial celestial bodies that probably today make up the Kuiper Belt. Several asteroids were pushed inwards and some of them were captured between Jupiter and the Sun becoming today’s trojans such as the Patroclus-Menoetius pair.

Dr. William Bottke, also of SwRI and another author of the research, explained that the observations of the Kuiper Belt indicate that in ancient times binary asteroids were common. Today only some of them exist within Neptune’s orbit, posing the problem of the interpretation of their survival. Some models of the solar system’s evolution suggest that if the instability had been delayed for many hundreds of millions of years the collisions inside the disk of small primordial objects would have disrupted those pairs so there would be none left that could end among Jupiter trojans. This means that the Patroclus-Menoetius pair’s existence favors the model that indicates an earlier instability.

The study of asteroids can reveal various information about the solar system’s history and is one of the reasons why they’re also the target of an increasing number of space missions. We just had the second anniversary of the launch of NASA’s OSIRIS-REx space probe, which will go and recover samples of asteroid 101955 Bennu, but the most ambitious one is Lucy, another NASA mission that in the next decade will go and closely study some trojan asteroids including Patroclus and Menoetius.