Trojans in our Solar System 

In 1772 Lagrange identified five points of balance between three bodies: the Sun, a planet, and a small object. In this context, stability refers to the fact that at these points the small object will be equally attracted by the two bodies, with the same intensity. The objects (asteroids) share the same orbit as the planet are called Trojans, and are located in two stable Lagrange points, called L4 and L5, where: L4, is 60º in front of the planet and L5, 60º behind it. 

According to Lagrange, large amounts of dust and asteroids (the size of meters to kilometers) should accumulate at these points around Jupiter, and such asteroids were first observed in 1906. With the advance of technology and new observations, hundreds of Trojans have been identified in the Solar System. However, it is not yet very clear how these objects are formed or why it seems that there are more objects at one Lagrange point than at the other. Despite the existence of these equilibrium points, it is not clear how dust and rocks accumulate in those regions, nor what are the dynamics of their evolution.

Moreover, considering that the solar system formed from, the Solar System formed from a primordial disk that had a large gas (99%) and little dust (1%), the origin of the Trojans must be closely related to this gas-dust interaction.

To answer this question, an international group of astronomers, led by Matías Montesinos, research associate of the Max Planck Tandem Group and collaborator of the Núcleo Milenio de Formación Planetaria, NPF, theoretically studied this interaction, thus reconstructing the origin and remote past of the Trojans around a Jupiter-type planet.

Six of the eight authors of this work belong to the NPF. Besides Montesinos, Juan Garrido-Deutelmoser, postgraduate student; Johan Olofsson, associate researcher; Jorge Cuadra, associate researcher; Amelia Bayo, director of NPF; and Mario Sucerquia, postdoctoral researcher participated. The research was published in the prestigious scientific journal Astronomy & Astrophysics.

 “We modeled the evolution of a protoplanetary disk through hydrodynamic simulations, taking into account the interactions of a Jupiter-type planet with the gas and dust content of the disk. In addition, we considered an energy equation, in which the gas is heated by the star, and cooled gradually as it radiates, allowing us to model more realistically the thermodynamics and dynamics of the process,” explains Montesinos.

The researchers concluded that the dust does indeed accumulate in the Lagrangian points, which occurs in a short period of time. In about 10,000 years, dust should accumulate at L4 and L5, in an amount approximately equal to a few moon masses. This dust is accumulated by the gravitational interaction between Jupiter and the primordial gas of the protoplanetary disk. 

“We also notice certain peculiarities in the final formation of the Trojans. For example, we discovered a natural asymmetry where L5 accumulates more mass than L4 (mass than L4, which was an unexpected result). In addition, we found that the dust reservoir for the assembly of a Trojan is only in the same orbital region of the planet. That is, the Trojans “trapped” in a Lagrangian point do not come from regions far from the planet (e.g., the outer edges of the disk), but from zones co-rotating with it, in the same orbit as the planet“, indicates Montesinos. The astrophysicist adds that this means, for example, that these Trojans would share the chemical composition of the planet they accompany, at least to the first order.

Amelia Bayo, who is also an academic in the Instituto de Física y Astronomía at Universidad de Valparaíso, says that one of the things that she finds most interesting about the work is that it connects very directly the “world” of exoplanet research, with what we can see in other solar systems, with our solar system. “We know for example that there are many planetary systems that are super different from our solar system, but it seems that the accumulation of dust at these points and with these asymmetries should be a common feature of the planetary zoo,” she says.

Future work

An interesting point for researchers is to consider the interactions with other planets in the Solar System that could influence the formation of the Trojans.

“As future observational work, it would be interesting to try to detect these primordial Trojans in certain young systems. There are many disks around young stars, which resemble what the Solar System once was. In some of them, cavities, supposedly formed by planets, have been observed, but it has been very difficult to detect Trojans. Even though most of the planets, supposedly responsible for the opening of those cavities, have not yet been detected, , there should be two swarms of Trojans, one in front of the planet at L4, and another following behind at L5. Finding these two large accumulations of dust inside a cavity would be an indirect clue to the presence of a hidden planet in it,” concludes Matias Montesinos.

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