The migration of Jupiter may have destabilized the orbit of other celestial bodies in the past. We have already discussed planet formation several times.
An interstellar cloud of gas and dust collapses under its own gravity, most of its mass eventually concentrates in the center and forms a star, and the dust grains orbiting it collide and combine to form larger and larger masses until they become planets. But the story of Earth’s evolution has another step that we don’t yet fully understand: the formation of the Moon.
The giant impact hypothesis
Throughout history, several different hypotheses have been proposed to explain the origin of the Moon. Some suggested that it was ejected from the Earth in the form of a large blob of molten rock when our planet was still a large, rapidly rotating ball of magma. However, it is impossible for the Earth to rotate at a high enough speed for this to happen.
Another less extravagant idea was that the Moon formed in another region of the solar system and was captured by the gravity of our planet. This hypothesis was discarded after the Apollo missions brought back tens of kilograms of lunar rocks, since their chemical and isotopic analysis revealed that the Moon is made of material too similar to that of the Earth for the two bodies to have formed in distant regions.
With this in mind, it seems that the so-called “giant impact hypothesis” is the one that seems to best fit the current properties of our satellite. In this scenario, a celestial body the size of Mars would have collided with the primordial Earth several tens of millions of years after it was formed and the impact would have launched into space a large amount of rocky debris belonging to the two planets. Over time, the rock fragments that were left circling the Earth would have coalesced to form the Moon.
Although this model is the one that best fits today’s observations, it still presents some unknowns. One of them is that the planet that collided with the early Earth must have collided at a very tight angle so that the collision ejected large amounts of material into space without destroying the two celestial bodies. But, was this collision the result of chance, or did something happen in the early solar system that facilitated this type of collision?
Migration of planets
Today’s solar system is divided into two distinct zones:
- Four rocky planets near the Sun in the inner solar system
- Four gas giants much farther away in the outer solar system.
But the planets were not always structured this way.
Current models of planet formation suggest that the gas giants formed much closer to the Sun than they do today, and that their mutual gravitational interactions gradually moved them farther away from our star over the next ten million years. Therefore, the authors of a new study simulated the dynamics of the early solar system to find out how the migration of the giant planets affected the rest of the celestial bodies.
In 50% of the simulations, the migration of gas giants disrupted the orbit of a rocky planetoid and put it on a collision path with Earth. However, most of these impacts did not result in the formation of a satellite. An Earth-Moon system such as we have today formed in only about 10% of the simulations.
Furthermore, the time between the beginning of the planetary migration and the impact with Earth in the simulations is about 20 million years. The magnitude of this figure is consistent with the impact date estimated from cosmochemic evidence, which suggests that it occurred between 30 and 60 million years after the gas began to dissipate in the protoplanetary disk.
Although a system such as the Earth and Moon did not form frequently in the simulations in this study, the authors conclude that the migration of giant planets is one possible mechanism that could have been triggered indirectly by the Moon.
Although the material that makes up the Moon partially “came out” of the Earth, it will not fall back onto our planet. In fact, the Moon moves 3.8 centimeters away from us every year.