What if there was a super-Earth in our solar system instead of the asteroid belt? This is exactly what planetary researchers have now tried out in simulations – with mostly fatal consequences. Depending on the position of Super-Earth, either the inner planets will be ejected from the system or the ice planets Uranus and Neptune will go off course. This could explain why our solar system has a conspicuous gap between Mars and Jupiter and is exotic in other respects as well.
For a long time, the solar system was considered the classic model for planetary systems around sun-like stars. But it is now clear that our homeland is really exotic – most planetary systems around foreign stars look completely different. Thus, most of them include super-Earths and mini-Neptunes – a category of planet sizes completely absent from the solar system. Instead, there is a distance of 17 Earth masses between Earth, the largest terrestrial planet, and Neptune, the smallest ice giant.
Challenge for theoretical models
Also strange: Most of the extrasolar gas giants orbit their stars at a short distance, in the solar system, however, Jupiter and Co are all in the outer region. In addition, there is a large gap between Mars and Jupiter – it almost seems as if an originally planned planet is missing here. “All of these anomalies are real challenges for our models of planet formation,” explains astrophysicist Stephen Kane of the University of California at Riverside.
So far, it has only been possible to partially explain why our solar system is such an exotic phenomenon. One of the possible causes is the migration of Jupiter in the early days of our system. “This planetary migration likely played the major role in the thinning of material between Mars and Jupiter,” says Kane. But also an unusual structure of the protoplanetary disc is postulated by some planetary researchers.
Extra planet between Mars and Jupiter
Now, to learn more about the hidden laws of our exotic system, Kane conducted an unusual experiment: in a series of simulations, he replaced the asteroid belt with a super-Earth and observed how this affected the orbits of the rest of the planets. “Studying the dynamic consequences of such an additional planet helps to test current planet formation theories,” explains the astrophysicist.
For his experiment, Kane used a common computer model of the solar system and added an additional planet of between one and ten Earth masses in various simulation runs. The extra planet was placed in different starting positions with a distance from the sun of two to four astronomical units (AU) – in the gap between Mars and Jupiter. The simulation then showed what happened in the roughly ten million years after the planet was inserted.
Fatal consequences for the inner planets
The simulations revealed that although there is enough space between Mars and Jupiter for another planet, an additional super-Earth would have fatal consequences for our solar system. The extra planet’s gravity and the orbital resonances it generates would significantly destabilize the entire system. The simulations only went reasonably well if the planet was barely heavier than Earth and its orbit was exactly three astronomical units – pretty much exactly in the middle of the gap between Mars and Jupiter.
On the other hand, it would be devastating if a super-Earth of seven Earth masses orbited at twice the distance between the sun and the earth. “In this case, the orbits of all four inner planets become so unstable that they are ejected from the solar system before the end of the simulation,” reports Kane. The first victims are Mars and Mercury, followed later by Venus and Earth. Even with other orbits of the additional planet, at least Mercury would be thrown out of the system.
Chaos at the Ice Giants
The large gas and ice giants in the outer planetary system are somewhat more stable, but even they would not be immune to severe turbulence – especially with a super-Earth at the outer edge of the gap: The extra planet then enters orbital resonance with Jupiter and Saturn, their orbits becoming increasingly so become more eccentric. “These shifts disrupt Neptune’s orbit, which is now also changing its eccentricity chaotically,” reports Kane. Eventually, as a result, Uranus will be thrown completely out of its orbit and thrown out of the solar system.
The ice giants in particular would also be affected in other orbits of the super-earth beyond three astronomical units. “They lose their trajectory stability in most cases,” reports Kane. However, the disruptive super-Earth would not be spared either: “These interactions usually result in the super-Earth being thrown out of the system,” explains the astrophysicist.
“Perfectly balanced like clockwork”
According to Kane, these results underscore how precisely balanced the solar system and its planets are. “They mesh like the finely balanced cogs of a watch,” says Kane. “Add a gear and the whole system collapses.” In other words, the large distance between Mars and Jupiter may not just be an accidental consequence of Jupiter’s migration. Instead, the gap also appears to be important to the stability of the planetary system.
However, the knowledge gained in the “additional planet experiment” could also help to make the models for planet formation more precise and to better understand extrasolar planetary systems, as the astrophysicist explains. (The Planetary Science Journal, 2023; doi: 10.3847/PSJ/acbb6b)
Quelle: University of California – Riverside
