New giant migration model threatens Planet Nine hypothesis

(ORDO NEWS) — Scientists have shown that a protoplanetary disk dispersing from within explains the changing orbits of giant planets in the young solar system.

The new model works more accurately if we assume that initially there were only four giants (and all remained in the system), and not five, as some scientists believe. And then the hypothetical Ninth Planet cannot be a departed giant.

According to the most popular hypothesis about the distribution of planets in our system, the giant planets formed closer to the Sun, and then flew off into their current orbits. In a new study published in Nature , an international team of astronomers suggests that this process could have started much earlier.

Stars are born in dense clouds of gas and dust. From the remaining matter around a young star, a huge rotating disk is formed, and planets form from the disk. The distribution of dust and gas in this protoplanetary disk influences the composition and evolution of the planets.

The assumption about the formation of giant planets near the Sun arose at the end of the last century. It was possible to answer the question of how this could happen in 2005.

Then, in the journal Nature , three articles were published describing the scenario for the migration of giants, the cause of which was the chaotic gravitational interactions of the planets.

An international team of scientists has succeeded in explaining the formation of the Oort cloud, the Trojan giant asteroids, the late heavy bombardment and other features of our system. The hypothesis has been called the Nice model, after the French city where a group of astronomers once started discussing this scenario.

To this day, the Nice model is considered the most likely scenario for the evolution of the solar system, but scientists continue to debate the causes of gravitational instability.

At first it was assumed that the migration of giants occurred hundreds of millions of years after the dissipation of the protoplanetary disk. But the results of new studies – including lunar regolith – indicate that this must have happened sooner.

“We wondered if the Nice model was really needed to explain the solar system,” says Sean Raymond of the University of Bordeaux.

And they came up with the idea that giant planets could fly apart due to the “recoil” effect from the protoplanetary disk. Perhaps there was no period of instability.”

It turned out that the dispersion of the disk from the inside to the outside could indeed provoke the instability of the Nice model. As a result, the team did not disprove, but strengthened the popular hypothesis.

New giant migration model threatens Planet Nine hypothesis 2
Illustration of the young solar system. The protoplanetary disk has not yet dissipated

In the new scenario, the giant planets, as before, form near the center. Having warmed up, the Sun begins to “blow off” the protoplanetary disk further and further to the vicinity of the system.

As the computer model showed, the movement of the border of the growing “hole” provokes the instability of the giants’ orbits, and they fly off to distant orbits. Unlike the classic Nice model, all of this must have happened in the first 10 million years of the system’s evolution.

Also, the new scenario explains the mixing of the matter of the inner and outer regions of the disk. The geochemistry of the Earth suggests that this should have happened at the stage of the formation of our planet.

The question of the number of planets in the young solar system remains open. The original model of Nice works better with five giants: during the period of instability, one of the giants flies out of the center and with this movement helps the remaining four (Jupiter, Saturn, Uranus and Neptune) find their orbits, while it flies out of the system or remains hypothetical (not yet found) Ninth planet.

In 2015, Caltech scientists presented circumstantial evidence for the existence of a massive body in the remote regions of the system. It was not possible to see it directly, but more and more new studies appeared – both supporting and criticizing the hypothesis of the Ninth Planet.

The question of its existence remains controversial, so in the new work, scientists do not focus on it. Nevertheless, they performed calculations for both four and five giants in the young system. Regardless of the original number, the new model also leaves a system with four giant planets.

“But if you start with four, then the final orbits more closely match the real ones,” says study co-author Seth Jacobson of the University of Michigan. It turns out that with the new scenario, the probability that the Ninth planet is the fifth giant that has flown away decreases.

Fortunately, the problem of the existence of the Ninth planet should be resolved soon. At the end of 2023, the powerful observatory named after Vera Rubin will start working. If the planet exists, a new telescope should be able to see it.


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