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Jupiter's migration key to our unique solar system

Early solar system Earth and the other terrestrial planets are late arrivals, forming only after the original planets of the inner solar system were destroyed, according to a new model explaining our solar system's unique architecture.

The study reported in the Journal PNAS, tries to explain why the solar system looks so different from all the other planetary systems scientists have observed so far.

"We are more or less a galactic anomaly," says the study's lead author Dr Konstantin Batygin of the California Institute of Technology.

"Systems like the solar system are a minority, so when we ask ourselves how common are 'truly' Earth-like planets, with Earth masses, hard surfaces, and small atmospheres, the answer is they're not very common at all."

The new model by Batygin and co-author Dr Greg Laughlin of the University of California Santa Cruz, shows the terrestrial planets Mercury, Venus, Earth, and Mars, must have formed hundreds of millions of years after the Sun and the rest of the solar system.

"They were forming when there was no more gas around, which is why we have a rather clear atmosphere, in sharp contrast to extra solar planets that we observe," says Batygin.

The inward planetary migration of Jupiter early in the solar system's history may have been responsible for this anomaly.

Why are we different

Batygin and Laughlin saw that about half of all planetary systems orbiting Sun-like stars feature large populations of 'super-Earth' sized planets which are thought to have substantial gaseous atmospheres.

The authors also noted that these planets were on orbits far closer to their host stars than the 88 day orbit of Mercury, the nearest planet to the Sun.

"If the dominant mode of planet formation is one that produces super-Earths on short period orbits, is it natural to assume that such planets existed in the early solar system?" asks Batygin.

"If so, what happened to clear the inner solar system of these first generation of planets?"

Batygin and Laughlin developed a model showing how the inward planetary migration of Jupiter early in the solar system's history would have triggered the destruction of all the original planets of the inner solar system.

The authors found that the gravitational interactions caused by Jupiter's journey, sent the solar system's original inner planets crashing into each other, causing them to smash apart, with the resulting debris falling into the Sun.

Then, the formation of Saturn, caused a gravitational interaction which drew Jupiter back out again to its current orbit.

"When Jupiter and Saturn move in and then out of the inner solar system, it initiated a collisional cascade in the inner solar system," says Batygin.

"As it moves inwards, Jupiter would have picked up about ten Earth masses worth of planetesimals along the way which would have been crushed due to collisions among the planetesimals."

A new beginning

Over the following hundreds of millions of years, the small amount of material left behind from this catastrophic event, coalesced to form the four small terrestrial worlds of Mercury, Venus, Earth, and Mars.

"This explains why the solar system's inner planets are so small and contain such thin atmospheres, compared to those seen in other planetary systems," says Batygin.

The Jupiter planetary migration hypothesis was originally developed to explain the structure of the asteroid belt, why Mars' mass is smaller than that of the Earth, and to understand the distribution of bodies in the outer solar system, including the orbits of Uranus and Neptune.

"We were not focusing on Jupiter and Saturn's early migration, we were interested in what makes the solar system unique, but it turns out Jupiter and Saturn's migration was the answer," says Batygin.

"It's very satisfying to have arrived at an idea which has already been proposed to explain other features of the solar system."