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How Jupiter survived to become king of planets

Making Jupiter A new model could explain how giant gas planets like Jupiter can exist.

Planets such as Jupiter are thought to be very common in the Universe. But computational models of planetary system evolution to date have struggled to explain how these gas giants survive beyond the embryonic stage.

According to these models, gas giants should migrate inwards and eventually fall into their host star within about 100,000 years.

Now, new computer simulations, described in the journal Nature, indicate that planetary embryos heat their surrounding environments, countering the forces dragging them into their stars.

"We have found a powerful ingredient that counters inward migration," says one of the study's authors Dr Frédéric Masset of the National Autonomous University of Mexico.

Planets are formed from protoplanetary disks of gas, dust and rocky fragments called planetesimals that orbit around newly forming stars.

Although they have masses of tens to hundreds of times that of Earth, gas giants such as Jupiter and Saturn are mostly composed of huge gas envelopes around a small rocky core, which may not be much larger than the diameter of the Earth.

"For almost 30 years it's been known that planets forming in the protoplanetary disk are subjected to very strong tidal forces," says Masset.

"The embryonic planet generates a spiral wake in the protoplanetary disk. And due to quite subtle asymmetries, the net torque of this wake causes planets to lose angular momentum, causing their orbits to decay in towards the star."

But, according to Masset and colleagues' model, heat generated by impacting material accreting onto planetary embryos is strong enough to heat up the surrounding protoplanetary disk.

"This dramatically changes the force felt by the planet and may halt or reverse migration," says Masset.

As the protoplanetary disk is heated, regions of lower density are generated immediately in front of and behind the planet as it orbits the star.

"As it's heated, it becomes less dense, and as you have perturbations of density, then you have perturbations of force acting on the planet," says Masset.

According to Masset, the area with the lowest density develops in the wake of the planet's orbit, acting to pull the planet outwards.

"We have a slight asymmetry, it's slight but the effects are huge," says Masset."

"This under-dense region behind the planet produces a positive torque counteracting inward migration."

Our solar system

Masset believes the model also helps explain the architecture of our own solar system.

"You can speculate that the embryos that were in the region of Mars and the asteroid belt contributed to form the protoplanets that migrated outwards and provided the rocky cores of Jupiter and Saturn," says Masset.

"So it would be natural with our model that this region would be depleted, because it would correspond to the residue from the assembly of Jupiter and Saturn's cores."