Mystery still shrouds planetary formation

By Ker Than

updated 3:24 p.m. ET March 28, 2006

For scientists who spend time thinking about how planets form, life would be simpler if gas giants like Jupiter and Saturn didn’t exist.

According to the standard model of planet formation, called "core accretion," planets form over millions of years as enormous blocks of rock and ice smash together to form planetary embryos, called "protoplanets," and eventually full-fledged planets.

Most scientists agree that core accretion is how terrestrial planets such as Earth and Mars were created, but the model can’t convincingly explain how gas giant planets like Jupiter and Saturn came to be.

One major problem is that developing gas giants through core accretion takes too long. According to the best current models, the process requires several million years — longer than the typical observed lifetime of the stellar gas disks from which planets are born.

The other main difficulty is the so-called "migration" problem. Protoplanets are not sitting stationary in the gas disks as they bulk up. Due to gravitational interactions with the disks, the protoplanets swirl rapidly inwards toward their central stars in what scientists call "Type 1" migration. Models predict that this death spiral can take as little as 100,000 years.

This migration problem is the toughest challenge facing theorists trying to explain gas giant formation through core accretion, said Alan Boss, a planet formation expert at the Carnegie Institution of Washington.

"The migration problem is scary," Boss told Space.com. "[The models] are off by a factor of 10 or 100, so you really have to wonder if there’s going to be a solution here."

A new test
To address the migration problem, astronomers Paul Cresswell and Richard Nelson from Queen Mary, University of London recently developed a new computer model that takes into account the gravitational interactions of multiple protoplanets in a gas disk.

Previous simulations looked only at one or two protoplanets. But many star systems, including our own, contain several planets. Cresswell and Nelson wondered if gravitational interactions between many protoplanets at once were enough to slow Type 1 migration and give large protoplanets enough time to mature into gas giants.

Their model revealed that in most cases, it wasn’t.

The simulation, to be detailed in the journal Astronomy & Astrophysics, showed that in a very few cases — about 2 percent — a lone protoplanet can be ejected far away from the central star, thus lengthening its lifetime. The rest of the time, however, the gravitational interactions between the multiple bodies caused them to fall into orbital resonance with one other. The protoplanets migrated inward together in lockstep toward the central star.

The researchers propose several possible solutions for why their model doesn't produce lasting planets.

Perhaps around a real star, several generations of protoplanets form, and only those that develop later — as the inner regions of the gas disk begins to dissipate — survive into planetary adulthood.

"It might just be that the last batch of protoplanets is the ones that don’t fall into the star," Cresswell told Space.com.

But in one sense this would make it harder to form gas giants, since relatively little gas would be left in the disk to form a thick atmosphere. However, it might still be possible if the planet could draw upon material sweeping in from outside its orbit, the researchers write.

Another solution might be that large parts of the gas disk are turbulent, and not smooth as the computer model assumed. The turbulence could be the result of magnetic field instabilities in the disk and would impede inward migration.

But it’s unclear whether this could actually work in real life.

"Turbulence may be one way of stopping it, but we don’t know how to make the turbulence," Boss said. "It only works in magnetically active areas of a disk, but most areas of a disk are thought to be magnetically inactive because it’s so dense that it’s shielded from outside radiation."

For Boss, the new study just accentuates again the problems involved with explaining gas giant formation with the core accretion model.

"If it works, it may not work as frequently as we would hope it to in order to explain all the planets we see out there," Boss said.

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