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.
A rival theory
Boss is the main proponent of a controversial and relatively new theory of planet formation called "disk instability."
Unlike the core accretion model, in which giant gas planets are created by first forming rocky cores and then hoarding gas to form an atmosphere, disk instability says that large planets form from large, loosely packed clumps of dust and gas whose central regions coalesce over time into cores that then grow relatively quickly.
Type 1 migration isn’t a problem for the disk instability model because Jupiter-sized clumps can form in as little as 1,000 years. Scientists think that once a protoplanet reaches about 10 Earth-masses, it has enough gravitational heft to carve a path for itself through the gas disk and avoid getting sucked into the star.
Boss thinks that core accretion and disk instability models are not mutually exclusive. It could be that core accretion works better under certain circumstances, and disk instability in others. Some scientists have even tried to combine the two into a hybrid theory.
In a talk to be given later this week at the NASA Astrobiology Science Conference in Washington, Boss will discuss how disk instability might better explain gas giant planet formation around small, dim red dwarf stars.
Red dwarfs have masses that are only one tenth to half that of the sun and are so gravitationally weak that gas giant formation through core accretion around them would likely take more than 10 million years. Disk accretion, in contrast, could yield gas giant planets in as little as 1 million years, Boss said.
New tests on the horizon
As technology improves and telescope resolutions get better, scientists might be able to catch planet formation as it happens and finally put an end to the debate or call it a draw.
In preparation for that day, Hannah Jang-Condell, a Carnegie Institution fellow, has developed a new method that could potentially turn up evidence supporting core accretion.
Jang-Condell’s method, which will also be presented at this week’s conference, involves looking for gravitational "dimples" in a star’s gas disk that are formed by giant protoplanets that might be actively accreting material to form gas atmospheres.
Importantly, the new method would be sensitive enough to detect protoplanets that are 10 to 20 Earth-masses — a fraction of Jupiter’s size. Scientists think that a protoplanet must be at least 10 Earth-masses to have enough gravity to become a gas giant.
The detection of such protoplanets would thus suggest that the first step of gas giant formation through core accretion is possible. Even if theorists couldn’t explain how the protoplanets formed or survived, they would at least know they were on the right track.
"If we do see the signature of a 10 to 20 Earth-mass planet, that at least tells us that something of that mass can form before the disk dissipates," Jang-Condell explained.
Boss points out, however, that the proposed test won’t be able to tell whether a detected protoplanet is actively accreting material or not, but only that it is there.
"It could also be consistent with a failure of core accretion to make a gas giant," Boss said. "It would still be making a Neptune-mass object, but you couldn’t whether it goes on to make a gas giant.
"It would be good indication that core accretion could happen, but it’s not like it’s a 100 percent acid test — it’s not quite that robust, but it’s still worth doing for sure."
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