Mystery still shrouds planetary formation

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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