Can Magnetic Fields Help Planetesimals Form?

Astronomers still don’t fully understand how planets form, especially ultra-dense, iron-rich planets like Mercury. How do trillions of tiny dust grains clump together to make pebbles, planetesimals, and eventually the cores of rocky planets?

Arrested (Dust) Development

When iron-rich dust grains in a protoplanetary disk collide, they stick together to form porous dust aggregates. Further collisions increase the size of aggregates, but only to a point; the growth of dust grains is limited by something called the bouncing barrier. The bouncing barrier kicks in when the once-fluffy dust aggregates become so compacted by collisions that incoming grains bounce off rather than stick. This limits the size of dust aggregates to just a couple of millimeters.

If the bouncing barrier stops dust grains from getting progressively larger, how do planets ever manage to form? One possibility is that the presence of an external magnetic field enhances the intrinsic magnetization of the iron in dust grains, which increases the attractive forces between them, making them “stickier.” If magnetic fields can help dust aggregates grow to centimeter size, other growth mechanisms like the streaming instability — a clumping mechanism caused by gas drag in a disk — can take over from there.

From simulations and observations, we expect the inner regions of protoplanetary disks to have strong magnetic fields — somewhere between 1 and 100 mT. How strongly do magnetic fields of this scale affect the growth of iron-rich dust aggregates?

Levitation and Magnetization

Maximilian Kruss and Gerhard Wurm (University of Duisburg-Essen, Germany) used laboratory experiments to explore the effects of magnetic fields on the bouncing barrier. Kruss and Wurm used a heat lamp to levitate micron-scale iron–silicate dust grains above a glass lens, which allows them to move and collide freely.

With no external magnetic field, the bouncing barrier stops the grains from growing larger than 2 mm. The authors find that for dust composed of equal amounts of iron and silicate, the bouncing barrier begins to shift when the external magnetic field strength reaches 2.2 mT. The size of the aggregates increases with increasing magnetic field strength, reaching 6 mm in length when the maximum field strength for the experimental setup (7 mT) is applied.

The Missing Link?

The magnetic field strength necessary to shift the bouncing barrier could be even lower; if dust grains in protoplanetary disks are more iron-rich than those used in this experiment, the required field strength could be below 1 mT — well within the expected range for protoplanetary disks.

It isn’t yet known what proportion of dust grains can be expected to be enriched in iron, but this experiment clearly shows that the presence of an external magnetic field encourages iron-rich dust grains to grow larger.

If magnetic fields can suppress the bouncing barrier so that the grains grow large enough for the streaming instability to take over, we may be able to explain how planet formation gets going on the very smallest scales, especially for iron-rich planets like Mercury.

Citation

“Seeding the Formation of Mercurys: An Iron-sensitive Bouncing Barrier in Disk Magnetic Fields,” Maximilian Kruss and Gerhard Wurm 2018 ApJ 869 45. doi:10.3847/1538-4357/aaec78