Protoplanetary disk accretion, mass budget and dispersal

5 problems

• Do magnetized winds or turbulence drive accretion in protoplanetary disks?

  Contested

  Gas in a disk can only fall onto the star if it loses its spin, either by passing it outward through turbulent stirring or by carrying it away in a magnetized wind (gas flung off the disk surface along magnetic field lines). Measurements suggest the disks are too calm for turbulence to do the job, but this is not settled.

• What clears protoplanetary disks after a few million years?

  Open

  The fraction of young stars that still have disks drops sharply over a few million years, which sets the deadline for forming gas giants. Ultraviolet and X-ray light from the star, magnetized winds and radiation from nearby massive stars could each remove the gas.

• Are the rings in protoplanetary disks carved by planets?

  Partially resolved

  Images of young disks show bright rings and dark gaps. Hidden young planets could carve them, but other processes can make rings too.

• How much gas do protoplanetary disks contain?

  Open

  Most of a disk's mass is hydrogen gas, which is nearly invisible, so astronomers weigh it through rarer molecules such as carbon monoxide. Those molecules give masses far below expectations, either because the disks are gas-poor or because the carbon monoxide has frozen onto grains or been destroyed.

• Do disks hold enough solids to build the observed planets?

  Open

  Weighing the dust in disks that are one to three million years old gives masses comparable to, or smaller than, the rock and metal locked in the planets found around older stars. Either planets form much earlier than assumed, the dust is being weighed wrongly, or disks are refilled from outside.