By Sabine Stanley, Ph.D., Johns Hopkins University
The planets we see today formed from gravitational accretion of smaller bodies that grew from a rotating disk surrounding the forming Sun. But where did the protoplanetary disk come from? We have to go back even further back in time to investigate that.
Molecular Clouds Give Birth to Nebulas
Before the protoplanetary disk, came the nebula. That nebula came from a molecular cloud.
Molecular clouds are seen throughout the galaxy. They are cold, dark, giant regions, with condensates of dust and molecular gasses. They can be over 200,000 astronomical units in size. These molecular clouds serve as nurseries for the creation of new stars.
When small perturbations in the motion of the gas and dust in these clouds happen to cause the density to get sufficiently high in a region, then that area begins to collapse under its own mutual gravitational attraction. It’s then called a cloud core.
When we zoom in on a cloud core, we see that the densest regions collapse the fastest, and so the cloud keeps fragmenting into smaller and smaller denser regions. Eventually, one of these cloud cores collapses enough to create a central region dense enough to form a proto-star, surrounded by a cloud of gas and dust collapsing in on the star. This spherical cloud is about 10,000 astronomical units in radius and is called a planetary nebula.
The nebula that preceded our protoplanetary disk didn’t stay a spherical cloud for long. The gas and dust particles that make up the nebula were randomly moving about. This gave the cloud a tiny bit of net rotation. As the cloud collapsed, its angular momentum had to be conserved. So the cloud began spinning faster and faster.
This happened for exactly the same reason that a figure skater pulls in their arms when they are spinning in order to speed up their rotation. By contracting, the cloud was essentially pulling in its arms.
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Why the Protoplanetary Disk?
Rotation of the cloud gives particles a centrifugal force away from the axis of rotation. This horizontal centrifugal force counteracts a bit of the gravitational force pointing toward the center of the cloud. This slows the horizontal motion of the particles. But the vertical motion, from above or below the disk and toward the midplane, continues.
That means cloud particles reach the midplane of the cloud before they reach the center of the cloud. So they still have horizontal velocities that put them in orbits around the center of the disk, and prevent them from plummeting into the center, where the Sun is forming.
So spinning disks naturally form around a growing star. In fact, we see similar disks orbiting other proto-stars as well. By studying the ages of the stars in those disks, we find out that it takes only one to 10 million years for an initial cloud, about 20,000 astronomical units in diameter, to collapse to the point where there is a functioning star at the center surrounded by a thin disk of about 200 astronomical units in extent.
Protoplanetary disks typically have masses that are only a few percentage of their star’s mass. They are also very thin, with a thickness of only about 10% of their radii. The disks contain gasses, like hydrogen and helium, and also micron-sized dust particles.
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How Did We Get From Dust in the Disk to the Boulder-Sized Planetesimals?
It turns out that micron-sized dust particles just don’t have enough mass for gravity to be an effective attractive force. So, instead, these objects grew by sticking together during random collisions due to electromagnetic attractions, rather than gravity.
Think of the electrostatic force that allows you to stick a balloon to your hair. Van der Waals forces are another example, which occur because of the electric dipole charge distributions in some molecules. Eventually, these sticking forces allowed objects to grow large enough, about one meter in size, for gravity to take over as the dominant attractive force.
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Barriers to Growth
However, it needs to be pointed out that there are two known barriers to growth that are open questions about this process. First, there is the bouncing barrier. Experiments and simulations suggest that when rocky grains grow to about one millimeter in size, they begin bouncing off each other, rather than merging through these electromagnetic forces. For icy grains, the same thing happens around 10 centimeters in size.
This suggests that objects in the protoplanetary disk can’t get any larger than this. Except that we know that they can.
Another barrier happens at the one meter size. Objects smaller than one meter interact significantly with the gas in the disk, whereas objects larger than about one meter don’t. The problem is that the gas is orbiting more slowly than the dust in the disk because it is subject to forces from radiation pressure in addition to gravitational forces.
That means that the objects larger than about one meter in size will feel a constant headwind from the gas as they orbit. This headwind causes the larger objects to lose orbital energy, spiraling inwards. Meter-sized boulders migrate inwards so rapidly that they should be lost into the Sun before they can grow significantly. And yet, planets formed.
Possible solutions to these barriers to growth have been proposed. Perhaps turbulence in the disk caused clumping that allowed material to grow faster, or maybe magnetic fields played a role. These two barriers to growth remain the biggest unsolved mysteries.
Common Questions about Protoplanetary Disk
Molecular clouds are cold, dark, giant regions, with condensates of dust and molecular gasses.
Molecular clouds are present throughout the galaxy, and can be over 200,000 astronomical units in size. In fact, these molecular clouds serve as nurseries for the creation of new stars.
Protoplanetary disks contain gasses, like hydrogen and helium, and also micron-sized dust particles.
When small perturbations in the motion of the gas and dust in molecular clouds occur to cause the density to get sufficiently high in a region, then that area begins to collapse under its own mutual gravitational attraction. This is known as a cloud core. Inside a cloud core, the densest regions collapse the fastest, and so the cloud keeps fragmenting into smaller and smaller denser regions. Eventually, one of these cloud cores collapses enough to create a central region dense enough to form a proto-star, surrounded by a cloud of gas and dust collapsing in on the star. This spherical cloud is about 10,000 astronomical units in radius and is called a planetary nebula.