Harvard Astronomy 201b

How do planets form?

In Uncategorized on April 12, 2011 at 1:15 am

This is NOT a question to which we yet “know” the answer. Instead, we know that the answer isn’t likely to be exactly the same for all planets (e.g. maybe gas giants form differently from rocky smaller planets?).  We also think we know about some of the processes that may be important.

Here’s a quick list of considerations (many of which are shown in this “game” video!):

1.  Disk formation. It is widely agreed that the starting point for planet formation is the disk that forms as material accretes from a molecular cloud core onto a disk around a forming star. (This illustration of “10-steps to star formation,” shows where disk formation fits into the larger star/disk/planet formation process.)  One very popular analytic theory of disks around young stars, and their associated outflows, is called the “X-wind” model.  The X-wind model is due to Frank Shu and colleagues, and it assigns a key role to the magnetic field, in slowing down the rotation of the disk and in generating bipolar outflows.  (This link illustrates the difference between “X-wind” and “D-wind” (or “disk-wind”) models.  In the D-wind, the outflow comes from the disk, rather than the X-point, where the magnetic field of the star connects to the disk.)

1a. Dust v. Gas. Circumstellar disks contain plenty of gas AND dust.  Some theories focus on the sticking of dust grains together into “planetesimals” as the first important step in planet formation, while others focus on instabilities that can cause the fragmentation of the whole disk into over-dense blobs that could be the seeds for future planets.  Also, the relative amounts and distribution of dust and gas (due to their differing opacities) will effect the internal (e.g. “dead zone”, cf. Gammie 1996) and external (e.g. flaring, e.g. van Boekel et al. 2005) structure of the disk.  These arrangements change over time (see “Time Evolution”, below).

from "Dynamics of Protoplanetary Disks," Phil Armitage, ARA&A, 2011.

2. Onset of planet formation. It is NOT clear when in the lifetime of a circumstellar disks planets begin to form.  It is fair to say that there are many competing theories! This video gives an overview of what happens, according to a “consensus” view…details, however, are not shown.  It’s also fair to say that it’s widely believed in 2011 that planet formation takes place roughly at the same time as the central star (or its disk) forms, rather than fully afterwards.

2a. Role of migration. Under many theories, it is far easier to form planets farther from the star, where material has a better chance of sticking together (see “Snow line”, below), and there’s more of it.  So, many theories (Wikipedia) rely upon forming planets at large distances and then letting them “migrate” inward (or outward) due to angular momentum exchange within the disk, as or after they form.  In some theories, many “early” planets crash into the star having been gravitationally drawn there as they migrate inward, and the solar systems we see are just what’s “leftover” when migration ends.

3. Turbulence in disks. In some theories, turbulence is harmful to planet formation, because it can increase the velocity dispersion amongst solid particles, potentially inhibiting growth to larger “planetesimals” with frequent, destructive, collisions.  In other theories, turbulence is helpful, because it can create vorticies which offer regions of relatively reduced velocity dispersion, therefore making it easier for particles to stick to each other.

Schematic Overview of Key Processes in Protoplanetary Disks, from Armitage 2011 ARA&A.

4. “Snow line.” Ice can be very sticky.  (Witness wet hands trying to pick up ice cubes w/o any “sticking” issues!)  It is widely believed that ice helps dust stick together.  So, in order to dust particles to stick (at least a little) when they collide (note that they also tend to break apart), a nice icy coating helps.  So, a key question for any disk is “how far from the star do you have to be before key molecules are solids, rather than liquids”?  That distance is called the “snow line,” and for our solar system it’s about 2.7 AU, illustrating that it is unlikely that the Earth (at 1 AU) formed where we find it now.   The solid particles that form beyond the snow line can grow to be either rocky planets, or the graviatational seeds for gas giants.

from “The Genesis of Planets” by Doug Lin–illustration by Don Dixon ©Scientific American/Nature Publishing Group 2008

5. Time Evolution.  The structure of circumstellar protoplanetary disks changes over time.  There may be a very long period of planet formation/migration, but eventually, the material will be used up, and will also potentially erode due to radiation from the central star.  The figure below, from Williams & Cieza’s 2011 Annual Reviews article, gives a nice breakdown of key phases, and also shows the nature of gas and solids clearly.

The Evolution of a Circumstellar Disk

Full caption:  The evolution of a typical disk. The gas distribution is shown in blue and the dust in brown. (a) Early in its evolution, the disk loses mass through accretion onto the star and FUV photoevaporation of the outer disk. (b) At the same time, grains grow into larger bodies that settle to the mid-plane of the disk. (c) As the disk mass and accretion rate decrease, EUV-induced photoevaporation becomes important, the outer disk is no longer able to resupply the inner disk with material, and the inner disk drains on a viscous timescale (∼105 yr). An inner hole is formed, accretion onto the star ceases, and the disk quickly dissipates from the inside out. (d) Once the remaining gas photoevaporates, the small grains are removed by radiation pressure and Poynting-Robertson drag. Only large grains, planetesimals, and/or planets are left This debris disk is very low mass and is not always detectable.

5a. Debris disks.  As shown above, once all planets a solar system will have have formed and the gas is largely gone, there is still leftover particulate matter.  The disk of “leftovers” (e.g. asteroids and comets), which can grind themselves into smaller and smaller pieces through collisions, is known as a “debris disk.”  Here’s an artistic video “showing” the debris disk around the star b-Pic (based on observational data).  These debris disks are low-mass, but bright enough to be detected in the sub-mm.  Their appearance depends strongly on inclination, as shown in this figure complied for the JCMT/SCUBA-2 “Debris Disk Legacy Survey”:

FIGURE 1: Debris disks seen with SCUBA, including (l-to-r) τ Ceti, ε Eridani, Vega (α Lyr), Fomalhaut (α PSa) and η Corvi. The disks are shown to the same physical scale i.e. as if all at one distance; actual distances are 3 to 18 pc. Sketches at the bottom demonstrate the disk orientations, and the star symbols are at the stellar positions. The spectral types and stellar ages are (l-to-r) G8 V / 10 Gyr, K2 V / 0.85 Gyr, A0 V / ~ 0.4 Gyr, A3 V / 0.3 Gyr and F2 V / ~ 1 Gyr. The images are at 850 μm except for η Corvi at 450 μm.

Suggested reading:

  • The Smithsonian Submillimeter Array has blazed a path (to soon be followed by ALMA) in observing Protoplanetary Disks, and the Protoplanetary Disks Research Group web page at the Harvard-Smithsonian Center for Astrophysics has links to several key publications.
  • Fantastic 2008 Scientific American article on the “Genesis of Planets” by Doug Lin…here’s an excerpt from its introduction: “The study of planet formation lies at the intersection of astrophysics, planetary science, statistical mechanics and nonlinear dynamics. Broadly speaking, planetary scientists have developed two leading theories. The sequential accretion scenario holds that tiny grains of dust clump together to create solid nuggets of rock, which either draw in huge amounts of gas, becoming gas giants such as Jupiter, or do not, becoming rocky planets such as Earth. The main drawback of this scenario is that it is a slow process and that gas may disperse before it can run to completion.
    The alternative, gravitational-instability scenario holds that gas giants take shape in an abrupt whoosh as the prenatal disk of gas and dust breaks up—a process that replicates, in miniature, the formation of stars. This hypothesis remains contentious because it assumes the existence of highly unstable conditions, which may not be attainable. Moreover, astronomers have found that the heaviest planets and the lightest stars are separated by a “desert”—a scarcity of intermediate bodies. The disjunction implies that planets are not simply little stars but have an entirely different origin.
    Although researchers have not settled this controversy, most consider the sequential-accretion scenario the most plausible of the two.”

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