Expert: Courtney Seligman - 11/16/2008

Question
QUESTION: Can you explain some of the processes involved in the alleged planet formation from dust particles in  prot-planetary discs.

Thanks


ANSWER: You can find a detailed, albeit somewhat scattershot discussion of this topic on my website, http://cseligman.com, as "The Origin of the Solar System" (http://cseligman.com/text/ssevolve/ssorigin.htm), but here's a summary of the concepts involved.

The solar system started out as a flattened, disk-shaped cloud of gas and dust circling the forming Sun, 4500 million years ago. In the outer solar system, it was very cold, so about 2% of the mass was in the form of solids (mostly ices, but also carbon compounds, and some dusty/rocky material). In the inner solar system, it was too hot for ices to exist, and very close to the Sun, even rocky materials would be vaporized, so only a small fraction of 1% of the material was in a solid form.

Early on, the solid materials would be in the form of microscopic and dust-size specks, so gravity was of little or no use in pulling things together; but if you chopped a planet into dust-size specks, you'd get a lot of them, so the disk would have been full of 'dust', and collisions between the dust particles would have been very frequent. As a result, within a few tens or hundreds of thousands of years, you would build up objects tens or hundreds of miles in diameter in the inner solar system, and objects hundreds or thousands of miles in diameter in the outer solar system (where there was more material to build them, so collisions would have been more frequent).

At some point, the forming Sun began to blow the gases out of the solar nebula (as the disk is called), probably by transferring its rotational energy to the nebula (stars like the Sun rotate very fast while forming, but much more slowly within a short time after their formation, so something must usally happen, to get rid of their rotational energy). The inner planets, being still too small to hold onto any of the gases gravitationally, would have been left as a large number of asteroid- and moon-sized solid bodies. But the outer planets, being larger and cooler, must have become large enough to gravitationally attract gases about the same time the Sun started blowing them away, so they accumulated a few Earth masses (for Uranus and Neptune) to a few hundred Earth masses (for Jupiter) of gas, before it was all blown away, making them "gas giants". (It should be noted that the original mass of gas must have been tens of thousands of Earth masses, or more, so even for the Jovian planets, the amount of gas captured before the rest was blown away was only a small fraction of the total.)

Over the next few millions of years, most of the remaining solid bodies collided with each other and the forming planets, so that by fifteen or twenty million years after the start of this process (that is, about 4480 to 4485 million years ago), there were only a few large objects scattered here and there, plus a huge number of much smaller bodies.

As the remaining large objects collided, the Moon was formed (as the result of an object bigger than Mars colliding with the forming Earth to create the more-or-less finished Earth), the northern half of Mars was blown into space (as the result of an object a thousand miles across running into the forming planet), and various other large impacts which may or may not have left still-visible evidence must have occurred. By 4400 million years ago, virtually all the really large objects were already part of one planet or another, or scattered through regions, such as the asteroid belt, too far from any planet to be of concern.

At this point, the planets were essentially the same size and mass as now, but there was still a large number (tens of millions or billions) of rocky and icy bodies scattered throughout their respective regions (rocky close to the Sun, icy further away). It took about 400 million years (until around 4000 million years ago) for most (99.9999%) of these smaller bodies to be swept up by the planets, either by direct collisions, or by near-misses turned into direct collisions by the gravity of the planets. In the asteroid belt, none of the objects were large enough for gravity to be a significant effect, so the region was never swept clear of small debris; but in the orbits of the major planets, virtually all the material was cleared by then (in fact, that's part of the current definition of a major planet -- something that is not only large, but has also essentially cleared its orbital region of other bodies).

So, save for gravitational attraction of gases by the outer planets during the last stages of their main formation (3 to 7 million years after the start of things, so about 4490+ million years ago), more-or-less random collisions were the most important factor involved in forming the planets, from beginning to end.

It's not clear whether you're also interested in things like the melting and differentiation of the planets, or the formation of their atmospheres; but if so, let me know, and I'll be happy to discuss that, as well.

Courtney Seligman
Professor of Astronomy
Long Beach City College


---------- FOLLOW-UP ----------

QUESTION: How has one to imagine the building large bodied objects as the result from attraction/collisions between dust grains in proto-planetary disks?

ANSWER: During the earliest stages of the formation of the Solar System, there would have been huge numbers of small particles orbiting the Sun. Under such conditions, all the particles in a given region would have nearly the same motion, because any differences in motion would be quickly damped out by collisions with other particles. (The same thing is going on in the rings of Saturn, and as a result of the near uniformity of motion, the rings are more than a hundred thousand times thinner than they are broad.)

Since the particles are moving so nearly identically, their speed relative to each other is very slow (probably only a fraction of a mile per second), and when they collide with each other (which would just be random collisions, not influenced by gravity, until much later in the game), they would tend to stick together. In the rings of Saturn, such collisions can build up larger objects in just a few months; in the Solar Nebula, it might have taken longer, but there would certainly be objects the size of small rocks and boulders within a few years after things started colliding, and objects the size of small moons and asteroids within a few tens of thousands of years.

I'm not sure if this is exactly what you had in mind, but if not, just let me know.

Courtney Seligman

---------- FOLLOW-UP ----------

QUESTION: If in proto-planetary disks micron-sized dust particles form larger aggregates by low speed collisions, how come I don't observe this kind of phenomena in my living room where collisions between dust particles undoubtedly is much more likely then in nebular disks?

Answer
I can see that my answer didn't directly address your main concern. Thanks for the clarification.

When interstellar or interplanetary dust particles stick together, they do so because they are 'sticky'. Partly, this is related to what they're made of. For instance, in the outer solar system, the particles are mostly made of ices and carbon compounds, which are very sticky, compared to rocky dust. But partly, it's a question of low-pressure gas physics (and for dust in our atmosphere, the fact that if the particles get too large, they'll gravitationally settle out on the ground).

In a dense gas, molecules of the gas are continually bombarding dust particles, pushing them around (this can actually be seen, if the particles are small enough, as "Brownian motion", and was used as an argument by Einstein for the reality of atoms as physical bodies). Under these conditions, if two particles aren't very strongly bound together, they can be knocked apart, by the gas molecules colliding with them.

In a near-vacuum, however, if two particles collide (particularly if they collide slowly), there is a much greater tendency for them to stick together. This effect is used in coating surfaces by "sputtering" -- vaporizing a material in a vacuum, and allowing the particles to collide with the surface to be coated. A great deal of the time, the particles will strongly stick to the surface.

I neglected to address the effects of stickiness in my earlier answer, because I like to contrast the relative velocities in the early solar system, when the particles were all going around the Sun with nearly identical motions, to that in the current asteroid belt, where the lack of constant collisions allows the solid bodies to gradually develop very different motions (as a result of gravitational perturbations by the planets). The high-speed collisions that result tend to break the asteroids into smaller pieces, while the slower collisions in the early solar system tended to build things up into bigger pieces (and in the same way, to do so even now in the rings of Saturn, where the relative velocities are very low). But as you note, if the particles didn't have a tendency to stick together, they could still just bounce off each other.

I might note that aside from the 'natural' stickiness discussed above, when the solar system was forming the solid material in each region was mostly close to its melting temperature. In the outer solar system, where the planetesimals were made of ices and carbon compounds, raising the temperature by a few hundred degrees would have been adequate to completely melt or vaporize them; and some of the compounds on their surfaces would have been even closer to the temperature required to do this, making them stickier than if they were completely frozen.

In the inner solar system, the materials would have been much 'tougher' (more refractory) rocky materials, but the temperatures were much higher, as well. At the orbit of Mars, temperatures must have been close to 1000 Fahrenheit degrees, and at the orbit of the Earth, closer to 1500 degrees. Because of this, some materials that were perfectly solid at Mars' orbit would have been vaporized, or be very close to doing that, at our orbit. This is thought to be the reason why Mars has a lower density than the Earth. Forming further out, where it wasn't as hot, it could accumulate low-density oxides and silicates which would have been less abundant (as solids) at the higher temperatures near our orbit. And as a result, even the rocky debris would have had a stickiness far greater than that of the much cooler rocks we find at the surface of our world, today.

I hope this clears up the matter, but as before, if you have any further questions, feel free to let me know.

Courtney Seligman