Building Planets at PSI: The Origin of the Solar System

Page design by Gregg Herres and William K. Hartmann

Stars form in regions of dense nebulae, consisting of gas and dust. Darker clouds are regions of thicker dust and gas, which can shrink and form small groupings of stars. For its first 50 to 100 million years, the sun and solar system were probably embedded in a region that looked like this.

Painting copyright William K. Hartmann.

What do you mean by building planets?

At PSI, we have what is probably the best theoretical computer model in the world to represent the processes by which dust and asteroid-like particles aggregated into planets. This model has been developed over 20 years of research at PSI.

How are planets built?

In the original solar system, the sun was surrounded by a disk-shaped cloud of dust and gas after it formed, 4.55 billion years ago. In this cloud, or "solar nebula," innumerable particles of dust condensed out of the gas and orbited the sun in nearly circular orbits. Adjacent particles underwent collisions at relatively low speed, in the same way that high speed race cars moving around a circular track might nudge into each other. Our program represents the motions of these particles at different distances from the sun, and tracks the results of collisions based on actual physical and mechanical properties. When we run the program with different starting conditions, we can actually see the innumerable small particles aggregate into smaller numbers of big bodies, eventually producing a system of a few planets. The program allows us to study conditions under which planets form.

Inside the solar nebula. This view shows the scene in the region of the Earth less than a million years after the sun formed. Small grains of dust are aggregating into "planetesimals." Planets grew by collisional aggregation of these objects.

Painting copyright William K. Hartmann.

Planets were assembled out of small "building bodies" called planetesimals, which themselves aggregated from the dust in the solar nebula. This view shows one such planetesimal, in the solar nebula at about the distance of present-day Earth. The nebula is so dusty that the sun is highly reddened like the sun seen through dusty haze at sunset. The plane of the dust can be seen in front of the sun. A smaller planetesimal has just collided with this one at high speed, and the impact will make a crater (left).

Painting copyright William K. Hartmann.

When planetesimals collided too fast (more than 40 m/s) they broke each other apart. Under today's regime of high speed collisions in the asteroid belt (typically 5 km/s), asteroids are catastrophically fragmented into small pieces, some of which eventually reach Earth as meteorites.

Painting copyright William K. Hartmann.

How did PSI develop the program?

Work on this program began in the 1970s as a joint project among PSI scientists. Drs. Stuart Weidenschilling and Donald R. Davis concentrated on describing the celestial mechanics of the motions of such particles, moving around the sun in a gaseous nebula. This work was aided over the years by former PSI staff members as well, such as Drs. Richard Greenberg, Dominique Spaute, Clark Chapman, and John Wacker. At the same time, in the late 1970s, PSI researcher William Hartmann began a program at Ames Research Center to study the results of collisions at various speeds. For example, he found that rocky particles bounce apart at speeds less than 30-50 m/s, but shatter each other at larger speeds, and that coatings of "regolith," or pulverized surface rock, strongly effect the results of the collisions. This experimental program has been carried on and expanded at PSI by Drs. Eileen Ryan, Donald Davis, and Stuart Weidenschilling. During the late 1980s and 1990s, this program has been further developed, especially by Stuart Weidenschilling, who has included and refined many additional effects, such as gas drag on the particles, and expanded the program to include simultaneously various zones at different distances from the sun.

What can be learned from this work?

As recently as one or two generations ago, scientists were very uncertain on how planets formed. Some thought it took a rare catastrophe to make a star produce planets; others thought it was more common but could not explain the details.

The PSI model has been one important tool in the growing realization that planets probably form fairly commonly from dusty debris left over after stars form. Disk shaped nebulae of such debris have been seen around other newly formed stars. Our model give strong evidence that dust particles tend to aggregate, and once the aggregation starts, the big ones tend to sweep up the smaller ones. As this process continues, the gravity of the biggest planetary bodies tends to dominate, and they sweep up most of the other bodies, producing a system with a few planet-sized worlds. If two planets form at too nearly the same distance from the sun, they will eventually collide, so that the planetary system organizes itself to have one dominant planet in each zone.

With this type of program, we may be able to understand better how planets may (or may not) form around other stars with various properties. For example, is it a problem if the star is too big or too small? Can planets form around double stars? These questions await future answers.

The half-finished Earth. This reconstruction shows Earth at the time it had reached perhaps half its present size. A thin atmosphere and an ocean is beginning to form, but the heavily cratered surface can be seen through the clouds. The collision that formed blew out mantle material to form the moon probably happened not long after this stage. (See our web page on Origin of the Moon).

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Building Planets at PSI: The Origin of the Solar System