We don't have spacecraft to take us outside our solar system but astronomers have still been able to develop a good understanding of how our solar system formed and in turn, how others formed. In the last dozen years, the nearly 300 exoplanets have been discovered have added to our knowledge base.

Conventional knowledge said most solar systems were like our own but three Northwestern University researchers questioned that assumption and explored the question in detail. What they learned is that the solar system in which the Earth orbits our sun is actually uncommon.

Edward Thommes, Soko Matsumura and Frederic Rasio were the first to develop large-scale, sophisticated computer simulations to model the formation of planetary systems from beginning to end. Because of computing limitations, earlier models provided only brief glimpses of the process. The findings of their study titled, "Gas Disks to Gas Giants: Simulating the Birth of Planetary Systems," are detailed in the August 8, 2008 issue of Science magazine.

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CLICK IMAGE FOR FULL SIZE. This depiction shows the range of computer simulations used to explore the formation of extra-solar planetary systems. It shows a planet-forming circumstellar disk (the star is in the center) seen at a very highly-inclined angle (close to edge-on) in three different starting condition scenarios (left to right). From top to bottom are snapshots of the system at three intervals between the start of the universe and 500 million years of evolution, at which point the disk makes planets and is cleared out of material.

At left is, "Typical disk which grows gas giants," in which the initial disk mass is high but the viscosity (stickiness) of the disk material is low. This system forms planets early and often, with lots of migration of these planets towards to the central star (where they are destroyed). Big gas giants (similar to Jupiter) are formed in the inner planetary system (where the Earth lives in our solar system). This model closely mimics the types of planetary systems seen today outside our solar system.

At right is, "Typical disk which does not grow gas giants," the case in which the disk mass is low but the viscosity is high. In this case, planet formation is too slow to allow big gas giants to form before the disk is gone.
In the middle is the rare case in between these two extremes in which big gas giants form, but do not migrate to the inner planetary system, and the orbits of all of the planets in the system are circular and stable. This is like our solar system, but it is one of the uncommon results of the simulations. Credit: Ed Thommes, University of Guelph, Canada

The researchers used a range of computer simulations to explore the formation of extra-solar planetary systems. They were able to show the action of a planet-forming circumstellar disk in three different starting condition scenarios at different intervals from the beginning of the universe to 500 million years of evolution. They found that our solar system represents the rare case in which big gas giants form, but do not migrate to the inner planetary system, and the orbits of all of the planets in the system are circular and stable.

"We now know that these other planetary systems don't look like the solar system at all," said Frederic A. Rasio, senior author of the Science paper, and a theoretical astrophysicist and professor of physics and astronomy in Northwestern's Weinberg College of Arts and Sciences. "We now better understand the process of planet formation and can explain the properties of the strange exoplanets we've observed. We also know that the solar system is special and understand at some level what makes it special."

The computer simulations were performed on a supercomputing cluster operated by Northwestern's Theoretical Astrophysics Group and partially funded by a Major Research Instrumentation grant from the National Science Foundation (NSF). Rasio's research group on exoplanets also is funded by a grant from the NSF Division of Astronomy.