Planetary Formation

According to the condensation theory of planet formation, planets form out of a spinning disk of gas that surrounds a newborn star known as a protoplanetary disk. The disk and star both originate in a rotating, collapsing cloud of material, and this process of collapse produces different abundances of materials in the disk at different distances from the star. In the higher temperature regions, comparable to the region around the planet Mercury in our solar system, the only kinds of material that can condense from the gas to the solid state (in this case, microscopic dust grains) are metals. Farther out, about where Venus, Earth, and Mars are now, the gas temperatures are lower. At these distances and temperatures, rocky materials such as silicates can also begin to form dust grains. Even farther out, the temperature gets low enough for water ice to form, and even farther from the star, ices of other compounds such as ammonia and methane can condense. But how do young planetary systems go from making dust grains to making planets?

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"The answer to that question, "explains astronomer David Jewitt, "is a process called binary accretion, where collisions between pairs of objects let larger and larger structures get put together. Collisions between grains, which are small and sticky, quickly lead to the construction of pebbles. Collisions between pebbles lead to rocks. Collisions between rocks lead to boulders. Collisions between boulders lead to planetesimals, rocky bodies the size of asteroids (bodies that orbit between Mars and Jupiter and range in size up to about 1,000 kilometers). If this accretion process happens far enough from the star, significant quantities of ices will be included in the planetesimals. This is the likely origin of comets. Eventually, the objects are large enough for gravity to begin compressing and heating the planetesimal interiors. As planetesimals grow even larger, gravity pulls them into a spherical shape, and the heaviest elements sink to the center of the body. Iron and nickel will, in this way, form the dense metallic cores of the young planets. Eventually, full-sized terrestrial planets, the rocky cores of gas giants(such as Jupiter and Saturn), and moons are formed through the accretion process.

The timescale for the accretion process depends on the density of gas and dust in the protoplanetary disk, because higher densities mean more frequent collisions. In the inner part of our pre-solar system disk, the density was high, and it took only 100 million years to build the terrestrial planets. The formation of the outer planets is a more complicated story, and scientists are still not sure how the gas and ice giants formed. "There are basically two models for the formation of giant planets like Jupiter, says Jewitt. In the first model, an icy terrestrial planet grows by binary accretion up to a mass about 5-10 times that of Earth, at which point new process begins. "When that small core planet reaches the critical mass, "Jewitt explains, "it has enough gravity to start pulling in gas from the surrounding disk. You get very rapid flow of gas onto this core, taking the planet all the way up to Jupiter's or Saturn's mass." This idea is called the core accretion model. According to Jewitt among most solar system astronomers the core accretion model is the preferred idea for the formation of the large gas planets Jupiter and Saturn. The problem with the core accretion theory for these particular planets is that building up the rocky center takes an exceedingly long time. Near the end of the accretion phase, the disk begins losing its gas content when radiation from the Sun causes it to disperse. If the gas disperses before the core has a chance to reach its critical mass, the idea cannot work. That is why astronomers have developed a second theory, called the hydrodynamic instability model. This begins with an enormous disk of gas that collapses in on itself due to the influence of its own gravity. Just as the star and its disk were formed from a gravitationally unstable cloud, some astronomers claim that planets form from the gravitational collapse of gas within the disk itself. "Parts of the disk would just contract under their own gravity, says Jewitt. "The planets would form directly without needing a core.