Discussion: Origin of the Solar System

Cloud Collapse Simulations

Simulation of cloud collapse - face-on view   Simulation of cloud collapse - edge-on view

Inner vs. Outer Solar System

The makeup of these planetesimals depended on the temperature. The inner disk was too warm for water to freeze, so planetesimals contained sillicates and metals, but no ice. In the outer disk, beyond the `snow line', frozen water was included in planetesimals (as were other ices); as a result, there was more to work with, and planets could grow to about 15 times the mass of the Earth.   Solar nebula and frost line
Solar System Formation [U. Colorado]

In regions of the protoplanetary disk where a lot of hydrogen and helium was available, a planet 10 to 15 times the mass of the Earth could capture gas directly from its surroundings. Two planets did so, and they became Jupiter and Saturn.

The Age of the Solar System

Radioactive elements decay according to a definite time-table. For example, uranium-238 decays to lead-206 with a half-life of 4.6 billion years; if you start with a sample of pure uranium-238, and wait 4.6 billion years, half the atoms will have turned into lead-206.

Radioactive dating
The Cartoon History of the Universe

Meteorites -- fragments from the asteroid belt which fall to Earth -- have ages of 4.6±0.1 billion years. The oldest Moon rocks are only slightly younger: 4.4 billion years. An entirely separate chain of reasoning puts the Sun's age at 4.6 billion years. The evidence points to 4.6 billion years for the age of the entire solar system.

Joshua E. Barnes (barnes@ifa.hawaii.edu)
Last modified: October 9, 2006
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