Composition and Size of the Planets

At low pressures, such as those found in space, cooling materials generally condense directly from a gas into a solid. Different materials change into a solid at different temperatures. Ice-forming materials, such as water, ammonia, and methane, condense at low temperatures. Rock-forming materials condense at much higher temperatures.

When the solar system formed, the temperatures near the sun were very high. The temperature near Mercury's orbit was about 1200 K, and near Mars's orbit it was roughly 500 K. Water and other ice-forming materials vaporize at these temperatures, so they did not play a role in the early formation of the terrestrial planets. The terrestrial planets are relatively small and rocky. In part, that is because the inner solar system was too hot during their formation for ice-forming compounds to condense.

In contrast, ice-forming materials could condense in the outer solar system because it was much colder. Much more material was available, and these planets grew to many times the size of Earth. As these planets grew, their gravity increased, and they were able to capture the extremely abundant hydrogen and helium gas from the surrounding space. The gas giants are large and have low densities because the outer solar system was cool enough for ice-forming compounds to condense.

As Figure 29 shows, astronomers have been able to observe protoplanetary disks around distant newborn stars. Astronomers have also detected more than a hundred planets in orbit around distant stars. This evidence of planets forming around other stars, plus the results of computer simulations, provides support for the nebular theory.