Question

How did the gaseous planets come to have so much atmosphere and so little solid surface?

A good question, one still in debate. Simulations of our solar system’s formation suggest that the amount of rocky material present in the inner protoplanetary disk was enough required to form the terrestrial planets (Mercury, Venus, Earth, and Mars). However, we are still unsure how the gaseous planets may have formed from the same disk given that the density of matter in the disk grows thinner farther from the sun. Since we know that rocky bodies accrete rapidly, the terrestrial planets must have formed while nebular gas was still strewn across the solar system. However, these planets were so small that their low gravities could not hold on to much gas in their atmospheres. Therefore, we must infer that the rocky cores that must have formed in the vicinity of Jupiter and Saturn were already large enough to hold on to this gas. But how large was large enough? Calculations suggest that these rocky cores would have to have been about 10-20 times the mass of the Earth to begin to hold the super-light gases of hydrogen and helium. However, some simulations have suggested that there may not have been enough mass in the outer planetary disk to form such cores. While many alternative hypotheses have been suggested, scientists admit that we have only a dim understanding of the early solar system.
March 11, 2002