How long did it take for the planets in our Solar System to form? Much depends upon the surface density of the solar nebula protoplanetary disk, the gas and dust from which the planets emerged. And the problem with surface density — mass per area — in these settings is that it’s hard to observe with our current instrumentation. Looking at distant systems in the process of formation, we see mostly dust and miss larger objects.
Thus an estimate based on known factors is called into play. It produces the so-called minimum mass solar nebula. Using it, scientists can estimate solar nebula mass by starting with the rocky components of each planet, adding hydrogen and helium until the composition resembles that of the Sun. Spread that mass over the area of each planet’s orbit and you get disk masses that look like what we see in systems around other stars.
But there’s a problem. The low surface densities this model produces aren’t sufficient to allow the planets to form in a reasonable period of time. This is where Steve Desch (Arizona State University) goes to work. Here’s the problem he sees:
“I was thinking about planet formation and noticing that all the current models failed to predict how Jupiter could grow to its current size in the life time of the solar nebula. Given Jupiter’s composition and size, models predicted it would take many millions of years for it to form, and billions of years for Uranus and Neptune – but our solar system isn’t that old.”
Desch turned to a different model for considering the solar nebula. The ‘Nice model’ (named after the French city whose open markets — fabulous olives! — still have my head spinning) operates according to fundamentally different assumptions. For one thing, the model assumes the giant planets formed much closer together than their present position suggests, with Neptune forming at around 15 AU. Using this model to assume a much more tightly packed solar nebula, Desch was able to simulate a nebula with smooth variations in surface density with distance from the Sun, although that density falls off sharply at the outer edge.
In fact, using this model, all the planets could be accounted for, assuming you switched Uranus and Neptune in their places. Desch now thinks that for the first 650 million years of the Solar System, Neptune was closer to the Sun than Uranus. Planet formation, meanwhile, now seems to fit the kind of time restrictions imposed by the early nebula. Desch again:
“The surface density of the solar nebula isn’t what we originally thought – it is actually much higher – and this has implications for where we formed and for how fast planets grow. A higher surface density of the solar nebula means that Uranus and Neptune formed closer and faster, in only 10 million years instead of billions.”
Just as significant, Desch thinks he can explain why the solar nebula falls off so sharply in density at large distances from the Sun. The process at work is photoevaporation, with material at the outer edge of the disk being constantly removed because of the effect of ultraviolet radiation from nearby massive stars. The paper examining all this is Desch, “Mass Distribution and Planet Formation in the Solar Nebula,” Astrophysical Journal 671 (December 10, 2007), pp. 878–893 (abstract). Dr. Desch has also made the full text available via his Web site.