Well overdue for a little science, this time from Livescience.
A new theory of the solar system's formation has been proposed that helps explain some interesting issues regarding our system's gas giants, Jupiter, Saturn, Uranus and Neptune. Current theories of stellar evolution can't explain how these gas giant planets formed so far away from our sun - if they formed in their current positions, it would have taken longer for them to form than the age of our solar system.
The new theory proposed that the larger planets formed nearer the Sun and migrated outward debuted in 2005, but had some mathematical modelling problems that have now been addressed by Arizona State astrophysicist Steven Desch. His new idea is that these larger planets not only formed closer to the Sun than they are today and migrated outward into their present orbits over millions of years, but that Uranus and Neptune actually swapped spots in the solar planetary order in the process!
"The solar system is 4.6 billion years old. The formation of rocky planets, from collisions between ever-larger objects, is a fairly rock-solid theory. But how the outer giants developed remains an open question.
"Models predicted [Jupiter] 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 said. "Having a denser disk of gas bunched up around the sun could explain the two planets' formations, but only if they switched places.""
If Desch's math adds up correctly (and so far no one from peer review has come forward spotting anything) about 650 million years after the formation of our system, Neptune moved outside the orbit of Uranus, making it the most distant main sequence planet (Pluto being recently demoted to "dwarf" planet status). All of our system's planets are thought to have formed in a relatively quick period of time, roughly about 10 million years. Up to this point, no one has had a very satisfactory explanation on how this happened.
Pretty interesting stuff. Hate to think about the math involved in orbital modelling, makes my head hurt to even contemplate it.