Solving Solar System Quandaries Is Simple: Just Flip-flop The Position Of Uranus And Neptune

...the planets weren't always in the order they are today. Four billion years ago, early in the solar system's evolution, Uranus and Neptune switched places. This is the result of recent work by Steve Desch, assistant professor in the School of Earth and Space Exploration at Arizona State University. The work appears in this week's Astrophysical Journal. Desch based his conclusion on his calculations of the surface density of the solar nebula. The solar nebula is the disk of gas and dust out of which all of the planets formed. The surface density -- or mass per area -- of the solar nebula protoplanetary disk is a fundamental quantity needed to calculate everything from how fast planets grow to the types of chemicals they are likely to contain... "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," Desch recounts... The new findings have other profound implications. "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," says Desch... "The distribution of mass falls off very steeply because the outer edge is constantly being boiled away through the process of photoevaporation, by the ultraviolet radiation of nearby massive stars." Prior to this, researchers had not considered the effects of photoevaporation on the mass distribution of the solar system.

most or all are dead links, but I didn't check 'em.

Did Jupiter Bully Other Planets in Sibling Rivalry?
by Robert Roy Britt
8 December 1999
One possible explanation, discussed in Thursday's issue of the journal Nature, is that Uranus and Neptune formed much closer to the center of the action than their current positions might indicate. In this scheme, Jupiter and Saturn were bullies of a protoplanetary playground, shoving the other two future giants out of the way.
Jupiter gave birth to Uranus and Neptune
by Dr David Whitehouse
8 December 1999
Not too long ago, scientists regarded the orbits that the planets circle our Sun as being the ones they were born in. Now they are realising that this is not the case. Uranus and Neptune may have migrated outwards and Jupiter may have come in from the outer cold. Scientists have always been slightly puzzled by the positions of Uranus and Neptune because in their present locations it would have taken longer than the age of the Solar System for them to form. Scientists from Queen's University suggest that the four giant planets started out as rocky cores in the Jupiter-Saturn region, and that the cores of Uranus and Neptune were tossed out by Jupiter's and Saturn's gravity.
Jupiter's Composition Throws Planet-formation Theories into Disarray
by Robert Roy Britt
Nov 17 1999
Examining four-year-old data, researchers have found significantly elevated levels of argon, krypton and xenon in Jupiter's atmosphere that may force a rethinking of theories about how the planet, and possibly the entire solar system, formed. Prevailing theories of planetary formation hold that the sun gathered itself together in the center of a pancake-shaped disk of gas and dust, then the planets begin to take shape by cleaning up the leftovers. In Jupiter's current orbit, 5 astronomical units from the sun, temperatures are too warm for the planetesimals to have trapped the noble gases. Only in the Kuiper belt -- a frigid region of the solar system more than 40 AU from the sun -- could planetesimals have trapped argon, krypton and xenon.

While lead researcher Tobias Owen does not put much stock in the idea that Jupiter might have migrated inward to its present position, other scientists on the team say the idea merits consideration. Owen expects the probes will find similarly high levels of noble gases in Saturn, Uranus and Neptune. Hints of these gases have even been found in the thick atmosphere of Venus, another planet now begging more study.
Newfound Moons Tell Secrets of Solar System
by Henry Fountain
August 12, 2003
The fact that most of the satellites' orbits are retrograde and eccentric speaks volumes about their origins: They had to have come from elsewhere, and been captured by the planets at some point. If they formed at the same time as the planets, from the spinning nebular disk, their orbits would be nearly circular and in the same direction as the planets' rotation, like the "regular" moons... In the case of the irregular satellites, they could not have shifted from an orbit around the Sun to an orbit around one of the giant planets without slowing down -- through friction in an atmosphere, perhaps; the influence of gravity; or a collision with another object... But there are two other possibilities for capture, Dr. Nesvorny said. One is that rapid growth of the core led to a corresponding increase in gravity, enough to pull down a nearby object. The other is that captured objects were a result of a collision between two planetesimals, the force of the collision being enough to dissipate the energy of at least one of them. Either of these two theories may be a more likely explanation for the satellites of Uranus and Neptune, which formed differently from Jupiter and Saturn, without the large amounts of gas.

Retrograde satellites lose momentum to the parent body and slowly spiral inward, which puts an upper limit on the length of time the retrograde moons have spent as satellites, and obviously, will spend as satellites.

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