[SunkenCiv]

the aforementioned sidebars, in chrono order, excerpted, with one bit of emphasis added:

Birth of a Giant: How Did Jupiter Get So Big?
by Leslie Mullen
17 May 2001

To solve the problem of how gas giants form, Alan Boss, a planetary scientist at the Carnegie Institution of Washington and a member of the NASA Astrobiology Institute, has developed a different theory. Based on computer models, he believes planets like Jupiter could form as a result of instability in a star's protoplanetary disk... "I think this model of disk instability is an intriguing idea," said Hal Levison, principal scientist at the Southwest Research Institute. "This model could solve a lot of problems we have regarding Jupiter's formation, but we're quite far away as to knowing whether or not it is true. For instance, we don't know whether the clump stays there, or if it eventually destroys itself. It seems to me that the technology is not quite there yet to answer whether disk instability would lead to the formation of planets like Jupiter." ... "Models suggest that core accretion seems to need at least several million years to form Jupiter," said Boss, "yet most protoplanetary disks do not seem to exist that long. Maybe the solar nebula was particularly long-lived, though, in which case, solar systems like our own may be rare."

Solar System Makeover: Wild New Theory for Building Planets
by Robert Roy Britt
9 July 2002

The decades-old standard model holds that all planets begin as rocky objects, colliding and merging until a few reach the size of Mars or Earth. In a handful of cases, growth gets out of hand; gas is drawn to the rocky core and a giant planet develops. This process, called core accretion, takes about 8 million years to build a gas giant. Unlike gaseous Jupiter and Saturn, however, Uranus and Neptune contain large cores of rock and ice and only a thin shell of gas. Theorists now agree that beyond Saturn there was never enough material to build such planets using the crash-and-stick approach. Uranus and Neptune either formed closer in and migrated outward, or they were created by some other means. ...Boss' process builds bloated precursors to Neptune and Uranus almost overnight. Clumps of material develop in regions of gravitational instability in the disk of gas and dust that orbited the newborn Sun, and the dust settles for form central cores... At this stage, a planet-to-be would have been a loosely bound, rotating, banana-shaped object scrambling to condense into a smaller sphere. Meanwhile, another young star -- nearby, much larger and extremely hot -- bathed the outer regions of the nascent solar system in extreme ultraviolet radiation. Material was stripped from proto-Uranus and Neptune and "photo-evaporated" right out of the solar system. All the while, each of the two planets used its own gravity in a desperate attempt to gather its material into a denser object, a planet that would then become stable. "It was a race," Boss says... During these one million years, Earth and its rocky neighbors were unaffected as they crashed into each other and built their bulk (experts agree that collisional growth works for these so-called terrestrial planets)... Out to somewhere beyond Jupiter, the Sun's gravity worked to retain a sphere of gas that served as a protective halo against the harsh external radiation. Jupiter and Saturn formed by the same disk instability process, Boss says, and Jupiter kept its original bulk as it condensed into its final shape inside the zone of protection. Saturn straddled the two zones and became a mid-size gas planet. From here, Boss' scenario plays out based on ideas put forth by other researchers. The hot nearby star dies and the Sun is kicked out of the intense star-forming region, sent to dwell in a calmer part of the Milky Way.

Planet Puzzle: Theorists Wrestle with How They're Built
by Michael Schirber
7 March 2005

Richard Durisen of Indiana University... is advocating a middle road – a little gravitational instability mixed in with a little core accretion. "There are two camps in planet formation. Dick [Durisen] is trying to form a third camp," said Alan Boss of the Carnegie Institution of Washington. Boss is a leading figure in the gravitational instability camp.

[SunkenCiv]

more reprises, related to the Uranus/Neptune problem:

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.