Gas Giants Jump Into Planet Formation Early

On the other hand (from October 19, 2004):

This artist's concept illustrates how planetary systems arise out of massive collisions between rocky bodies. New findings from NASA's Spitzer Space Telescope show that these catastrophes continue to occur around stars even after they have developed full-sized planets, when they are as old as one hundred million years. For reference, our own Sun, at 4.5 billion years old, is far past this late stage of planet formation. In this image, a young star is shown circled by full-sized planets, and rings of dust beyond. These rings, also called "debris discs" arise when embryonic planets smash into each other. One of these collisions is illustrated in the inset above. Spitzer was able to see the dust generated by these collisions with its powerful infrared vision. Credit: NASA/JPL-Caltech/T. Pyle (SSC-Caltech)



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didn't check the links, probably some are goners:

How Jupiter Got Big
by Leslie Mullen
June 2, 2002)
The traditional view is that Jupiter first formed a rocky core, several times the size of Earth, which then attracted a still larger outer envelope of gas. This process is known as "accretion." But there are problems with this model. The major problem is that if the large, gaseous planet did form by the gradual accretion of material, it would have taken a very long time to develop. Current estimates range between 10 million and 1 billion years. However, recent observations of distant stars suggest that planets have at most a few million years or less to gather up as much dust and gas as they can before the protoplanetary disk that feeds them disappears.
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'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.
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.
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.




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