Retrograde satellites lose momentum to the parent body and slowly spiral inward, which puts an upper limit (possibly not considered by these researchers) on the length of time the retrograde moons have spent as satellites, and obviously, will spend as satellites.Newfound Moons Tell Secrets of Solar SystemThe 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.
by Henry Fountain
August 12, 2003
Now... What if the gas giants are the result of ejections from the sun's mass during the sun's early formation. Say the proto-star was spinning too quickly and didn't have the mass and thus gravitational pull to maintain its shape, it wobbled, it spun off balls of protogas, and spit them out in arcs, of which some managed to fall into stable orbits, while others escaped the solar system entirely, and still others eventually fell back into the sun.
If this is the case, the universe is probably filled with trillions of gas giants just floating between the stars.
I wonder, how dense are these gas planets? I mean, is the gas so dense as to the consistency of liquid? I'm wondering if, as you go into the planet to the core, if it becomes liquid and perhaps eventually as a solid...?
That may explain how these gas giants exist...
Birth of a Giant: How Did Jupiter Get So Big?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."
by Leslie Mullen
17 May 2001Solar System Makeover: Wild New Theory for Building PlanetsThe 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.
by Robert Roy Britt
9 July 2002Planet Puzzle: Theorists Wrestle with How They're BuiltRichard 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.
by Michael Schirber
7 March 2005
I'd always thought that, since binary star systems are so common, Jovian planets were simply stars that never achieved enough mass to light off.
I'm butting in here without a lot of knowledge or training, but it strikes me that planet formation looks a lot like weather, in terms of complexity. I'm not sure how you could expect to model it without a lot of histories on which to base your model, and even then, complexity would overwhelm any particular run of your model.
Hot Jupiters do not rule out alien Earths
New Scientist Space | 03/31/06 | Maggie McKee
Posted on 03/31/2006 8:21:28 PM EST by KevinDavis
http://www.freerepublic.com/focus/news/1607198/posts
Three week bump.
X-Planets FR 'blog
There is a book called "Rare Earth" that explains this.
We're going on a planet hunt
EurekAlert | 04/05/06 | Claire Bowles
Posted on 04/05/2006 10:53:38 PM EDT by KevinDavis
http://www.freerepublic.com/focus/f-bloggers/1610016/posts
