Posted on 03/29/2006 10:21:37 AM PST by SunkenCiv
For scientists who spend time thinking about how planets form, life would be simpler if gas giants like Jupiter and Saturn didn’t exist.
According to the standard model of planet formation, called "core accretion," planets form over millions of years as enormous blocks of rock and ice smash together to form planetary embryos, called "protoplanets," and eventually full-fledged planets.
Most scientists agree that core accretion is how terrestrial planets such as Earth and Mars were created, but the model can’t convincingly explain how gas giant planets like Jupiter and Saturn came to be.
One major problem is that developing gas giants through core accretion takes too long. According to the best current models, the process requires several million years—longer than the typical observed lifetime of the stellar gas disks from which planets are born.
(Excerpt) Read more at news.yahoo.com ...
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.
Heh. :') Or perhaps the heliocentric universe is finally dying off.
Planets in all the wrong places
The Christian Science Monitor | 03/06/06 | Michelle Thaller
Posted on 03/06/2006 8:16:39 PM EST by KevinDavis
http://www.freerepublic.com/focus/f-news/1591313/posts
Rogue Planet Find Makes Astronomers Ponder TheoryEighteen rogue planets that seem to have broken all the rules about being born from a central, controlling sun may force a rethink about how planets form, astronomers said on Thursday... "The formation of young, free-floating, planetary-mass objects like these is difficult to explain by our current models of how planets form," Zapatero-Osorio said... They are not linked to one another in an orbit, but do move together as a cluster, she said... Many stars in our own galaxy, the Milky Way, may have formed in a similar manner to the Orion stars, she said. So there could be similar, hard-to-see planets floating around free near the Solar System.
by Maggie Fox
October 5, 2000
I'll come back to this later,...spending my time this morning following the twists and turns of the translation mysteries of the Iraq documents....
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...
I look forward to it.
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.
Did Jupiter Bully Other Planets in Sibling Rivalry?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.
by Robert Roy Britt
8 December 1999Jupiter gave birth to Uranus and NeptuneNot 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.
by Dr David Whitehouse
8 December 1999Jupiter's Composition Throws Planet-formation Theories into DisarrayExamining 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.
by Robert Roy Britt
Nov 17 1999
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.
:')
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.
Considering the millions of meteorites that hit these gas planets, and don't come out the other side, I suspect there is a solid core by this time.
I'd agree wholeheartedly, at least insofar as I think looking for a single model for all planet formation probably is wrongheaded.
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
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