My first post :)

 Correct me if I'm wrong, but I seem to remember a new planet forms by space debris (such as asteroids) colliding into one another and forming larger debris, and repeating this process until a planet-sized object has formed.

 This seems counter-intuitive to me.  Logic (to me, at least) dictates that if two large objects moving at high speeds crash into one another, they would break up rather than stick together. 

 I took two astronomy courses in college, and either zoned out or didn't fully grasp the planet formation lectures.

 Thanks to all in advance!

Is your intuition correct? Yes, and no.

In a pre-solar nebula, the particles are near enough and orbiting the central mass similarly. Therefore, the collision speeds are slow to begin with. As planetesimals form, they dwarf the nearby matter, which is mostly gas and dust, so even high-speed collisions favor assimilation.

But in the later stages of planetary formation, high-speed collisions and some mass-loss are inevitable. Even so, however, when major impacts occur enough of the impactor and target are vaporized that we have dust and gas forming again, and failing to reach escape velocity from the remaining accreted mass, so debris forms rings that eventually spiral in and otherwise coalesce into the planetary bodies.

The early Solar System would have been a violent place, indeed. But gravity wins and except for small bodies ejected from the inner parts of the system by encounters with planets there would have been a gradual settling out that favored accretion.

This is not all based on models and computer simulations, by the way. We have actual samples of the early Solar System in our hands and labs: meteorites. Studying the petrology, lithology, and mineralogy of meteorites provides strong clues about planetary formation and conditions in the early Solar System -- and these largely match the models and simulations.

Important questions remain. For example, did the gas giants form where they are today, or closer in and then migrate outward? Precisely how did chondrules form? How much of the presolar nebula's mass went into forming comets, TNOs, and Oort-cloud objects?

Thanks for the informative reply.

I assume that all the "stuff" in the pre-solar nebula was fairl uniform; is there any evidence to suggest why a planet would become terrestrial vs. jovian? Did it take the jovian planets approximately the same amount of time to form as did the terristrial planets?

The general idea is that as things began to clump, the heavy stuff moved inward. As soon as enough of it was concentrated in the center to form the Sun, the gradient got steeper and more and more heavy stuff stayed sunward.

Then, once the Sun lit up, the solar wind pushed all the remaining lighter stuff (like gases) to the outer regions which is where the gas giants are. This may not be the whole story, however, because as we find exoplanets (other solar systems) we also find very large gas giants orbiting much nearer their host stars.

As to timing, if we assume they all started forming at the same time, the gas giants might actually form faster as they start to accrete. That is, the gas giants need not take as long to differentiate and cool down to form a solid crust, since they can conceivably be called "planet" as soon as they form and "clear their neighborhood" ...

Quite interesting...so if this: 

chipdatajeffB:

so debris forms rings that eventually spiral in and otherwise coalesce into the planetary bodies.

is the case, why did the rings of Saturn not spiral in, is it the mass of the planet itself??

That's an excellent question.

Apparently Saturn's rings formed very much more recently than the rest of the Solar System and/or they are somehow being replenished.

The Cassini spacecraft is helping to answer this question. It's already provided images showing that ice geysers on Saturn's satellite Enceladus are replenishing the outer rings. However, that doesn't seem to be enough (at the present rate of outflow from Enceladus).

This lends credence to the idea that a collision shattered one or more of Saturn's icy satellites, resulting in the rings (formed from the debris), relatively recently.

Computer models show that without some outside energy applied, the rings should be lost within about 250,000 years. So, again, either they're very recent (astronomically and geologically speaking) or something is keeping them aloft.

Another thing that Cassini has confirmed (and expanded greatly upon) is the Voyager finding that "shepherd moons" among Saturn's tiniest satellites have a stabilizing effect on the ring system overall, though they cause perturbations locally among the rings.

True ring science has been with us for several decades now (since Voyager gave us our first up-close-and-personal in the 1970s) but, in the words of Brent Spiner's character (the lead scientist at Area 51) in the movie Independence Day: "In the last few days since those guys (in this case Cassini) showed up (in 2004), things have really been exciting!"

Stay tuned. Here's a great site to learn about it: CICLOPS.

This is all very interesting to me. Saturn and it's rings were first things I ever saw through an observatory telescope, and it was quite literally breath-taking.  Such wonderful memories

 I digress...

 Do astronomers have a working theory as to: 1) why rings are mostly (exclusively?) the stuff of the outer planets?  and 2) Why don't moons have rings?  Perhaps the gravity of the host planet is too strong to allow for this?

chipdatajeffB:

Computer models show that without some outside energy applied, the rings should be lost within about 250,000 years.

 In this case, does "lost" mean pulled into Saturn by Saturn's own gravity, or flung in to space by other forces?

 And finally... would it be plausible for the rocks and ice that make up planetary rings to ever form into a planetoid or moon, like what happened in our pre-solar galaxy?

 

Neptune's 8th Ring:

Do astronomers have a working theory as to: 1) why rings are mostly (exclusively?) the stuff of the outer planets? 

Yes. It's largely because the outer planets are very much further apart, so there are fewer gravitational resonances to upset the orbits of ring particles, and because the larger planets each have a bevy of small satellites to contribute to the "shepherding" effect.

... and 2) Why don't moons have rings?  Perhaps the gravity of the host planet is too strong to allow for this?

Yes, and again there is the problem of gravitational resonances disrupting stable orbits of ring particles. Another way to put that is that rings can't form in regions of strong perturbational resonances, in the first place, or stay there for long if they form via a collision.

chipdatajeffB:

Computer models show that without some outside energy applied, the rings should be lost within about 250,000 years.

 In this case, does "lost" mean pulled into Saturn by Saturn's own gravity, or flung in to space by other forces?

It actually means Both: both sets of forces are at work.

 And finally... would it be plausible for the rocks and ice that make up planetary rings to ever form into a planetoid or moon, like what happened in our pre-solar galaxy?

Well, it happened with Earth and the Moon, but there were not perturbational gravitational impediments to that nearby. Saturn's rings, for example, lie within the planet's Roche limit, where a body large enough to have a stable orbit at that distance will be torn apart by the differential gravitational forces across its (radial to the planet) diameter. That works against the formation of rings, which is one reason scientists believe that Saturn's rings resulted from a collision between a large satellite and an interloper of some sort.

Traditional hypotheses on the formation of planets by a snowman is failed today. All becomes clearer that the planetary systems are formed namely in binary systems. And the relative positioning of planets is defined by influence of the second component.

 

Our solar system is no exception to this rule. The role of the second component in it make an underdeveloped Star - the Jupiter, which is  formed by special way. Then Jupiter promotes formation of other planets. http://www.thescienceforum.com/viewtopic.php?t=15901&start=0

Normal 0 false false false MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Обычная таблица"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman"; mso-ansi-language:#0400; mso-fareast-language:#0400; mso-bidi-language:#0400;}

Welcome to the Forum!

KrupS:

Traditional hypotheses on the formation of planets by a snowman is failed today.

Where do you get that idea? The method of planet formation described above is quite well supported by data and is the method supported by planetary geologists today.

All becomes clearer that the planetary systems are formed namely in binary systems. And the relative positioning of planets is defined by influence of the second component.

Actually, planetary orbits in binary systems would be less likely to be stable. Gravitational resonances in systems with two large bodies work to destabilize smaller objects, not to stabilize them.

Our solar system is no exception to this rule.

Apparently it is. Where is the evidence that the Sun was once part of a binary system?

The role of the second component in it make an underdeveloped star - the Jupiter, which is formed by special way.

The solar nebula hypothesis fully explains the formation of Jupiter. It is almost 100 times less massive than it would need to be to become a star, or to be considered a stellar companion to the Sun.

Then Jupiter promotoes formation of the other planets.

http://www.thescienceforum.com/viewtopic.php?t=15901&start=0

The information at this link doesn't seem to reinforce your point ... ??

Jupiter and Uranus have "smaller" rings. Can I say that they are older (pulling in already) and have less "shepherding effect" by the moons ?

Tks

Jupiter's ring system is completely different from Saturn's and may, indeed, be as old as the planet itself. While this ring system is as yet poorly understood compared to Saturn's, we know the following based on observations by various spacecraft (going all the way back to Pioneer flybys) and images from the HST:

• The rings are composed almost entirely of dust.
• There are several (4) distinctly separate rings, and may in fact be two completely different ring systems.
• The inner Halo ring has the shape of a torus and may be as much as 10,000km at its thickest point (this is several thousand times as thick as Saturn's rings).
• Dust particles remain in the rings for from 100 to 1,000 years, so the rings are being replenished continually.

Scientists think the origin of the rings is collisions between meteoroids and its innermost satellites. It also seems likely that one or more satellites have been destroyed in this manner.

Jupiter is considerably more massive than Saturn, so its gravity well is "steeper". This means that masses inside its Roche limit are more easily destroyed and are less stable (will be attracted to the planet more quickly).

The rings of Uranus are very different. It has at least 13 much smaller rings, and the particles are generally larger than those of Jupiter's rings, and comparable to those of Saturn's although the largest are yet smaller than Saturn's.

Uranus's rings are narrower (radially) and spaced quite far apart. Shepherding by several of its satellites is possible: the rings exist only in several orbits of very stable orbital resonance.

Some of the rings are distinctly different colors, the outermost of them thought to contain a much higher percentage of water ice particles.

The rings' orbital dynamics indicate they must be far younger than the planet itself, yet there is no consensus on an actual age for them.

Normal 0 false false false MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Обычная таблица"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman"; mso-ansi-language:#0400; mso-fareast-language:#0400; mso-bidi-language:#0400;} Normal 0 false false false MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Обычная таблица"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman"; mso-ansi-language:#0400; mso-fareast-language:#0400; mso-bidi-language:#0400;}

I have opened extremely simple and universal way of the formation of planetary systems. According to this mechanism as  soon all Solar system, as satellite systems of planets-giants are formed the namely as binary systems. The basic idea is stated below.

 

chipdatajeffB:

Actually, planetary orbits in binary systems would be less likely to be stable. Gravitational resonances in systems with two large bodies work to destabilize smaller objects, not to stabilize them.

 

 

It is this thesis is the root cause of failures of the modern version of nebulyarnoy hypothesis. But if you turn it upside down, everything becomes easy. Gravitational perturbations necessary for the formation of ordered systems of celestial bodies.

When rock collides with cobble-stones, stone destroys  the rock to thousands-millions of small fragments. But the fragments remain in a gravitationally bound cluster. The cluster tooks a volume many times greater than the originally rock.   The cluster absorbs small stones and motes  more effective, than a solid body of the same weight.

KrupS:

I have opened extremely simple and universal way of the formation of planetary systems.

You can speculate all you like, but don't expect to convince anyone without evidence.

It is this thesis is the root cause of failures of the modern version of nebulyarnoy hypothesis.

You say it has failed, but planetary scientists would not agree. What makes you think it has failed?

Perhaps I am misunderstanding you here. Do you mean the whole planetary nebula theory is incorrect, or only a portion of it?

chipdatajeffB:

Perhaps I am misunderstanding you here. Do you mean the whole planetary nebula theory is incorrect, or only a portion of it?

 

I mean that only a portion is incorrect. Mainly the final stage.

Normal 0 false false false MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Обычная таблица"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman"; mso-ansi-language:#0400; mso-fareast-language:#0400; mso-bidi-language:#0400;}

chipdatajeffB:

You can speculate all you like, but don't expect to convince anyone without evidence.

 

It is not correct. I have a reliable logic chain of many reasons.

 

KrupS:

chipdatajeffB:

Perhaps I am misunderstanding you here. Do you mean the whole planetary nebula theory is incorrect, or only a portion of it?

 

I mean that only a portion is incorrect. Mainly the final stage.

Then I did misunderstand you. Explain how the final stage works in your model.

KrupS:

chipdatajeffB:

You can speculate all you like, but don't expect to convince anyone without evidence.

 

It is not correct. I have a reliable logic chain of many reasons.

 

Logic alone is not enough. You need to support the logic with data. If it's purely theoretical (no observational data to back it up) then you need to support it with physics and mathematics that work (i.e., show the idea is possible).

 

In my variant the protoplanetary disc was created as a result of collision in space two protostars - giant gas-dust balls. As a result of this impact their dense cores are stored as unity objects, and shells dissipate. The binary system the Sun – the Jupiter had created as a result of this collision . Gas-dust protoplanetary disk had created  in addition. The other planets was formed due to Jovian gravitation force.

 

As near as we can tell, collisions between stars are extremely rare.

If a protostellar nebula condensed with multiple stars, any merger would like take place very early -- long before the separate stars accumulated enough mass to be called prestellar, and almost certainly before they could light up and be called stars. Computer models used to depict different protostellar nebulae show that when multiple dense concentrations in the nebula grow larger than brown-dwarf size, ejection of one all but the most massive is quite likely, and that it happens very early.

Astronomers must revise these models to account for the creation of so-called "hot Jupiters" which have been discovered in exoplanetary systems (they are, according to most current models) too near their host stars to be accommodated by the models.

Perhaps there is room for such a collision, but it certainly complicates the existing situation rather than simplifying it.

As to the other planets forming due to Jovian gravitational force, we need not rely solely on models to understand why this would not work: Jupiter has  been observed to alter the orbits of comets and asteroids; we have calculated its effect on the orbits of the Gas Giants and even Mars and the Earth; we have visual evidence (the Asteroid belts, the Kirkwood Gap, etc.) that it acts to clear out the Solar System, rather than to create it.

Jupiter certainly did play a major role in the formation of the Solar System; of that there can be no doubt.