Replenishment of Planetary Rings

[dougettinger]

I understood that planetary rings were a mystery since they should "evaporate" or dissipate by collisions or fall into the parent planet over only millions of years. Then I read recently, there is some thinking due to Cassini probe findings that the rings may be as old as the solar system. So which is the current thinking ? If the the outer planet have been collecting and keeping material for rings over the life of the solar system why does not Jupiter have the biggest ring system ?

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

I understood that planetary rings were a mystery since they should "evaporate" or dissipate by collisions or fall into the parent planet over only millions of years. Then I read recently, there is some thinking due to Cassini probe findings that the rings may be as old as the solar system. So which is the current thinking ? If the the outer planet have been collecting and keeping material for rings over the life of the solar system why does not Jupiter have the biggest ring system ?

I'm not sure there's much to add beyond what we have already discussed. Since all the gas giants have different sorts of ring systems, I don't think many believe that their formation is as simple as just collecting material over time. Satellite collisions are probably part of the explanation, and the reason that Jupiter has a smaller ring system is simply that no sufficiently large collisions occurred.

[dougettinger]

I understood that planetary rings were a mystery since they should "evaporate" or dissipate by collisions or fall into the parent planet over only millions of years. Then I read recently, there is some thinking due to Cassini probe findings that the rings may be as old as the solar system. So which is the current thinking ? If the the outer planet have been collecting and keeping material for rings over the life of the solar system why does not Jupiter have the biggest ring system ?

I'm not sure there's much to add beyond what we have already discussed. Since all the gas giants have different sorts of ring systems, I don't think many believe that their formation is as simple as just collecting material over time. Satellite collisions are probably part of the explanation, and the reason that Jupiter has a smaller ring system is simply that no sufficiently large collisions occurred.

What amazes me is how scientists sometimes flip the coin. On one side of the coin collisional debris if corralled by gravity will last almost forever without being accreted - such as the asteroid belt, the Trojan asteroids, and the outer planet ring systems. On the other side of the coin the collisional debris from the Earth-Moon collision magically accretes to form the Moon. Am I missing some factor that decides "to be or not to be" ?

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

What amazes me is how scientists sometimes flip the coin. On one side of the coin collisional debris if corralled by gravity will last almost forever without being accreted - such as the asteroid belt, the Trojan asteroids, and the outer planet ring systems. On the other side of the coin the collisional debris from the Earth-Moon collision magically accretes to form the Moon. Am I missing some factor that decides "to be or not to be" ?

Well, they're all very different situations. In the asteroid belt, the density is extremely low. There is no mechanism to cause accretion. In the ring systems, the density is still too low for gravitational accretion in the presence of tidal forces and satellite perturbations. Also, debris from a collision would start out in orbit around the gas giant, and needs only spread out. The Earth-Moon system is nothing like these. In that case, there was a presumed collision between two planet-sized bodies. You have a lot of concentrated mass, and so much energy released that the splatter is molten. So you end up with a large accreted body (the Moon), and lots of debris. But the Earth-Moon system doesn't provide a stable environment for small moons or rings, so any that might have been produced early on would not have lasted long.

[dougettinger]

What amazes me is how scientists sometimes flip the coin. On one side of the coin collisional debris if corralled by gravity will last almost forever without being accreted - such as the asteroid belt, the Trojan asteroids, and the outer planet ring systems. On the other side of the coin the collisional debris from the Earth-Moon collision magically accretes to form the Moon. Am I missing some factor that decides "to be or not to be" ?

Well, they're all very different situations. In the asteroid belt, the density is extremely low. There is no mechanism to cause accretion. In the ring systems, the density is still too low for gravitational accretion in the presence of tidal forces and satellite perturbations. Also, debris from a collision would start out in orbit around the gas giant, and needs only spread out. The Earth-Moon system is nothing like these. In that case, there was a presumed collision between two planet-sized bodies. You have a lot of concentrated mass, and so much energy released that the splatter is molten. So you end up with a large accreted body (the Moon), and lots of debris. But the Earth-Moon system doesn't provide a stable environment for small moons or rings, so any that might have been produced early on would not have lasted long.

I am trying to understand the nuances, and you are explaining them quite well. When the planets were first forming they each needed rocky, ferretic cores to begin the larger accretion process. So during the core building stage why did not a core start in the asteroid belt region as well as in the orbits of Mars, Jupiter, and Saturn ? Jupiter at that stage could not cause significant tidal forces. At one time scientists thought the asteroid belt was the result of a collision. Why is the asteroid belt collision hypothesis being abandoned in favor of the asteroid belt being the perimeter edge to the formation of ices in the latter stages of the protostar disk ?

I believe that some compounds found in asteroids (meteorites) require water for their formation and some compounds require very high temperatures. Don't these findings still present a problem for most asteroid hypotheses ?

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

I am trying to understand the nuances, and you are explaining them quite well. When the planets were first forming they each needed rocky, ferretic cores to begin the larger accretion process. So during the core building stage why did not a core start in the asteroid belt region as well as in the orbits of Mars, Jupiter, and Saturn ? Jupiter at that stage could not cause significant tidal forces.

I wouldn't say that planets formed cores during the accretion process. They started as homogeneous clumps of accreted material, brought together by a combination of fluid dynamic forces and self gravity. As structure developed, tidal forces became stronger. This would produce both positive and negative feedback effects- zones where material tended to concentrate, speeding up accretion, and zones where orbits were less stable, interfering with accretion. The asteroid belt is a region of unstable orbits, and I don't think anybody can say to what extent Jupiter (or the primordial Jupiter) was influencing things in the very early Solar System. A planet may have formed in the region that is now the asteroid belt. It may have moved to another location, or it may have been ejected. Models do a pretty good job of describing reasonable evolutionary scenarios for the planets once they had accreted and most of the nebular material either used or swept away. Earlier than that, they stop working. The environment was just too chaotic to be analyzed by projecting the models backwards.

At one time scientists thought the asteroid belt was the result of a collision. Why is the asteroid belt collision hypothesis being abandoned in favor of the asteroid belt being the perimeter edge to the formation of ices in the latter stages of the protostar disk ?

The position of the asteroid belt in terms of the ice zone isn't relevant to its nature or formation. It is simply understood as a region where tidal instabilities interfere with accretionary processes. Collisional processes are still important- there were once larger bodies in the asteroid belt, which have since fragmented- something that is continuing even now.

I believe that some compounds found in asteroids (meteorites) require water for their formation and some compounds require very high temperatures. Don't these findings still present a problem for most asteroid hypotheses ?

No- they support it. Water is present in the asteroid belt, and always has been. Many asteroids are differentiated, or are debris from differentiated bodies. So there was plenty of heat- many asteroids had molten interiors.

[dougettinger]

I am trying to understand the nuances, and you are explaining them quite well. When the planets were first forming they each needed rocky, ferretic cores to begin the larger accretion process. So during the core building stage why did not a core start in the asteroid belt region as well as in the orbits of Mars, Jupiter, and Saturn ? Jupiter at that stage could not cause significant tidal forces.

I wouldn't say that planets formed cores during the accretion process. They started as homogeneous clumps of accreted material, brought together by a combination of fluid dynamic forces and self gravity. As structure developed, tidal forces became stronger. This would produce both positive and negative feedback effects- zones where material tended to concentrate, speeding up accretion, and zones where orbits were less stable, interfering with accretion. The asteroid belt is a region of unstable orbits, and I don't think anybody can say to what extent Jupiter (or the primordial Jupiter) was influencing things in the very early Solar System. A planet may have formed in the region that is now the asteroid belt. It may have moved to another location, or it may have been ejected. Models do a pretty good job of describing reasonable evolutionary scenarios for the planets once they had accreted and most of the nebular material either used or swept away. Earlier than that, they stop working. The environment was just too chaotic to be analyzed by projecting the models backwards.

At one time scientists thought the asteroid belt was the result of a collision. Why is the asteroid belt collision hypothesis being abandoned in favor of the asteroid belt being the perimeter edge to the formation of ices in the latter stages of the protostar disk ?

The position of the asteroid belt in terms of the ice zone isn't relevant to its nature or formation. It is simply understood as a region where tidal instabilities interfere with accretionary processes. Collisional processes are still important- there were once larger bodies in the asteroid belt, which have since fragmented- something that is continuing even now.

I believe to have gathered two main points from this discussion. 1) The computer modeling has difficulty creating the initial seeds for acretion of the nebular material and creating from the beginning chaos the overall organization that we witness now; computer modeling does quite well if the starting point is chosen carefully. 2) The asteroid belt region may have easily possessed larger bodies that were finally ejected by tidal interactions after Jupiter gained a certain size, but not before creating some collisional debris. I hope my summary meets your approval. I am not trying to quote anybody; I am just trying to understand.

I believe that some compounds found in asteroids (meteorites) require water for their formation and some compounds require very high temperatures. Don't these findings still present a problem for most asteroid hypotheses ?

No- they support it. Water is present in the asteroid belt, and always has been. Many asteroids are differentiated, or are debris from differentiated bodies. So there was plenty of heat- many asteroids had molten interiors.

I certainly agree that water is present in the asteroid belt.

If there is enough heat from the protorstar disk interior to produce molten interiors and molten materials that have solified, then how did water survive on these smaller bodies without being boiled away into space? Certain compounds on asteroids can only be explained if water was present. Does this logic make the data somewhat anomalous ? Unless, as you mentioned one or more bodies were large enough and had time enough to have already differentiated and formed hydrous compounds on their crusts before collision(s) occurred. I have arrived at a satisfactory conclusion, if you also find this conclusion satisfactory (?)

By the way, I believe this conclusion does bear on the timeline for a mature Jupiter and Saturn to reach resonance and "rock the boat" in the asteroid orbital region.

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

I believe to have gathered two main points from this discussion. 1) The computer modeling has difficulty creating the initial seeds for acretion of the nebular material and creating from the beginning chaos the overall organization that we witness now; computer modeling does quite well if the starting point is chosen carefully. 2) The asteroid belt region may have easily possessed larger bodies that were finally ejected by tidal interactions after Jupiter gained a certain size, but not before creating some collisional debris.

There are models that do fine in producing the initial seeds of accretion, they just can't be used to reduce to the state of the current Solar System. That will always be the case, since the details of the initial nebula are unknown and always will be, and the system is exquisitely sensitive to initial conditions. I'd say that any large bodies formed elsewhere in the system may or may not have been involved in significant collision events. That includes hypothetical bodies formed in or near what is now called the asteroid belt.

If there is enough heat from the protorstar disk interior to produce molten interiors and molten materials that have solified, then how did water survive on these smaller bodies without being boiled away into space?

The primary heat source for planets and large asteroids is interior radioactive decay. Bodies can maintain molten cores for a long time, while still collecting water on their cold surfaces. Water is generally assumed to have been deposited in the inner system by comets.

By the way, I believe this conclusion does bear on the timeline for a mature Jupiter and Saturn to reach resonance and "rock the boat" in the asteroid orbital region.

I think otherwise for two reasons. First, there were important tidal resonances within the accretion disc long before any planets could be considered mature. All you need is a certain degree of mass concentration, not completely formed planets. Second, the position of the asteroid belt has probably shifted along with all the other planetary orbits. What we see today is the endpoint of a complex evolutionary process; working it backwards with any precision is very difficult. But that doesn't argue against a well understood qualitative theory of formation.

[dougettinger]

If there is enough heat from the protorstar disk interior to produce molten interiors and molten materials that have solified, then how did water survive on these smaller bodies without being boiled away into space?

The primary heat source for planets and large asteroids is interior radioactive decay. Bodies can maintain molten cores for a long time, while still collecting water on their cold surfaces. Water is generally assumed to have been deposited in the inner system by comets.

Radioactive decay is the primary heat source of planetary interior heat. The primary heat source external to the surfaces came from the heat of gravitational collapse and then subsequently from the radiative heat of the Sun when its core began to fuse hydrogen. This is why the inner terrestrial planets are more rocky with less ices and little atmosphere. Hypothetically, the water and other lighter elements and compounds came from colliding comets to support the current given conditions. Hence, I am not so sure water could collect on the surfaces of these gravitationally deficient smaller bodies.

What is amazing and hard to believe is how iron, its other associated, neighboring elements, and the elements with higher atomic numbers that decay readily become so concentrated in the inner cores of the inner planets and outer gas giants. I understand how these heavier elements would differentiate to reside in the center cores; but, I do not understand their proportions. Their must higher proportions to the other elements in these planetary bodies and asteroids contrast greatly to the known proportions of elements found in the Sun and interstellar molecular clouds that form stars. If the primordial materials for star making were well mixed within the main elements of hydrogen and helium making up the protostar disk's gases and dust, how did these very heavy metals become so concentrated in the cores of planetary bodies ? Something is amiss.

By the way, I believe this conclusion does bear on the timeline for a mature Jupiter and Saturn to reach resonance and "rock the boat" in the asteroid orbital region.

I think otherwise for two reasons. First, there were important tidal resonances within the accretion disc long before any planets could be considered mature. All you need is a certain degree of mass concentration, not completely formed planets. Second, the position of the asteroid belt has probably shifted along with all the other planetary orbits. What we see today is the endpoint of a complex evolutionary process; working it backwards with any precision is very difficult. But that doesn't argue against a well understood qualitative theory of formation.

My intuitions lead me to believe that the orbital radii indeed moved inward and roughly together as the central mass grew larger and more concentrated. During the T Tauri stage as the remaining protostar disk was being evacuated the outer planets should have continued to sweep up more material and grow in mass. However, their velocities increased in order to conserve angular momentum of the overall system. Centrifugal and gravitational forces could then have remained balanced to roughly preserve the outer planet orbital radii. Is this inductive reasoning remotely plausible ? To have numerous orbital crossings, in and out orbital movements against other orbits, and interchanges of orbits is troubling to me. Chaos beyond what we currently see should reign in these cases.

Doug Ettinger
Pittsburgh, PA

[wonderboy]

I understood that planetary rings were a mystery since they should "evaporate" or dissipate by collisions or fall into the parent planet over only millions of years. Then I read recently, there is some thinking due to Cassini probe findings that the rings may be as old as the solar system. So which is the current thinking ? If the the outer planet have been collecting and keeping material for rings over the life of the solar system why does not Jupiter have the biggest ring system ?

I'm not sure there's much to add beyond what we have already discussed. Since all the gas giants have different sorts of ring systems, I don't think many believe that their formation is as simple as just collecting material over time. Satellite collisions are probably part of the explanation, and the reason that Jupiter has a smaller ring system is simply that no sufficiently large collisions occurred.

Has it not been suggested that jupiter was part of a planetery collision/slingshot type affair in the early period of the solar system. I don't have the link but I did read it somewhere. If that happened i'm pretty sure it would have had a say in its ring system.


Paul.

[dougettinger]

You may be thinking of Jupiter's and Saturn's resonance creating perturbing forces that slinged some the primary bodies being formed in the asteroid belt region by accretion - leaving behind just small collisional debris. Chris is suggesting that any bodies captured by Jupiter did not collide with each other thereby creating no rings of collisional debris.

My personal hypothesis deviates from the mainstream. "Gas giant rings do dissapate over time. The rings are replenished by gas and dust that are captured in the outer solar system gravity field from passing interstellar materials. As the dust and gases are collected and fall inward their density becomes critical enough to gather around the gas giant gravity fields. However, Saturn, being the first gas giant to encounter this interstellar material creates the largest, most significant, newest, and longer lasting rings. This includes smaller bodies less than 1 km in diameter that can collide with each when caught in Saturn's gravity field." Any of you are welcome to hack away at this hypothesis, if you wish.

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

Has it not been suggested that jupiter was part of a planetery collision/slingshot type affair in the early period of the solar system. I don't have the link but I did read it somewhere. If that happened i'm pretty sure it would have had a say in its ring system.

There is a good case to be made that early in the development of the Solar System- about the first 600 million years- complex gravitational resonances resulted in significant changes in planetary orbits. That sort of environment probably resulted in quite a few collisions, but I don't think this explains ring systems as we observe them now. That's because it seems likely that Saturn's rings are fairly new, having developed in the last 500 million years or so. Less is known of other ring systems, but it is similarly likely that they formed long after the planetary orbits became stable. In fact, if ideas about ring system age are accurate, ring systems have probably come and gone several times in the last 4.7 billion years.

[Chris Peterson]

My personal hypothesis deviates from the mainstream. "Gas giant rings do dissapate over time. The rings are replenished by gas and dust that are captured in the outer solar system gravity field from passing interstellar materials.

I have a number of issues with this. First, there is no need to introduce interstellar materials, because there is an adequate amount of material from our own Solar System to both create and maintain the ring systems around gas giants. Second, Saturn's rings are almost pure water ice; if it was accumulated from an interstellar source, where is the other material? We don't see any sign of interstellar water in isolation from dust and complex molecular clouds. Third, if enough interstellar material were passing through the Solar System at a low enough velocity to be captured, we'd see meteoritic evidence of this on Earth. But no meteorite has ever been found that doesn't date to the formation of the Solar System. Interstellar material should statistically be significantly older or newer. Finally, the only interstellar material it would be possible for a gas giant to trap would have to have nearly zero velocity with respect to the Sun- something that I can not think of any way to explain.

[dougettinger]

My personal hypothesis deviates from the mainstream. "Gas giant rings do dissapate over time. The rings are replenished by gas and dust that are captured in the outer solar system gravity field from passing interstellar materials.

I have a number of issues with this. First, there is no need to introduce interstellar materials, because there is an adequate amount of material from our own Solar System to both create and maintain the ring systems around gas giants. Second, Saturn's rings are almost pure water ice; if it was accumulated from an interstellar source, where is the other material? We don't see any sign of interstellar water in isolation from dust and complex molecular clouds. Third, if enough interstellar material were passing through the Solar System at a low enough velocity to be captured, we'd see meteoritic evidence of this on Earth. But no meteorite has ever been found that doesn't date to the formation of the Solar System. Interstellar material should statistically be significantly older or newer. Finally, the only interstellar material it would be possible for a gas giant to trap would have to have nearly zero velocity with respect to the Sun- something that I can not think of any way to explain.

Thanks for the critique of my hypothesis. I superbly enjoy these discussions.

1) The Oort Cloud and what supplied the outer solar system with Kuiper Belt objects is still very hypothetical. Hence, the reason for non-planar, highly elliptical comets is also very hypothetical. The objects in our own solar system for the most part have seem to find stable resting places with only slight perturbations from its other brethren. The present objects are mostly in equilibrium with each other at least over time scales of tens of millions of years. Incoming materials from interstellar space if they exist would most likely be swept up by the large gravity fields of the gas giants with greatly reduced amounts finding their way to the inner solar system and eventually falling into the Sun or infrequenlty falling onto the terrestrial planets.

2) You make an excellent point about the how the ices and dusts are able to be separated in Saturn's rings. It might be the simple case of the heavier dust particles falling into the planet first similar to the operation of a centrifuge.

3) Their are isotopic anomalies of materials found on both comets and asteroids. One very hypothetical argument is that a nearby supernova blew material into our solar system causing younger isotopes of aluminum. It is definitely true the age of the oldest rocks on earth and in most meteorites / asteroids date to a certain peroid of time. Perhaps what we are seeing are the oldest remnants of the earliest collisions orbiting the inner solar system and being perturbed occasionally from the asteroid belt that was created in this earliest period. The chance of finding younger isotopes is remote, especially within the inner solar system because they are mostly intercepted by the outer planets. But finding these younger isotopes is much improved by space probes exploring the outer solar system.

4) As we well know, interstellar space is full of dust and gases cast off from novae, supernovae, and regular star making processes such the Herbig-Haro and T Tauri stages. Observations of GMC's, nebulae, and other galaxies attest to this fact. There is nothing to stop my thinking that our solar system or any other star with a planetary system can act like a giant sucking fan 80 AU's in diameter traveling through these dusts and gases and gathering some in its path. Of course, the trajectories of any captured gas and dust would necessarily have to be almost parallel and moving in the same direction as the Sun's path. Perhaps other tangential trajectories could be possible; we are not talking about a simple two body case here.

Again, as always, I welcome your comments.

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

1) The Oort Cloud and what supplied the outer solar system with Kuiper Belt objects is still very hypothetical.

I don't really understand what "very hypothetical" means. I'd describe it differently. We observe long period comets, and develop a theory that there must be a distant source for these. So we theorize the Oort cloud. This theory makes a number of predictions- details of orbital dynamics of comets, details of long period comet rates, and so forth. These predictions appear to match observations to a reasonable degree. In addition, numerical models describing the formation of the Solar System provide plausible scenarios for the existence of the Oort cloud. Thus, we ought properly refer to the Oort cloud as a theory, and one that is well supported by observation. Also arguing in its favor are the general lack of alternative explanations for long period comets.

Incoming materials from interstellar space if they exist would most likely be swept up by the large gravity fields of the gas giants with greatly reduced amounts finding their way to the inner solar system and eventually falling into the Sun or infrequenlty falling onto the terrestrial planets.

This is not consistent with basic orbital dynamics. Jupiter has a significant gravitational focusing effect extending on the order of 10 planetary diameters. That is, it is likely to capture material that is in solar orbit and comes within about that distance. The distance is much smaller for material which is not in solar orbit, and is likely to have a higher velocity. The size of Jupiter's gravitational focusing region is tiny compared to the surface of a sphere around the Solar System. If material were coming into the system from interstellar space, we would expect to see the same density anywhere in the Solar System.

2) You make an excellent point about the how the ices and dusts are able to be separated in Saturn's rings. It might be the simple case of the heavier dust particles falling into the planet first similar to the operation of a centrifuge.

Again, this is not consistent with orbital dynamics. Denser material in orbit does not decay faster.

3) Their are isotopic anomalies of materials found on both comets and asteroids.

Anomalies are easily explained by a variety of physical mechanisms. These all produce material that appears younger (often because it is younger). The important point is that we don't observe any older material, except in the form of interstellar dust grains embedded in 4.7 billion year old, primordial material. There is no bulk material older than the Solar System. That is completely inexplicable if we are being bombarded with interstellar material.

4) As we well know, interstellar space is full of dust and gases cast off from novae, supernovae, and regular star making processes such the Herbig-Haro and T Tauri stages. Observations of GMC's, nebulae, and other galaxies attest to this fact. There is nothing to stop my thinking that our solar system or any other star with a planetary system can act like a giant sucking fan 80 AU's in diameter traveling through these dusts and gases and gathering some in its path. Of course, the trajectories of any captured gas and dust would necessarily have to be almost parallel and moving in the same direction as the Sun's path. Perhaps other tangential trajectories could be possible; we are not talking about a simple two body case here.

If interstellar space has that much material, we could detect it. For instance, the Oort cloud would be easily detected if it weren't so small. If material at that density permeated interstellar space, it would be obvious because of scatter, absorption, and radiation. Additionally, there is no suggestion that supernovas produce anything larger than dust particles, so how would icy bodies the size of those making up Saturn's rings (centimeters to meters) form? The surface density of material dissipating from supernovas is far too low to support gravitational accretion.

[bystander]

Is Halley's comet an alien interloper?
New Scientist - 10 May 2010

OUR sun may have stolen the vast majority of its comets from other stars. The theft could explain the puzzling profusion of objects in a huge reservoir surrounding the sun called the Oort cloud.

The Oort cloud is a collection of comets thought to orbit the sun in a roughly spherical halo about 50,000 times as far from the sun as Earth - at the outer edge of the solar system. How did the comets get there? In the standard picture, they formed much closer to the sun, then migrated outward in a two-stage process.

First, the gravity of the giant planets flung them into elongated orbits to form a population called the scattered disc. Objects in the scattered disc come about as close to the sun as Neptune, but venture dozens of times further out, to more than 1000 times the Earth-sun distance. That far from the sun, the gravitational pull of the galaxy becomes significant, so many of the scattered-disc objects get pulled out to populate the Oort cloud.

There is a problem with this picture, however. Simulations have long predicted that this process could only populate the Oort cloud with 10 times as many comets as are currently in the scattered disc, while estimates based on observed comets suggest the ratio is more like 700 to 1.

Southwest Research Institute (SwRI)
Harold F. (Hal) Levison

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[dougettinger]

1) The Oort Cloud and what supplied the outer solar system with Kuiper Belt objects is still very hypothetical.

I don't really understand what "very hypothetical" means. I'd describe it differently. We observe long period comets, and develop a theory that there must be a distant source for these. So we theorize the Oort cloud. This theory makes a number of predictions- details of orbital dynamics of comets, details of long period comet rates, and so forth. These predictions appear to match observations to a reasonable degree. In addition, numerical models describing the formation of the Solar System provide plausible scenarios for the existence of the Oort cloud. Thus, we ought properly refer to the Oort cloud as a theory, and one that is well supported by observation. Also arguing in its favor are the general lack of alternative explanations for long period comets.

By "very hypothetical" I meant the Oort Cloud in not detectable and I understand clearly why detectability is difficult. Also, the Kuiper Belt objects have very little data in regards to composition to support the current theories.
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Incoming materials from interstellar space if they exist would most likely be swept up by the large gravity fields of the gas giants with greatly reduced amounts finding their way to the inner solar system and eventually falling into the Sun or infrequenlty falling onto the terrestrial planets.

This is not consistent with basic orbital dynamics. Jupiter has a significant gravitational focusing effect extending on the order of 10 planetary diameters. That is, it is likely to capture material that is in solar orbit and comes within about that distance. The distance is much smaller for material which is not in solar orbit, and is likely to have a higher velocity. The size of Jupiter's gravitational focusing region is tiny compared to the surface of a sphere around the Solar System. If material were coming into the system from interstellar space, we would expect to see the same density anywhere in the Solar System."

If incoming objects had as one of their velocity vector products, a velocity vector comparable to the orbital velocity and in the orbital plane of a planet in their area of entry and the other velocity vector product was parallel and comparable to the Sun's translational velocity, then it should be possible for an object to be captured by the Sun's gravity field. Then as the objects continue in their captured orbit and spiral inward they can easily come within the necessary planetary radii to be captured by an outer planet. I am suggesting that over thousands and millions of years this scenario is plausible. And please consider that the density of materials in interstellar space in front of the Sun's path do vary as the Sun orbits the galaxy and weaves in and out of the spiral arms. If you cannot accept the existence of such objects or their proper entry velocity vector for capture, then just assume the workings of orbital mechanics in this case from the point of entry. Is this capture possible just based on orbital mechanics ?
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2) You make an excellent point about the how the ices and dusts are able to be separated in Saturn's rings. It might be the simple case of the heavier dust particles falling into the planet first similar to the operation of a centrifuge.

Again, this is not consistent with orbital dynamics. Denser material in orbit does not decay faster.

My thinking is that denser materials form heavier objects on average will seek smaller orbital radii compared to their neighboring lighter objects with the same velocities in order to conserve angular momentum after they are perturbed into a planetary orbit. This separation would continue and then the heavier objects would eventually fall into the capturing planet leaving behind the icy rings. Is their an error in this thinking ?
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3) Their are isotopic anomalies of materials found on both comets and asteroids.

Anomalies are easily explained by a variety of physical mechanisms. These all produce material that appears younger (often because it is younger). The important point is that we don't observe any older material, except in the form of interstellar dust grains embedded in 4.7 billion year old, primordial material. There is no bulk material older than the Solar System. That is completely inexplicable if we are being bombarded with interstellar material.

The Later Bombardment of the early solar system would have produced inside the inner solar system wide spread collisional debris from the earliest of times, 4.6 billion years. Some residue from this bombardment would have become molten and show the date of the Late Bombardment Period, 3.9 billion year ago. Later time periods for aluminum isotopes in comets have been discovered and are supposely the result of a nearby Supernova. Material from older time periods more than 4.6 billion years are not readily available for analysis. These materials have been either absorbed in collisions or are for the most part captured in the outer solar system where the analysis of isotopes is rather skimpy.
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4) As we well know, interstellar space is full of dust and gases cast off from novae, supernovae, and regular star making processes such the Herbig-Haro and T Tauri stages. Observations of GMC's, nebulae, and other galaxies attest to this fact. There is nothing to stop my thinking that our solar system or any other star with a planetary system can act like a giant sucking fan 80 AU's in diameter traveling through these dusts and gases and gathering some in its path. Of course, the trajectories of any captured gas and dust would necessarily have to be almost parallel and moving in the same direction as the Sun's path. Perhaps other tangential trajectories could be possible; we are not talking about a simple two body case here.

If interstellar space has that much material, we could detect it. For instance, the Oort cloud would be easily detected if it weren't so small. If material at that density permeated interstellar space, it would be obvious because of scatter, absorption, and radiation. Additionally, there is no suggestion that supernovas produce anything larger than dust particles, so how would icy bodies the size of those making up Saturn's rings (centimeters to meters) form? The surface density of material dissipating from supernovas is far too low to support gravitational accretion.

I will accept the idea that supernovas produce only dust particles, but keep in mind that no detection is possible for settling this theory. But to my surprise the theories of an early chaotic solar system and the Nice theory have produced fairly large orbs that have been ejected into interstellar space. Perhaps every star system does the same thing. Also, a recent paper claims that very cool brown dwarfs are just now being detected within 9 light years of Earth. The paper is suggesting that the solar neighborhood is rife with these dim bodies. My suggestion is that the density measurements of materials in interstellar space, even the local neighborhood, does not reveal perhaps a surprisingly large population of larger bodies eager to become planetisimals of passing star systems. Can orbital mechanics make this possible ?
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Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

By "very hypothetical" I meant the Oort Cloud in not detectable and I understand clearly why detectability is difficult.

I don't like that definition. "Detectability" is not a good criterion for something's existence. In a very real sense, the Oort cloud is "detectable" in our observations of comet sources, and in numerical simulations. Direct detection isn't a requirement. Nobody has seen dark matter, but it is widely accepted as real. The Oort cloud is hypothesized, and its existence is less certain than dark matter, but it is nevertheless well supported by several lines of evidence, and most people consider its existence reasonably likely. I wouldn't describe it as "very hypothetical".

If incoming objects had as one of their velocity vector products, a velocity vector comparable to the orbital velocity and in the orbital plane of a planet in their area of entry and the other velocity vector product was parallel and comparable to the Sun's translational velocity, then it should be possible for an object to be captured by the Sun's gravity field.

Sure. But this has to represent a very small fraction of the material in interstellar space. If there is enough stuff traveling slowly in the plane of the ecliptic, there is enough to be passing through slowly from out of the ecliptic, and enough to be visible optically in all directions. And that simply doesn't seem to be the case.

My thinking is that denser materials form heavier objects on average will seek smaller orbital radii compared to their neighboring lighter objects with the same velocities in order to conserve angular momentum after they are perturbed into a planetary orbit. This separation would continue and then the heavier objects would eventually fall into the capturing planet leaving behind the icy rings. Is their an error in this thinking ?

I don't know any physical mechanism that would sort objects into different orbits based on either their density or their mass. I don't really understand the mechanism you are suggesting; it doesn't sound physically plausible to me.

Material from older time periods more than 4.6 billion years are not readily available for analysis. These materials have been either absorbed in collisions or are for the most part captured in the outer solar system where the analysis of isotopes is rather skimpy.

My point is, if we are encountering enough interstellar debris to maintain planetary rings, this material should be readily found. It is known that the influx rate of ring material for Saturn is about the same as the meteor influx rate at Earth. If interstellar material is replenishing Saturn's rings, we certainly would have meteorites from interstellar sources. But out of tens of thousands of samples, we have not a single one older than the Solar System.

I will accept the idea that supernovas produce only dust particles, but keep in mind that no detection is possible for settling this theory. But to my surprise the theories of an early chaotic solar system and the Nice theory have produced fairly large orbs that have been ejected into interstellar space.

Not at all. Perhaps a few planetary bodies or minor planets were ejected. But by the time where the Nice Model is invoked (once a mature star and planetary system have formed) the mass remaining to be ejected is very small- a few Earth masses perhaps. Every star system out there could eject this much material and the odds of another star ever encountering any of it would be vanishingly small. To replenish Saturn's rings from interstellar material would require stellar masses of debris in every cubic light year. No theory of stellar formation suggests that so much condensed material is ejected. Almost all the nebular mass that is lost during star formation is still in gas or dust form.

Also, a recent paper claims that very cool brown dwarfs are just now being detected within 9 light years of Earth. The paper is suggesting that the solar neighborhood is rife with these dim bodies. My suggestion is that the density measurements of materials in interstellar space, even the local neighborhood, does not reveal perhaps a surprisingly large population of larger bodies eager to become planetisimals of passing star systems. Can orbital mechanics make this possible ?

Generally, no. The odds of any extraterrestrial body being so closely matched in velocity to the Sun that it can be captured are very tiny. In order for there to be enough material with such a velocity, there would need to be vastly more at other velocities. If this much material were present, we would readily detect it.

(It would be helpful if you could isolate your latest responses from the material surrounded by quote tags. It is difficult to figure out what to respond to when your new comments are embedded in quotes.)

[dougettinger]

Chris Peterson wrote: "My point is, if we are encountering enough interstellar debris to maintain planetary rings, this material should be readily found. It is known that the influx rate of ring material for Saturn is about the same as the meteor influx rate at Earth. If interstellar material is replenishing Saturn's rings, we certainly would have meteorites from interstellar sources. But out of tens of thousands of samples, we have not a single one older than the Solar System."

[Another thing to consider besides the size of the sampling is the time span of the sampling. This time span is very miniscule in relation to the time for one Sun's orbit around the galaxy or even thourgh one spiral arm. It is very possible that incoming materials ahead of the Sun's path for thousands of years have come from star deaths that are younger than 4.6 billion years. Also, there may be the case that for the past millions of years that the space traveled in front of the Sun's path has been very sparse of materials. Perhaps you know more about materials in interstellar space that are trailing the Sun's path (?)

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

Another thing to consider besides the size of the sampling is the time span of the sampling. This time span is very miniscule in relation to the time for one Sun's orbit around the galaxy or even thourgh one spiral arm.

That is true. The meteorite record extends back only a few thousand years, for the most part.

It is very possible that incoming materials ahead of the Sun's path for thousands of years have come from star deaths that are younger than 4.6 billion years.

Possible, although that strikes me as unlikely.

Also, there may be the case that for the past millions of years that the space traveled in front of the Sun's path has been very sparse of materials.

Also possible. But the problem with periodic passages through denser regions of interstellar space is that we should be able to detect such regions, and we don't. To replenish Saturn's rings with extrasolar material would require either very high density (which would produce periodic impact events on all planets and moons- something not observed) or extended periods of travel through very large clouds with at least the density of the Oort cloud, and such large clouds should be observable- but we don't see them. What we do see is large clouds of dust and gas, and certainly the Solar System moves through such regions as it orbits within the Milky Way. Such areas may have some sort of effect on climate, but probably can't provide material for impacts or accretion, simply because the particles are too small. And there is still the extreme statistical unlikelihood that we would encounter material with less than the solar escape velocity, meaning that even if present, very little could be captured by any bodies in the Solar System.

[dougettinger]

I propose an interesting exercise. Let's assume that there are bodies in interstellar space that range in size from 1 km to Pluto size to Neptune size. They are dark and well distributed so that the infrared signature is not noticed. Then assume that these bodies were the missing Dark Matter and compute, accordingly, their numbers and distribution. Statistically, could our size solar system then easily encounter and capture these bodies? Of course, it would be also be assumed that these bodies are both cast-offs from forming star systems and from normal condensation processes inside GMC's not requiring nebular disks.

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

I propose an interesting exercise. Let's assume that there are bodies in interstellar space that range in size from 1 km to Pluto size to Neptune size. They are dark and well distributed so that the infrared signature is not noticed. Then assume that these bodies were the missing Dark Matter and compute, accordingly, their numbers and distribution.

Missing matter or missing dark matter? It is pretty well established that dark matter cannot be made up of bodies like you describe.

Statistically, could our size solar system then easily encounter and capture these bodies?

No. If the density was high enough, we would start seeing collisions. The ratio of collisions to captures would be huge.

[dougettinger]

I propose an interesting exercise. Let's assume that there are bodies in interstellar space that range in size from 1 km to Pluto size to Neptune size. They are dark and well distributed so that the infrared signature is not noticed. Then assume that these bodies were the missing Dark Matter and compute, accordingly, their numbers and distribution.

Missing matter or missing dark matter? It is pretty well established that dark matter cannot be made up of bodies like you describe.

Statistically, could our size solar system then easily encounter and capture these bodies?

No. If the density was high enough, we would start seeing collisions. The ratio of collisions to captures would be huge.

Let me ask the question a different way. All known observed or unobserved matter is only 5 % of all matter that includes Dark Matter. Disregarding Dark Matter, statistically, what ratio of unobserved (as I have previously described) to observed matter could our size solar system easily encounter and capture as it is traveling through the galaxy. And let me be very generous - let's assume that a major size object (larger than Pluto) is captured every 50 million years. Please keep in mind that this is only a thought exercise and is not based on any real data collection or accepted theories.

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

All known observed or unobserved matter is only 5 % of all matter that includes Dark Matter.

Normal matter represents 4.6% of the total energy in the Universe. Dark matter represents 23%. So normal matter represents 20% of all matter, not 5%. Approximately half of the normal matter is accounted for in observations, but new reports suggest that much more has been recently seen, and that the whole issue of unobserved normal matter may soon go away.

Disregarding Dark Matter, statistically, what ratio of unobserved (as I have previously described) to observed matter could our size solar system easily encounter and capture as it is traveling through the galaxy. And let me be very generous - let's assume that a major size object (larger than Pluto) is captured every 50 million years.

I don't see how a question like that can be answered. Since there is no evidence at all of Pluto sized bodies existing in interstellar space, any assumption as to the density of such objects would be purely a guess. You could put an upper bound on that guess by considering what density would be detectable (far less than the density of the Oort cloud), but there is no lower bound. And then there is the problem of managing a capture at all- something that would require the body be traveling on the same plane as the Solar System, have a low velocity relative to the Sun, and would still involve a complex double encounter with one of the gas giants and the Sun.

Since you ask for a statistical estimate, I'd say all the above translate to a number very near zero.

[dougettinger]

Normal matter represents 4.6% of the total energy in the Universe. Dark matter represents 23%. So normal matter represents 20% of all matter, not 5%. Approximately half of the normal matter is accounted for in observations, but new reports suggest that much more has been recently seen, and that the whole issue of unobserved normal matter may soon go away.

Yes, you are correct. My 5 % figure is really the approximate percentage of all matter in the mass-energy universe.

I don't see how a question like that can be answered. Since there is no evidence at all of Pluto sized bodies existing in interstellar space, any assumption as to the density of such objects would be purely a guess. You could put an upper bound on that guess by considering what density would be detectable (far less than the density of the Oort cloud), but there is no lower bound. And then there is the problem of managing a capture at all- something that would require the body be traveling on the same plane as the Solar System, have a low velocity relative to the Sun, and would still involve a complex double encounter with one of the gas giants and the Sun.

I am still exploring the idea of celestial bodies being captured by our solar system. Thanks for your patience. The Oort cloud of spherical shape is postulated to extend as far as 50,000 AU's or almost one light year which is considered to be the boundary of the Sun's gravity field for such objects. The projected area of this sphere commands a respectable swath of space as it moves through the galaxy. Supposely, the materials of the Oort cloud formed much closer to the forming Sun and eventually migrated to these large distances by various means.

Now for my point - if the Sun's gravity field could hold these objects that were formed 1/1000th closer to the Sun and possessing varying trajectories, then why cannot this same gravity field capture similar sized objects with similarly varying trajectories from outside this sphere of influence when they are intersected by the Sun's path ? Due to the randomness of velocity vectors many of these objects should have nearly tangential vectors with the Oort cloud sphere and/or disk. The primary velocity vector, that of the Sun's direction, should be quite similar knowing how spiral galaxies work.

Allow me to add another point of discussion - A question arises as to how these type of celestial bodies are formed between the stars where there is not sufficient heat from a forming protostar disk to create ice balls and/or molten cores. Scientists are now discovering very low mass brown dwarfs even with their own planets. So has anyone really established a lower boundary for what the smallest body can form in a GMC without having a typical size protostar disk ?

Doug Ettinger
Pittsburgh, PA