Comet Lifetimes

[dougettinger]

What is the estimated lifetime of a comet ? For a short period comets (orbiting inside the Kuiper Belt) of typical size, several kilometers in diameter, how long does it take to bake or outgass the lighter volatiles and convert it into another asteroid without a coma ? If we know the orbital period and an estimate of the amount of material lost in the coma each time it comes close to the Sun, a rough answer should be possible.

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

What is the estimated lifetime of a comet ? For a short period comets (orbiting inside the Kuiper Belt) of typical size, several kilometers in diameter, how long does it take to bake or outgass the lighter volatiles and convert it into another asteroid without a coma ? If we know the orbital period and an estimate of the amount of material lost in the coma each time it comes close to the Sun, a rough answer should be possible.

A few thousand years. Also, not enough is known about the composition of comets to generalize about what is left once the volatiles burn away. There are a couple of asteroids that are suspected to be extinct comets (based on their orbits and the existence of debris trails), but there don't seem to be enough of them if comets usually leave cores. So simply dissolving into gravel-like debris (which has a short lifetime in the inner Solar System) may be the usual way comets end their lives.

[neufer]

What is the estimated lifetime of a comet ?

• Fate of comets

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1) Departure/ejection from Solar System

<<If a comet is traveling fast enough, it may leave the Solar System; such is the case for Hyperbolic comets. To date, comets are only known to be ejected by interacting with another object in the solar system, such as Jupiter.>>
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2) Volatiles exhausted

<<Jupiter family comets (JFC) and long period comets (LPC) appear to follow very different fading laws. The JFCs are active over a lifetime of about 10,000 years or ~1,000 revolutions while the LPCs disappear much faster. Only 10% of the LPCs survive more than 50 passages to small perihelion, while only 1% of them survive more than 2,000 passages. Eventually most of the volatile material contained in a comet nucleus evaporates away, and the comet becomes a small, dark, inert lump of rock or rubble that can resemble an asteroid.>>
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3) Breakup/disintegration

<<Comets are also known to break up into fragments, as happened with Comet 73P/Schwassmann-Wachmann 3 starting in 1995. This breakup may be triggered by tidal gravitational forces from the Sun or a large planet, by an "explosion" of volatile material, or for other reasons not fully explained.>>
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4) Collisions

<<Some comets meet a more spectacular end—either falling into the Sun, or smashing into a planet or other body. Collisions between comets and planets or moons were common in the early Solar System: some of the many craters on the Earth's Moon, for example, may have been caused by comets. A recent collision of a comet with a planet occurred in July 1994 when Comet Shoemaker-Levy 9 broke up into pieces and collided with Jupiter.

Many comets and asteroids collided into Earth in its early stages. Many scientists believe that comets bombarding the young Earth (about 4 billion years ago) brought the vast quantities of water that now fill the Earth's oceans, or at least a significant proportion of it. Other researchers have cast doubt on this theory. The detection of organic molecules in comets has led some to speculate that comets or meteorites may have brought the precursors of life—or even life itself—to Earth. There are still many near-Earth comets, although a collision with an asteroid is more likely than with a comet.

It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to the Earth's Moon, some of which may have survived as lunar ice.>>

[dougettinger]

Thanks, Art, for listing all the reasons for the dimise of comets. The one I was primarily interested in was how long the volatiles survive.

Doug Ettinger
Pittsburgh, PA

[dougettinger]

What is the estimated lifetime of a comet ? For a short period comets (orbiting inside the Kuiper Belt) of typical size, several kilometers in diameter, how long does it take to bake or outgass the lighter volatiles and convert it into another asteroid without a coma ? If we know the orbital period and an estimate of the amount of material lost in the coma each time it comes close to the Sun, a rough answer should be possible.

A few thousand years. Also, not enough is known about the composition of comets to generalize about what is left once the volatiles burn away. There are a couple of asteroids that are suspected to be extinct comets (based on their orbits and the existence of debris trails), but there don't seem to be enough of them if comets usually leave cores. So simply dissolving into gravel-like debris (which has a short lifetime in the inner Solar System) may be the usual way comets end their lives.

You and Art are leading me to the next question. Art has increased the lifetime value to 10,000 years for the Jupiter comets. Given the lifetime of the solar system being 4.6 billion years give or take a few 10,000 years, where are the short period and JPC comets coming from ? Obviously, the long period comets must come from the perturbed Oort Cloud. There is no other place left for them to survive as long as the age of the solar system. I originally thought many, many years ago that comets came from sporadic collisions throughout the age of the solar system, but the planetary system seems to be stable enough over the last few million years to avoid frequent collisions that might possibly produce comets.

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

You and Art are leading me to the next question. Art has increased the lifetime value to 10,000 years for the Jupiter comets. Given the lifetime of the solar system being 4.6 billion years give or take a few 10,000 years, where are the short period and JPC comets coming from ?

Short period comets are presumed to come from the same place as long period comets: the Oort cloud. It only takes a small change in velocity to change a long period comet into a short period one. That velocity change can occur when a comet is gravitationally perturbed by one of the outer planets- generally Jupiter.

[dougettinger]

So without the Oort Cloud and its necessary perturbations theorists are in deep trouble. I value inductive reasoning as well as any other good theorist. But, it would be nice to actually prove the cloud's existence.

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

So without the Oort Cloud and its necessary perturbations theorists are in deep trouble. I value inductive reasoning as well as any other good theorist. But, it would be nice to actually prove the cloud's existence.

"Proof" is a concept that doesn't really exist in science. All we have to work with is evidence. In the case of the Oort cloud, the evidence for its existence is strong enough that few really doubt it's the source of comets- especially since nobody has even come up with an alternative explanation that reaches the level of testable theory.

[neufer]

In the case of the Oort cloud, the evidence for its existence is strong enough that few really doubt it's the source of comets- especially since nobody has even come up with an alternative explanation that reaches the level of testable theory.

Confidence in Oort cloud comets may be "fading":

<<Oort noted that the number of returning comets was far less than his model predicted, and this issue, known as "cometary fading", has yet to be resolved. No known dynamical process can explain this undercount of observed comets.>>

Short period comets are presumed to come from the same place as long period comets: the Oort cloud. It only takes a small change in velocity to change a long period comet into a short period one. That velocity change can occur when a comet is gravitationally perturbed by one of the outer planets- generally Jupiter.

Short period comets are presumed to come from the observable scattered disc.

<<The Kuiper belt was initially believed to be the source of the Solar System's ecliptic comets. However, studies of the region since 1992 have revealed that the orbits within what is now called the Kuiper belt are relatively stable, and that these comets originate from the more dynamic scattered disc.

Comets can loosely be divided into two categories: short-period and long period—the latter being believed to originate in the Oort cloud. There are two major categories of short-period comets: Jupiter-family comets and Halley-family comets. The latter group, which is named for its prototype, Halley's Comet, are believed to have emerged from the Oort cloud but to have been drawn into the inner Solar System by the gravity of the giant planets. The former type, the Jupiter family, are believed to have originated from the scattered disc. The centaurs are thought to be a dynamically intermediate stage between the scattered disc and the Jupiter family.

There are many differences between SDOs and Jupiter-family comets, even though many of the latter may have originated in the scattered disc. Although the centaurs share a reddish or neutral coloration with many SDOs, their nuclei are bluer, indicating a fundamental chemical or physical difference. One hypothesis is that comet nuclei are resurfaced as they approach the Sun by subsurface materials which subsequently bury the older material.


The eccentricity and inclination of the scattered disc population compared to the classical and 5:2 resonant Kuiper belt objects

The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater than 30 astronomical units. These extreme orbits are believed to be the result of gravitational "scattering" by the gas giants, and the objects continue to be subject to perturbation by the planet Neptune. While the nearest distance to the Sun approached by scattered objects is about 30–35 AU, their orbits can extend well beyond 100 AU. This makes scattered objects "among the most distant and cold objects in the Solar System". The innermost portion of the scattered disc overlaps with a torus-shaped region of orbiting objects known as the Kuiper belt, but its outer limits reach much farther away from the Sun and farther above and below the ecliptic than the belt proper.

Because of its unstable nature, astronomers now consider the scattered disc to be the place of origin for most periodic comets observed in the Solar System, with the centaurs, a population of icy bodies between Jupiter and Neptune, being the intermediate stage in an object's migration from the disc to the inner Solar System. Eventually, perturbations from the giant planets send such objects towards the Sun, transforming them into periodic comets. Many Oort cloud objects are also believed to have originated in the scattered disc.

The first scattered disc object to be recognised as such was a (15874) 1996 TL66, originally identified in 1996 by astronomers based at Mauna Kea in Hawaii. Three more were identified by the same survey in 1999: 1999 CV118, 1999 CY118 and 1999 CF119. The first object presently classified as a scattered disc object to be discovered was (48639) 1995 TL8, found in 1995 by Spacewatch. As of 2009, over 100 scattered disc objects have been identified, including 2007 UK126 (discovered by Schwamb, Brown, and Rabinowitz), (84522) 2002 TC302 (NEAT), Eris (Brown, Trujillo, and Rabinowitz) Sedna (Brown, Trujillo, and Rabinowitz) and 2004 VN112 (Deep Ecliptic Survey). Although the numbers of objects in the Kuiper belt and the scattered disc are hypothesized to be roughly equal, observational bias due to their greater distance means that far fewer scattered disc objects have been observed to date.

Known trans-Neptunian objects are often divided into two subpopulations: the Kuiper belt and the scattered disc. A third reservoir of trans-Neptunian objects, the Oort cloud, is believed to exist, although no confirmed direct observations of the Oort cloud have been made. Some researchers further suggest a transitional space between the scattered disc and the inner Oort cloud, populated with "detached objects".

The Kuiper belt is a relatively thick torus (or "doughnut") of space, extending from about 30 to 50 AU comprising two main populations: the classical Kuiper belt objects (or "cubewanos"), which lie in orbits untouched by Neptune, and the resonant Kuiper belt objects; those which Neptune has locked into a precise orbital ratio such as 3:2 (the KBO goes around twice for every three Neptune orbits) and 2:1 (the object goes around once for every two Neptune orbits). These ratios, called orbital resonances, allow KBOs to persist in regions which Neptune's gravitational influence would otherwise have cleared out over the age of the Solar System, since the objects are never close enough to Neptune to be scattered by its gravity. Those in 3:2 resonances are known as "plutinos", because Pluto is the largest member of their group, whereas those in 2:1 resonances are known as "twotinos".

In contrast to the Kuiper belt, the scattered disc population can be disturbed by Neptune. Scattered disc objects come within gravitational range of Neptune at their closest approaches (~30 AU) but their farthest distances reach many times that. Ongoing research suggests that the centaurs, a class of icy planetoids that orbit between Jupiter and Neptune, may simply be SDOs thrown into the inner reaches of the Solar System by Neptune, making them "cis-Neptunian" rather than trans-Neptunian scattered objects. Some objects, like (29981) 1999 TD10, blur the distinction[21] and the Minor Planet Center (MPC), which officially catalogues all trans-Neptunian objects, now lists centaurs and SDOs together.[8]

The Minor Planet Center classifies the trans-Neptunian object 90377 Sedna as a scattered disc object. Its discoverer Michael E. Brown has suggested instead that it should be considered an inner Oort cloud object rather than a member of the scattered disc, because, with a perihelion distance of 76 AU, it is too remote to be affected by the gravitational attraction of the outer planets. Thus, an object with a perihelion greater than 40 AU could be classified as outside the scattered disc.

The scattered disc is a very dynamic environment. Because they are still capable of being perturbed by Neptune, scattered disc objects' orbits are always in danger of disruption; either of being sent outward to the Oort cloud or inward into the centaur population and ultimately the Jupiter family of comets. For this reason Gladman et al. prefer to refer to the region as the scattering disc, rather than scattered. Unlike Kuiper belt objects (KBOs), the orbits of scattered objects can be inclined as much as 40° from the ecliptic.

Although motions in the scattered disc are random, they do tend to follow similar directions, which means that SDOs can become trapped in temporary resonances with Neptune. Examples of resonant orbits within the scattered disc include 1:3, 2:7, 3:11, 5:22 and 4:79.


Simulation showing Outer Planets and Kuiper Belt: a) Before Jupiter/Saturn 2:1 resonance b) Scattering of Kuiper Belt objects into the solar system after the orbital shift of Neptune c) After ejection of Kuiper Belt bodies by Jupiter

The scattered disc is still poorly understood: no model of the formation of the Kuiper belt and the scattered disc has yet been proposed that explains all their observed properties. According to contemporary models, the scattered disc formed when Kuiper belt objects (KBOs) were "scattered" into eccentric and inclined orbits by gravitational interaction with Neptune and the other outer planets. The amount of time for this process to occur remains uncertain. One hypothesis estimates a period equal to the entire age of the Solar System; a second posits that the scattering took place relatively quickly, during Neptune's early migration epoch.

Models for a continuous formation throughout the age of the Solar System illustrate that at weak resonances within the Kuiper belt (such as 5:7 or 8:1), or at the boundaries of stronger resonances, objects can develop weak orbital instabilities over millions of years. The 4:7 resonance in particular has large instability. KBOs can also be shifted into unstable orbits by close passage of massive objects, or through collisions. Over time, the scattered disc would gradually form from these isolated events.

Computer simulations have also suggested a more rapid and earlier formation for the scattered disc. Modern theories indicate that neither Uranus nor Neptune could have formed in situ beyond Saturn, as too little primordial matter existed at that range to produce objects of such high mass. Instead, these planets, and Saturn, may have formed closer to Jupiter, but were flung outwards during the early evolution of the Solar System, perhaps through exchanges of angular momentum with scattered objects. Once the orbits of Jupiter and Saturn shifted to a 2:1 resonance (two Jupiter orbits for each orbit of Saturn), their combined gravitational pull disrupted the orbits of Uranus and Neptune, sending Neptune into the temporary "chaos" of the proto-Kuiper belt. As Neptune traveled outward, it scattered many trans-Neptunian objects into higher and more eccentric orbits. This model states that 90% or more of the objects in the scattered disc may have been "promoted into these eccentric orbits by Neptune's resonances during the migration epoch...[therefore] the scattered disc might not be so scattered.">>

[dougettinger]

Neuf, thanks for the update. Intuitively, I prefer the scattered disk (caused by collisions and perturbations over the life of the solar system) idea over the Oort Cloud theory. Also, I am deeply troubled by the Nice Theory where Neptune and Uranus cross orbits and migrate outward. As planetary bodies gain mass during their early formation they should move inward rather evenly together increasing their velocities to preserve angular momemtum with resonances having little affect. After the orbital sequence of the primary planets is established it should remain as is. Why could not the so-called cloud of KBO's and comets beyond Neptune be remnants of the original protostar disk that were not cleared or removed either by planetary formation or by the early solar winds ?

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

Intuitively, I prefer the scattered disk (caused by collisions and perturbations over the life of the solar system) idea over the Oort Cloud theory.

They aren't actually all that different. The Kuiper belt, scattered disc, and Oort cloud are all components of a broader theory of planetary system formation. Also, the origin of comets need not be explained by any single theory. It is certain that not all short period comets originate in the scattered disc, for instance, because we can calculate the statistical perturbation rate for long period comets being converted to short period comets.

Also, I am deeply troubled by the Nice Theory where Neptune and Uranus cross orbits and migrate outward. As planetary bodies gain mass during their early formation they should move inward rather evenly together increasing their velocities to preserve angular momemtum with resonances having little affect. After the orbital sequence of the primary planets is established it should remain as is.

The Nice model works, with 100% certainty. That is, it is entirely physical and describes a process that is possible and can easily occur. Your suggestion of how things ought to behave is not supported by simple physical dynamics. The only thing "theoretical" about the Nice model is whether it actually happened in our own solar system, not whether it could happen. That is certain.

Why could not the so-called cloud of KBO's and comets beyond Neptune be remnants of the original protostar disk that were not cleared or removed either by planetary formation or by the early solar winds?

That's exactly what they are presumed to be.

[dougettinger]

Intuitively, I prefer the scattered disk (caused by collisions and perturbations over the life of the solar system) idea over the Oort Cloud theory.

They aren't actually all that different. The Kuiper belt, scattered disc, and Oort cloud are all components of a broader theory of planetary system formation. Also, the origin of comets need not be explained by any single theory. It is certain that not all short period comets originate in the scattered disc, for instance, because we can calculate the statistical perturbation rate for long period comets being converted to short period comets."

The scattered disk is shown to extend to about 150 AU's in the diagram presented by Art. Wikipedia has determined the Kuiper Belt to be between 30 and 55 AU's with the scatted disk estending to over 100 AU's. Unless there is a typo the distant Oort cloud begins at about 50,000 AU's. This is a significant, remote distance from the scattered disk which for me makes it much less a component that can be blended into a theory that explains what makes the Kuiper belt and the scattered disk.

Also, I am deeply troubled by the Nice Theory where Neptune and Uranus cross orbits and migrate outward. As planetary bodies gain mass during their early formation they should move inward rather evenly together increasing their velocities to preserve angular momemtum with resonances having little affect. After the orbital sequence of the primary planets is established it should remain as is.

The Nice model works, with 100% certainty. That is, it is entirely physical and describes a process that is possible and can easily occur. Your suggestion of how things ought to behave is not supported by simple physical dynamics. The only thing "theoretical" about the Nice model is whether it actually happened in our own solar system, not whether it could happen. That is certain.

I will certainly take you word on this matter. It sounds like you are very familiar with the Nice model and perhaps have been involved in the computer programming to support it. I have played with very crude computer models in the distant past and know that crossing orbits can become very chaotic and that the good timing can be a factor in avoiding chaos.

Why could not the so-called cloud of KBO's and comets beyond Neptune be remnants of the original protostar disk that were not cleared or removed either by planetary formation or by the early solar winds?

That's exactly what they are presumed to be.

This is all well. But, I have a difficult time envisioning how the earlier, stronger, solar winds with their radiative pressures were indeed strong enough to push outward from the inner solar system collisional debris even as large as a typical meteorite or a small asteroid. And then supposely continue to push materials farther away from the regions of the outer gas giants and yet not disturb the asteroid belt. I know this is not your field, but perhaps you can explain briefly how the necessary forces are derived to evacuate dust and smaller bodies from the inner solar system after the Sun started to fuse hydrogen (?)

I know that ultra thin solar sails can be propelled and comet comas can be affected by radiation pressure and solar winds, but how are rocks propelled or pushed away by these forces ?

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

This is all well. But, I have a difficult time envisioning how the earlier, stronger, solar winds with their radiative pressures were indeed strong enough to push outward from the inner solar system collisional debris even as large as a typical meteorite or a small asteroid.

I'm not aware of any theory that suggests solar wind or radiation pressure have ever pushed material outward (or that they do so now). Do you have more information about where you have heard this? In fact, solar radiation causes particles to spiral closer to the Sun.

The general picture of how the Solar System formed has gas and dust coming together by self-gravitation, with the gravitational energy being converted to heat and fusion starting. Gravity provided an inward directed force, and solar wind and radiation provided an outward force. The radius any particular material ended up at was determined by an equilibrium between these two. In the inner system, material accreted into planets or planetesimals, which cleared the area of other material. Farther out, most material was perturbed out of orbit, as well. That left the Kuiper belt, scattered disc, and possibly the Oort cloud as the residual accretion disc, but with surface densities too low to allow further accretion.

[dougettinger]

This is all well. But, I have a difficult time envisioning how the earlier, stronger, solar winds with their radiative pressures were indeed strong enough to push outward from the inner solar system collisional debris even as large as a typical meteorite or a small asteroid.

"I'm not aware of any theory that suggests solar wind or radiation pressure have ever pushed material outward (or that they do so now). Do you have more information about where you have heard this? In fact, solar radiation causes particles to spiral closer to the Sun."

The Sun is believed to have been a classic T Tauri Star. I am quoting Wikipedia (T Tauri wind), "Disk material forms around the star. The disk collimates the intense stellar wind into two oppositely directed beams producing what is referred to as a bipolar flow, which can cause the formimg star to lose up to 0.4 mass of the Sun, and can start to disrupt the cloud." ------- " The matter ourflows are observed from a disk region with an outer radius of less than 0.5 AU. The outflowing matter initially moves almost along the disk until being accelerated up to V greater than 100 km/s and only afterwards begins to collimate. Inner region of the wind is collimated into the jet at a distance of less than 3 AU from the disk mid plane. The V(z) gas velocity component in the jet decreases with increasing distance from the jet axis.

The question for me is whether the intense stellar winds can push away heavier objects such as asteroids of varying sizes (?) And then what happens to material in the disk that is not collimated into bipolar jets beyond 3 AU ? In the Nice model simulation the solar system is already evacuated of material beyond the 1st phase of orbits of the all the outer planets. How did this happen ? The outer planets should only be able to sweep away bands of material thereby creating gaps similar to what happens in Saturn's rings.

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

The Sun is believed to have been a classic T Tauri Star. I am quoting Wikipedia (T Tauri wind), "Disk material forms around the star. The disk collimates the intense stellar wind into two oppositely directed beams producing what is referred to as a bipolar flow, which can cause the formimg star to lose up to 0.4 mass of the Sun, and can start to disrupt the cloud."...

The question for me is whether the intense stellar winds can push away heavier objects such as asteroids of varying sizes (?) And then what happens to material in the disk that is not collimated into bipolar jets beyond 3 AU ? In the Nice model simulation the solar system is already evacuated of material beyond the 1st phase of orbits of the all the outer planets. How did this happen ? The outer planets should only be able to sweep away bands of material thereby creating gaps similar to what happens in Saturn's rings.

I would not describe the jets of a T Tauri star as ejecting material in the sense that you might envision solar wind ejecting material. The jets are an outflow of material that gained its energy (and angular momentum) by falling inwards in the accretion process. They are focused, and out of the plane of accretion. While they no doubt play an important role in distributing angular momentum, and disrupting nebular material out of the plane of formation, I don't see much impact on planetary formation or planetary orbits.

[dougettinger]

My concern is not about the affect of T Tauri winds on the planetary formation or planetary orbits. How does the inner and outer solar system disk become evacuated leaving behind only the planets, satellites and minor planetisimals ?

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

My concern is not about the affect of T Tauri winds on the planetary formation or planetary orbits. How does the inner and outer solar system disk become evacuated leaving behind only the planets, satellites and minor planetisimals ?

The planets form by a combination of fluid dynamic effects and self-gravity, which naturally clears the region of their orbits (they are consuming that material during formation). Once you have orbiting, discrete masses, gravitational resonance largely clears everything else. Solar winds blow away material off the ecliptic, although that is only effective to a certain distance, so you'd expect a somewhat spherical halo of residual debris to persist. And in fact, that's what we seem to observe.

[dougettinger]

My concern is not about the affect of T Tauri winds on the planetary formation or planetary orbits. How does the inner and outer solar system disk become evacuated leaving behind only the planets, satellites and minor planetisimals ?

The planets form by a combination of fluid dynamic effects and self-gravity, which naturally clears the region of their orbits (they are consuming that material during formation). Once you have orbiting, discrete masses, gravitational resonance largely clears everything else. Solar winds blow away material off the ecliptic, although that is only effective to a certain distance, so you'd expect a somewhat spherical halo of residual debris to persist. And in fact, that's what we seem to observe.

The T Tauri winds are reputed to affective only within 3 AU, somewhat beyond the orbital distances of asteroids. And, certainly, the quieter solar winds have little affect on the known asteroids that supposely have survived the age of the solar system. In the Nice model this leaves orbital regions between the asteroid belt and present day Saturn's orbit with no mechanism for clearing away the dust and gases in the ecliptic or invariant plane.

Given discrete and fairly large orbiting masses I do not understand how gravitational resonances clear out discrete masses from 1 km size to the size of Ceres; these resonances supposely provide the reason for preventing the given asteroid population from coalescing which counters the reason for clearing discrete masses from other orbital regions. Most asteroids are irregular indicating that collisions have occurred, and, obviously, the collisions have not caused accretion which also counters the reason for clearing away discrete masses from other orbital regions.

It is not clear to me how fluid dynamic effects, self-gravity, and resonances can clear away, for instance, a 1 km orbiting object that may either be midway between Mercury and Venus, or midway between Venus and Earth , or midway between Earth and Mars. What are the actual principles of physics that allow this to happen ? Physics principles would tell me that these midway objects should remain in their given orbits. Resonances may perturb an orbit into a more elliptical orbit or more inclined orbit; but is difficult to envision it being ejected from the system. I realize there are anomalous orbiting asteroids between Mars and Mercury, but their overall density is insignificant.

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

It is not clear to me how fluid dynamic effects, self-gravity, and resonances can clear away, for instance, a 1 km orbiting object that may either be midway between Mercury and Venus, or midway between Venus and Earth , or midway between Earth and Mars.

I think you are confounding things that happen at different stages of stellar system formation. Fluid dynamic effects are only present while there is still a dense accretion disc. Such discs often have many times the mass of the final planetary system, because most material ends up getting ejected before it ever accretes. Within a dense accretion disc, fluid dynamics and self-gravity begin the planetary accretion process. Clearing due to gravitational resonances probably occurs later, when the accretion disc has largely disappeared.

I don't really know how to define the physics any clearer. Accretion disc systems are regularly modeled using numerical simulations based on fluid dynamics and classical mechanics, and they reliably show that planets form in discrete and separate orbits, consuming much of the disc material in the process. In rich star forming regions, stellar winds from hot, massive stars may stop the accretion process of other stars by blowing away material, as well. Other models based on fairly straightforward physics (such as the Nice model) demonstrate the sort of effects that resonances can have in any developed planetary system, new or old.

Models don't prove that nature actually works this way, of course, but they demonstrate the physical plausibility of these theories. When combined with actual observations of developing star systems (of which we now have many examples), they provide a lot of evidence that we really do have a good idea about how stars and planets form, at least in a broad sense.

[dougettinger]

It is not clear to me how fluid dynamic effects, self-gravity, and resonances can clear away, for instance, a 1 km orbiting object that may either be midway between Mercury and Venus, or midway between Venus and Earth , or midway between Earth and Mars.

I think you are confounding things that happen at different stages of stellar system formation. Fluid dynamic effects are only present while there is still a dense accretion disc.

I just might be confounding the different stages and don't understand how they are sequenced or overlap. I have read that a typical protostar disk is about 3 times the mass of the star that is created from this disk. The exact ratio may be controversial but is roughly this scale. I first must deal with this mass accreting to form the star. This disk may be 200 AU's in diameter and is continually collapsing toward the protostar. If we assume a star the size of the Sun, then in some manner 2 solar masses along with its angular momentum must somehow disappear by a combination of processes such as HH objects, T Tauri winds, magnetic braking, etc. These considerations have very little to do with protoplanetary disks and the formation of planets. The masses required to form the planets is insignificant to the 2 solar masses of excess matter in the protostar disk. The forces of gravity and fluid dynamics provided by the overall disk are certainly important in forming the seeds and the growth by accretion of protoplanetary disks.

A definite milestone is the fusing of hydrogen inside the star and the creation of T Tauri winds that disrupt the protostar disk and terminating accretion onto the star. So where in this timeline are planets born, then accreting, and finally sweeping all the material in their orbital region ? Wikipedia breaks down the timeline for planetary formation into three stages: 1) runaway accretion whereby there is preferential growth of larger bodies at the expense of smaller ones. 2) oligarchic accretion whereby the largest accrete until the planetisimals are exhausted creating planetary embryos inside gaps separated by rings of planetesimals. 3) merger stage where the embryos dissipate the rings and become massive enough to perturb each other causing chaotic orbits with a few planets surviving.

I am assuming that planets formation times overlap both the protostar accretion stage and the T Tauri stage. Is this correct ?

I have issues with these listed planetary formation stages: 1) Fluid dynamics should maintain the material in a fluid mode preventing any accretion. 2) As larger growing bodies collide with each other dispersion is more likely than accretion. How do all these oligarchics find and collide with each over a distance of 1, 2, or 3 AU radii ? How are the rings between the gaps gathered ? Can resonances of terrestrial size planets be that effective over a large distance of several AU's ? An argument has already been posited that collisions between planetary bodies is very improbable and near misses are a little more likely. 3) I really need to be shown the computer simulation can produce 2 to 5 planets with stable rounded orbits adequately spaced (as referenced and described in Wikipedia). Models can work if your are going from A to D and you first pick intermediate destinations such as B and C; then make the model go from A to B; then from B to C, etc. One can produce expected results but possibly deceive himself.

Such discs often have many times the mass of the final planetary system, because most material ends up getting ejected before it ever accretes.

What processes eject the dust, planetesimals and the bodies from 1 km to Moon size from the final planetary system ? I presume that we are not discussing T Tauri winds at this particular time. Are they ejected from the solar system or do they end up in the Kuiper Belt or Oort Cloud or both ?

Within a dense accretion disc, fluid dynamics and self-gravity begin the planetary accretion process. Clearing due to gravitational resonances probably occurs later, when the accretion disc has largely disappeared.

I don't really know how to define the physics any clearer. Accretion disc systems are regularly modeled using numerical simulations based on fluid dynamics and classical mechanics, and they reliably show that planets form in discrete and separate orbits, consuming much of the disc material in the process. In rich star forming regions, stellar winds from hot, massive stars may stop the accretion process of other stars by blowing away material, as well. Other models based on fairly straightforward physics (such as the Nice model) demonstrate the sort of effects that resonances can have in any developed planetary system, new or old.

Models don't prove that nature actually works this way, of course, but they demonstrate the physical plausibility of these theories. When combined with actual observations of developing star systems (of which we now have many examples), they provide a lot of evidence that we really do have a good idea about how stars and planets form, at least in a broad sense.

You make a good point. Stellar winds and even explosions from neighboring stars may blow away material from a protostar disk, thereby changing the expected parameters. But obvious parameters such as protostar disk size and density rule. Obviously, different size stars are a result of the protostar disk size and density; apparently the incoming accreted material from a protostar disk can overcome the forces of T Tauri winds above a certain disk size and density.

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

Yikes! Please fix your quoting... it is very hard to figure out what to respond to. None of your new comments should be inside quote tags!

I just might be confounding the different stages and don't understand how they are sequenced or overlap.

I think the first stage is stellar formation from the gravitational collapse of dust and gas. Conservation of angular momentum causes the collapsed material to spin, and this in turn flattens collapsing material into a disc. Fusion begins, and this stops the inflow of material from within (and outside) the accretion disc, and the mass of the star is established. The stellar winds don't blow away the accretion disc, they just prevent the star from absorbing additional material. The star may be in this state for tens of millions of years while planets form in the remaining dust disc. There are observations of many stars in this state. T Tauri stars retain accretion discs where we might assume that planetary formation is in process (at least, for non-binary systems).

I have issues with these listed planetary formation stages: 1) Fluid dynamics should maintain the material in a fluid mode preventing any accretion.

Why? Because the dense, cool dust disc behaves as a fluid, you have regions that are laminar, and regions that are turbulent. Where you have eddies, you have increased self-gravitation- a positive feedback mechanism that will enhance the formation of accreted bodies. We see the fluidic aspect of this in Saturn's rings. There is no accretion because the surface density is too low for self gravity to bring material together, but that wouldn't be the case in a dense dusty accretion disc.

2) As larger growing bodies collide with each other dispersion is more likely than accretion.

Why? You only have collisions within an individual radial zone, and that assumes multiple bodies are created in the same orbit. I think it is much more likely that only a single body exists in any one orbit. You don't have collisions between bodies in different orbits. I see accretion as the dominant process, not collisions.

What processes eject the dust, planetesimals and the bodies from 1 km to Moon size from the final planetary system ?

Most material is ejected during the star formation process. I don't think that much mass is lost from the cool accretion disc during planet formation. Anything that does get ejected will simply be because it gains enough energy through gravitational interaction with another body. If you put a few hundred objects in low eccentricity orbits using an n-body simulator, it doesn't take long for most of them to get ejected. Such a system is just too unstable. But in the case of the Solar System, the total ejected mass is assumed to be pretty small.