Oort Cloud (split from: Lecture 11: Comets and Meteors)

[dougettinger]

I have asked this question previously, but perhaps some new data has changed the answer. Is there any physical evidence to collaborate the existence of the Oort Cloud which is supposely the source of comets that keep visiting the inner solar system ?

I just wonder how less plausible it would be that comets may come from an endless sink of condensed debris in interstellar space. Does the solar wind lose it power to drive dust and gases outward any farther than the 1000 AU's more or less where the Oort Cloud resides ? Or maybe some debris from supernova remnants condenses into small planetoids and pervades the entire region where large massive stars have lived briefly and then exploded ? And then other older star systems continuously sweep and collect some of these smaller bodies as they orbit the galaxy ?

Doug Ettinger, Pittsburgh, PA 02/15/2011

[Chris Peterson]

I have asked this question previously, but perhaps some new data has changed the answer. Is there any physical evidence to collaborate the existence of the Oort Cloud which is supposely the source of comets that keep visiting the inner solar system?

I'd say you answered your own question. The existence of comets is very strong evidence for the Oort cloud.

I just wonder how less plausible it would be that comets may come from an endless sink of condensed debris in interstellar space.

That is not a possibility. A dense enough background of material to provide all the comets in our system would be dense enough to create extinction in observations, and would produce an IR background that is not seen. Furthermore, if comets were coming from interstellar space they would almost all be in extremely hyperbolic orbits; in fact, cometary orbits are always elliptical or just barely hyperbolic, indicating they were perturbed out of orbits around the Sun.

Does the solar wind lose it power to drive dust and gases outward any farther than the 1000 AU's more or less where the Oort Cloud resides?

Certainly, it must fall off in accordance with an inverse square law.

Or maybe some debris from supernova remnants condenses into small planetoids and pervades the entire region where large massive stars have lived briefly and then exploded ? And then other older star systems continuously sweep and collect some of these smaller bodies as they orbit the galaxy?

It is dynamically very difficult for any star system to collect interstellar material. If such material existed, only a very tiny fraction would be captured.

[dougettinger]

I have asked this question previously, but perhaps some new data has changed the answer. Is there any physical evidence to collaborate the existence of the Oort Cloud which is supposely the source of comets that keep visiting the inner solar system?

I'd say you answered your own question. The existence of comets is very strong evidence for the Oort cloud.

Due to the short life of comets and the long life of the solar system, and due to the difficulty of planetoids being captured from interstellar space if they even exist, astronomers are certainly forced to invent something like the Oort Cloud. Supposely, there are no other options. I am merely exploring a possible option and not trying to be controversial.

I just wonder how less plausible it would be that comets may come from an endless sink of condensed debris in interstellar space.

That is not a possibility. A dense enough background of material to provide all the comets in our system would be dense enough to create extinction in observations, and would produce an IR background that is not seen. Furthermore, if comets were coming from interstellar space they would almost all be in extremely hyperbolic orbits; in fact, cometary orbits are always elliptical or just barely hyperbolic, indicating they were perturbed out of orbits around the Sun.

I am not sure how one delinates between an original, perturbed proto-star disk object and a recently captured object that may have highly eccentric elliptical orbits about the Sun. There is a definite range of parameters, although narrow, where interstellar objects may be captured.

You mention the lack of a certain IR background as definite proof of no comets in interstellar space. Just looking at the Sombero Galaxy's rim in optical wavelengths reveals very thick and opaque dust and gases. How does anyone know whether these dusty regions also have larger objects than just particle size material ? In our galaxy one can see hidden stars through dusty regions by use of infared wavelengths. Are we sure that infared astronomy has enough resolution to reveal comet size objects ?

Does the solar wind lose it power to drive dust and gases outward any farther than the 1000 AU's more or less where the Oort Cloud resides?

Certainly, it must fall off in accordance with an inverse square law.

I have no good feel for the solar wind's power. And probably near the end of a protostar's formation the solar winds were more powerful. But does the solar wind have enough push to affect cometary size objects even at a distance of 5 Au ? And some of the dwarf planets in the Kuiper Belt are pretty hefty.

Or maybe some debris from supernova remnants condenses into small planetoids and pervades the entire region where large massive stars have lived briefly and then exploded ? And then other older star systems continuously sweep and collect some of these smaller bodies as they orbit the galaxy?

It is dynamically very difficult for any star system to collect interstellar material. If such material existed, only a very tiny fraction would be captured.

Let's just say that many cometary-like bodies were orbiting the galaxy in the same direction and with similar velocities as the Sun. What chances do exist for capture if these planetoids were overtaken by the Sun's gravity field and were within 50 to 100 AU from the Sun? It is not too preposterous to imagine that a very high proportion of the bulk density of the Milky Way is orbiting the galaxy in a similar fashion as our Sun. And if red and brown dwarf stars with planets have condensed from star dust then why not even smaller bodies ?

Doug Ettinger, Pittsburgh, PA 02/16/2011

[Chris Peterson]

Due to the short life of comets and the long life of the solar system, and due to the difficulty of planetoids being captured from interstellar space if they even exist, astronomers are certainly forced to invent something like the Oort Cloud.

Such "invention" is the very heart of science. The purpose of a theory is to explain observation. In the case of the Oort cloud, there is more to support it than just the existence of comets. The frequency of comets, the ratio of short to long period comets, and the orbits of comets are all consistent with the existence of the Oort cloud. And recently, we have actual observations of Oort cloud like structures around other stars.

I am not sure how one delinates between an original, perturbed proto-star disk object and a recently captured object that may have highly eccentric elliptical orbits about the Sun. There is a definite range of parameters, although narrow, where interstellar objects may be captured.

I don't think you realize just how narrow that range is. First, the Sun can't capture any objects; as I've pointed out before, a capture requires three bodies. Yet we observe from the orbits of most comets that they haven't been perturbed by Jupiter.

You mention the lack of a certain IR background as definite proof of no comets in interstellar space. Just looking at the Sombero Galaxy's rim in optical wavelengths reveals very thick and opaque dust and gases. How does anyone know whether these dusty regions also have larger objects than just particle size material ? In our galaxy one can see hidden stars through dusty regions by use of infared wavelengths. Are we sure that infared astronomy has enough resolution to reveal comet size objects ?

You don't need to see comet sized objects. What you would see is a dense enough background of objects to produce an IR signature. Just like you can't see air molecules, but you can see the presence of an atmosphere looking through just a few kilometers of it.

I have no good feel for the solar wind's power. And probably near the end of a protostar's formation the solar winds were more powerful. But does the solar wind have enough push to affect cometary size objects even at a distance of 5 Au ? And some of the dwarf planets in the Kuiper Belt are pretty hefty.

The solar wind doesn't even have a significant effect on objects close to the Sun, unless they are smaller than a few centimeters. It would have essentially no effect on distant objects, regardless of size.

Let's just say that many cometary-like bodies were orbiting the galaxy in the same direction and with similar velocities as the Sun. What chances do exist for capture if these planetoids were overtaken by the Sun's gravity field and were within 50 to 100 AU from the Sun? It is not too preposterous to imagine that a very high proportion of the bulk density of the Milky Way is orbiting the galaxy in a similar fashion as our Sun. And if red and brown dwarf stars with planets have condensed from star dust then why not even smaller bodies ?

As previously noted, we'd see observational evidence of such bodies. There is no mechanism for such bodies to be captured by the Sun, except through an interaction with Jupiter, and we know that many comets don't show such an interaction.

There is no need to explain comets as extrasolar in origin, no evidence to support the idea that they are, and a good deal of evidence arguing against it.

[dougettinger]

Chris, thanks for your prompt and determined follow-up and patience.

I am curious to know what observations were made to see such Oort Cloud structures around other stars. Are these stars nearby such as less than 15 ly's ? Should we not see this structure around our closest star system that is 4 ly's away ?

How does the comet's orbit reveal only an interaction with the Sun and not with the outer gas giants ? What proportion of comets do show an interaction with the outer planets ? Could not the initial capture be an N-body case and then the huge mass of the Sun on the comet's first pass whip it into a huge elliptical orbit that no longer appears to have an interaction with the outer planets ?

Doug Ettinger, Pittsburgh, PA 02/15/11

[Chris Peterson]

I am curious to know what observations were made to see such Oort Cloud structures around other stars. Are these stars nearby such as less than 15 ly's ? Should we not see this structure around our closest star system that is 4 ly's away ?

I don't have a reference off-hand. I seem to recall there was a related APOD in the last year or so. I believe it was an icy zone around a protostar.

How does the comet's orbit reveal only an interaction with the Sun and not with the outer gas giants ?

It is a statistical analysis, and mainly concerns the inclination of the comet. Long period comets with high inclinations are unlikely to have interacted with Jupiter.

Could not the initial capture be an N-body case and then the huge mass of the Sun on the comet's first pass whip it into a huge elliptical orbit that no longer appears to have an interaction with the outer planets ?

Yes, but again you can apply a statistical analysis. Since so few passing objects would undergo such an interaction, it would require an absurdly high interstellar density of objects- which we should observe, and do not.

[dougettinger]

I am curious to know what observations were made to see such Oort Cloud structures around other stars. Are these stars nearby such as less than 15 ly's ? Should we not see this structure around our closest star system that is 4 ly's away ?

I don't have a reference off-hand. I seem to recall there was a related APOD in the last year or so. I believe it was an icy zone around a protostar.

I also recall that APOD. Why cannot we see this ice ring or ice signature around stars nearby to the Sun ?

How does the comet's orbit reveal only an interaction with the Sun and not with the outer gas giants ?

It is a statistical analysis, and mainly concerns the inclination of the comet. Long period comets with high inclinations are unlikely to have interacted with Jupiter.

Since many comets have high inclinations it then is assumed that the comet sink or source is spherical like the Oort Cloud is envisioned. Comets could not have been perturbed into such high inclinations by the outer massive planets. However, high inclinations and hyperbolic (non-periodic) comets suggests to me that they could have been captured from interstellar space. As these interstellar comets spiral around the Sun some become perturbed by the outer planets as claimed by Wikipedia.

I quote from Wikipedia, "If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun ( a few tens of kilometers per second). If such objects entered the solar system, they would have positive total energies, and would be observed to have geniunely hyperbolic trajectories." The main caveat is the assumed absolute velocity of these comets. They could have been propelled by the blasts of supernovae and easily have velocities of 250 km/s, close to the Sun's speed. If the trajectories are closely parallel then the capture process would almost imitate an Oort Cloud being perturbed.

Could not the initial capture be an N-body case and then the huge mass of the Sun on the comet's first pass whip it into a huge elliptical orbit that no longer appears to have an interaction with the outer planets ?

Yes, but again you can apply a statistical analysis. Since so few passing objects would undergo such an interaction, it would require an absurdly high interstellar density of objects- which we should observe, and do not.

The interstellar densities of objects could be much higher than expected or measured. The conjunction of two young supernova remnants and an IMC could possibly create an incoming hail storm of comets if the Sun was in the vicinity with its voracious mouth of 100 AU diameter. Like a whale feeding on plankton it navigates closer to the higher concentrations. Likewise, the Sun, not by happenstance, but by being in the right place in the spiral galaxy feeds on matter being created by the birth and death of massive stars occurring in nearby star nurseries.

Doug Ettinger, Pittsburgh, PA 02/15/2011

[Chris Peterson]

I also recall that APOD. Why cannot we see this ice ring or ice signature around stars nearby to the Sun?

If recent exoplanet discoveries have taught us anything, it's that there's a huge variability in the structure of planetary systems. One possibility is that most stars don't have Oort clouds. Another is that we can only detect the ice (for now, anyway) during the early stages of the system, when everything is warmer. Objects in the Oort cloud of a mature stellar system are likely to be very cold, and have a weak IR signal compared with the background.

Since many comets have high inclinations it then is assumed that the comet sink or source is spherical like the Oort Cloud is envisioned. Comets could not have been perturbed into such high inclinations by the outer massive planets. However, high inclinations and hyperbolic (non-periodic) comets suggests to me that they could have been captured from interstellar space. As these interstellar comets spiral around the Sun some become perturbed by the outer planets as claimed by Wikipedia.

An interstellar object wouldn't spiral around the Sun. It would be in a hyperbolic orbit, and would only pass through the system once. If it was out of the ecliptic, it would probably not be perturbed significantly by Jupiter, and would therefore not be captured. It is certainly possible for a body to enter on the ecliptic, lose some momentum to Jupiter, and then be captured by the Sun (meaning it would end up in an elliptical orbit around the Sun). But again, you have to consider the statistics. If you assume that all (or most) comets were captured in this way, you have to explain where the vastly greater number of comets are that should be passing through the system all the time in very hyperbolic orbits.

I quote from Wikipedia, "If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun ( a few tens of kilometers per second). If such objects entered the solar system, they would have positive total energies, and would be observed to have geniunely hyperbolic trajectories." The main caveat is the assumed absolute velocity of these comets. They could have been propelled by the blasts of supernovae and easily have velocities of 250 km/s, close to the Sun's speed. If the trajectories are closely parallel then the capture process would almost imitate an Oort Cloud being perturbed.

If they are propelled by a supernova, they will have even greater energies. The point of the article is that interstellar bodies orbiting the galaxy at the same radius as the Sun will have a similar orbital velocity. But what is "similar"? You need to understand that at Oort cloud distances, or a fraction of a light year, the solar escape velocity is measured in centimeters per second. That's compared with an orbital velocity of 250 km/s. So in order to have an orbit that looks like a typical long period comet, the interstellar debris would have to be matched in velocity with the Sun by something like one part in 25 billion. There is no mechanism that could explain that. Even if there were a debris field close to this, there would still be a statistical distribution, and we should see strongly hyperbolic cometary orbits- but we don't. Virtually all long period comets have an eccentricity very close to 1.

The interstellar densities of objects could be much higher than expected or measured.

I don't think so. The density required to produce the observed cometary flux would be high enough that we'd seem to be in a nebula, with stars more than a few light years away showing significant extinction. In addition, that much mass, heated by all the stars in the area, would raise the background well above the observed 3 K. I don't believe that the regional mass density required to explain comets could go unobserved with our current technology.

It is also worth noting that we have captured and analyzed dust from comets, and it all dates to the beginning of the Solar System. So if comets are interstellar, they are coming from a source almost exactly the same age as the Sun, which seems extremely unlikely. Any interstellar material co-orbiting the galaxy with the Sun would not be associated with the region of our Sun's formation- the perturbations associated with our numerous galactic orbits over 4.7 billion years have thoroughly scrambled things up (which is why the stars that formed with the Sun are no longer around us).

[dougettinger]

I also recall that APOD. Why cannot we see this ice ring or ice signature around stars nearby to the Sun?

If recent exoplanet discoveries have taught us anything, it's that there's a huge variability in the structure of planetary systems. One possibility is that most stars don't have Oort clouds. Another is that we can only detect the ice (for now, anyway) during the early stages of the system, when everything is warmer. Objects in the Oort cloud of a mature stellar system are likely to be very cold, and have a weak IR signal compared with the background.

We are now will add some options: 1) Most stars may not have Oort clouds. 2) Oort Cloud structures are too cold to be detected by IR signals. 3) Interstellar comets, if they exist, may be too cold to be detected by IR signals.

It is very important to know all the possible details about the ice ring or ice region found around this protostar disk shown in the APOD. I would appreciate knowing how to reference this APOD. Some likely important details would be the age of the proto-star, the size of the star and/or proto-star disk, at what distance from the star the ice ring resides, and whether the ice region is more cylindrical or spherical.

Some minerals from outer solar system comets were detected and analyzed to be derived only from very hot environments found in the inner regions of a proto-star disk. So knowing the approximate distance of this ice ring in the APOD will enable us to figuratively -connect the dots. How these hot comets travel to the outer regions of the solar system is a question for later consideration. Maybe they travel from somewhere else.

Doug Ettinger, Pittburgh, PA 02/16/2011

[dougettinger]

I also recall that APOD. Why cannot we see this ice ring or ice signature around stars nearby to the Sun?

Since many comets have high inclinations it then is assumed that the comet sink or source is spherical like the Oort Cloud is envisioned. Comets could not have been perturbed into such high inclinations by the outer massive planets. However, high inclinations and hyperbolic (non-periodic) comets suggests to me that they could have been captured from interstellar space. As these interstellar comets spiral around the Sun some become perturbed by the outer planets as claimed by Wikipedia.

An interstellar object wouldn't spiral around the Sun. It would be in a hyperbolic orbit, and would only pass through the system once. If it was out of the ecliptic, it would probably not be perturbed significantly by Jupiter, and would therefore not be captured. It is certainly possible for a body to enter on the ecliptic, lose some momentum to Jupiter, and then be captured by the Sun (meaning it would end up in an elliptical orbit around the Sun). But again, you have to consider the statistics. If you assume that all (or most) comets were captured in this way, you have to explain where the vastly greater number of comets are that should be passing through the system all the time in very hyperbolic orbits.If they are propelled by a supernova, they will have even greater energies. The point of the article is that interstellar bodies orbiting the galaxy at the same radius as the Sun will have a similar orbital velocity. But what is "similar"? You need to understand that at Oort cloud distances, or a fraction of a light year, the solar escape velocity is measured in centimeters per second. That's compared with an orbital velocity of 250 km/s. So in order to have an orbit that looks like a typical long period comet, the interstellar debris would have to be matched in velocity with the Sun by something like one part in 25 billion. There is no mechanism that could explain that. Even if there were a debris field close to this, there would still be a statistical distribution, and we should see strongly hyperbolic cometary orbits- but we don't. Virtually all long period comets have an eccentricity very close to 1.

Firstly, I am not capturing comets at Oort cloud distances. The solar system is capturing them at Kuiper Belt distances or even distances near the outer planets. And, yes, the velocities of the comets would be close to the Sun's velocity - perhaps somewhat less and are overtaken by the Sun's gravity field. Also, existing Kuiper Belt objects and the outer planets would enhance the N-Body capture mechanism. The overall capture mechcanism would be quite similar to the capture of an irregular satellite around the outer orbits of the gas giants, but at velocities initially closer the Sun's velocity.

And then there is the issue of the statistical distribution of cometary orbits. There is no cosmic law that says there is an even distribution of comets throughout the galaxy, if they exist. Great Periods of Dying may argue that they enter the solar system in spurts. If comets normally last periodic orbits for 10 to 20 thousand years, we cannot possibly see with any good detail the galaxial regions from thence we came from that many years. I propose that cometary objects may be captured by our solar system in a very uneven distribution that totally negates a statistical distribution study of existing cometary orbits. When comets are eventually captured the solar system has about 20,000 years to play with them before they expire or are ejected into hyperbolic orbit.

Doug Ettinger, Pittsburgh, PA 02/16/2011

[dougettinger]

I also recall that APOD. Why cannot we see this ice ring or ice signature around stars nearby to the Sun?

The interstellar densities of objects could be much higher than expected or measured.

I don't think so. The density required to produce the observed cometary flux would be high enough that we'd seem to be in a nebula, with stars more than a few light years away showing significant extinction. In addition, that much mass, heated by all the stars in the area, would raise the background well above the observed 3 K. I don't believe that the regional mass density required to explain comets could go unobserved with our current technology.

It is also worth noting that we have captured and analyzed dust from comets, and it all dates to the beginning of the Solar System. So if comets are interstellar, they are coming from a source almost exactly the same age as the Sun, which seems extremely unlikely. Any interstellar material co-orbiting the galaxy with the Sun would not be associated with the region of our Sun's formation- the perturbations associated with our numerous galactic orbits over 4.7 billion years have thoroughly scrambled things up (which is why the stars that formed with the Sun are no longer around us).

Hello, Chris. Our discussions have become rather lengthy. I have broken this topic in three parts. This part is the third.

Interstellar comets, if they exist, that may impinge the gravity field of our solar system may not come from the outpouring of a recent star death or nebula. They may have come from supernova remnants or nebulas that are well behind us in time and in our Sun's orbit or also are right ahead of us in our tour around the galaxy. Or the next encounter of comets may be several thousand years ahead of current time. The occasional encounter of a single comet gives us that infrequent non-periodic comet.

We see regional mass densities all the time with dust and gas clouds that are created by supernova remnants and nebula from a star's death. I am not sure that our astronomical measurements can distinguish between dust and comet size objects. Comets may be like schools of fish in the sea; they are difficult to detect unless you know specifically where to look.

If the age of all measured materials in comets to date indicate the age of our solar system then certainly this argues well for all comets originating from the Sun's proto-star disk. However, I remember articles that tried to explain some isotopes taken from one or more comets that were a dramatically different age from our solar system. The explanation was that the Sun's orbit passed through the remnants of a supernova and gathered these either shorter or longer lived isotopes. I am not sure that the study of the age of cometary materials is complete. I also remember that the isotopes of water in comets are different than that on Earth which may argue that cometary materials were not heated by the central regions of the proto-star; and therefore, comets did not come the central regions of our solar system. So what heated these cold bodies initially ?

I am like you, Chris. I have fun trying to connect all the dots. Remember that simple game?

Doug Ettinger, Pittsburgh, PA, 02/16/2011