Thread: Is the idea of the Oort cloud dated?

Is the idea that the Ort cloud peters out at around a light year from Earth, starting to look unlikely in view of modern observations?
Oort cloud - Wikipedia, the free encyclopedia

Over the billions of years of the systems existence thousands of stars must have passed by very close, throwing anything orbiting out at those kinds of distances into interstellar space.
List of nearest stars - Wikipedia, the free encyclopedia

Then there is evidence that in the past, our system was travelling through much denser interstellar medium, exposing any distant material to much stronger interstellar winds. Distant objects in the cloud would only be held in place by gravity less than 10^-11g.
Local Bubble - Wikipedia, the free encyclopedia

Is it more likely that the Ort clouds of all the stars are fully mixed into interstellar space, and the Ort cloud is only the local part of true interstellar debris?

This recent paper presents one view on the matter, suggesting a rather substantial depletion over time.

Originally Posted by John Galt

This recent paper presents one view on the matter, suggesting a rather substantial depletion over time.

I can find very little evidence that the density of the Ort cloud does drop away with distance, as the paper you pointed me to does look to assume.

I wonder what percentage of the Oct cloud objects are truly orbitling the sun, the expected orbital velocity is tiny.
If Neptune's orbit is 30 AU and Ort cloud stretches out to 50,000 AU, then the Ort cloud has a volume a billion times larger than the solar system.
I just can't help thinking it now makes more sense to assume it is a galaxy wide sea of debris, that gets a bit denser near stars.

I may be misreading the paper, but I don't see why you say the percentage drops off with distance.

Returning to the central point you make, it seems to me fundamentally flawed. In essence you appear to be making this contradictory argument.

From time to time comets appear on trajectories that reveal they have not come from interstellar space, we therefore posit a cloud of such comets around the sun some of which are disturbed by passing stars and Galactic tides so that plunge into the inner solar system. Since passing stars and Galactic tides can disturb comets they must have pretty well mashed them all out of anythin we might call the Oort cloud.

I think that is faulty reasoning, but am prepared to be persauded.

Originally Posted by John Galt

I may be misreading the paper, but I don't see why you say the percentage drops off with distance.

Returning to the central point you make, it seems to me fundamentally flawed. In essence you appear to be making this contradictory argument.

From time to time comets appear on trajectories that reveal they have not come from interstellar space, we therefore posit a cloud of such comets around the sun some of which are disturbed by passing stars and Galactic tides so that plunge into the inner solar system. Since passing stars and Galactic tides can disturb comets they must have pretty well mashed them all out of anythin we might call the Oort cloud.

I think that is faulty reasoning, but am prepared to be persauded.

I agree no comet has been spotted with a velocity consistent with an interstellar origin, but the numbers are pretty close.
Comet west Comet West - Wikipedia, the free encyclopedia came from a miniumum 1.1 lightyears out, assuming it was not slowed on aproach the solar system in some manner.
What are the chances of an object 1.1 lightyears out having an orbit that brings it that close to the sun, one in millions?
So how many object are there out there that have a more circular orbit?

If you assume most of the material that is orbiting the sun, is in highly excentric orbits, then these objects spend millions of years travelling slowly near aphelion, and only a few years near the sun.
As the solar system is only one billionth of the Ort cloud, the chances of true interstellar material in the cloud entering the inner solar system is very low, unless it is captured first by hitting a dense pocket of gas or something.
When you multiple all these probabilities together you still get high densities of objects at a lightyear out, and very vonerable to the tiniest external forces.

As the Sun is a larger than average star, it is likely to capture more than it loses in interactions with other stars on close approach.

I just don't see any evidence to prove the point either way.
However, if you assume every star has regular close encounters with other stars at high speed, spraying their Ort clouds in every direction, then a sea of interstellar debris looks more likely than a spherical cloud.

Originally Posted by PetTastic

I just can't help thinking it now makes more sense to assume it is a galaxy wide sea of debris, that gets a bit denser near stars.

If that's truly the case, we would be talking about a lot of matter that is damn near impossible to detect. Could it possibly be another clue to finding the elusive Dark Matter? Maybe Dark Matter is just ordinary matter after all.

<Never mind, just another crazy thought>

Anyway, I hope I am wrong, as interstellar space being full of rocks would mean we are trapped on this planet.

Assuming that the sun is losing mass constantly.
And, that less mass means less gravitational force.
For the ort cloud to not fall into the plane of the disks which the remaining planets occupy, then it would seem that the sin's gravitational force never extended into the ort cloud, and as time passes will have constantly diminishing effect of the cloud(if it really is a spherical "cloud".

Or, enough to bring the interstellar ort cloud bodies near to the solar system, creating a denser positioning of the objects, then
Will this density of objects tend to drift apart ---disperse into the greater galaxy?

(gee I hope that makes sense)

Originally Posted by PetTastic

Anyway, I hope I am wrong, as interstellar space being full of rocks would mean we are trapped on this planet.

Why would you think that? Even the Oort cloud is mostly empty space and your chance of encountering any rocky icy lumps of matter if you were traveling through it, would be slim to none.

Originally Posted by PetTastic

Originally Posted by John Galt

This recent paper presents one view on the matter, suggesting a rather substantial depletion over time.

I can find very little evidence that the density of the Ort cloud does drop away with distance, as the paper you pointed me to does look to assume.

I wonder what percentage of the Oct cloud objects are truly orbitling the sun, the expected orbital velocity is tiny.
If Neptune's orbit is 30 AU and Ort cloud stretches out to 50,000 AU, then the Ort cloud has a volume a billion times larger than the solar system.
I just can't help thinking it now makes more sense to assume it is a galaxy wide sea of debris, that gets a bit denser near stars.

I decided to feed that into Google.

It seems that 50,000 AU is approximately 0.79 light years. Alpha Centauri is 4.37 light years away. So I can see your point that the Oort cloud extends outward far enough that it covers a considerable fraction of the space in between stars.

Originally Posted by Google Converter

[COLOR=#878787 !important]1 Astronomical Unit =[/COLOR]
1.58128451 × 10^-5 light years

Anyway, I hope I am wrong, as interstellar space being full of rocks would mean we are trapped on this planet.

I think you will find that we have always been trapped on this planet and it takes an excess of optimism and imagination to think that will change anytime soon. I don't think the problem is space being full of rocks - after all, rocks are resources.

Originally Posted by Bad Robot

Originally Posted by PetTastic

Anyway, I hope I am wrong, as interstellar space being full of rocks would mean we are trapped on this planet.

Why would you think that? Even the Oort cloud is mostly empty space and your chance of encountering any rocky icy lumps of matter if you were traveling through it, would be slim to none.

If we ever hope to reach the nearest stars in less than a 100 year travel time, we would need to travel at more than 1/100 of the speed of light.
Hitting a 1 kg rock at that speed would be a 4.5 10¹² J impack. (kiloton of TNT)

Originally Posted by PetTastic

Originally Posted by John Galt

I may be misreading the paper, but I don't see why you say the percentage drops off with distance.

Returning to the central point you make, it seems to me fundamentally flawed. In essence you appear to be making this contradictory argument.

From time to time comets appear on trajectories that reveal they have not come from interstellar space, we therefore posit a cloud of such comets around the sun some of which are disturbed by passing stars and Galactic tides so that plunge into the inner solar system. Since passing stars and Galactic tides can disturb comets they must have pretty well mashed them all out of anythin we might call the Oort cloud.

I think that is faulty reasoning, but am prepared to be persauded.

I agree no comet has been spotted with a velocity consistent with an interstellar origin, but the numbers are pretty close.
Comet west Comet West - Wikipedia, the free encyclopedia came from a miniumum 1.1 lightyears out, assuming it was not slowed on aproach the solar system in some manner.
What are the chances of an object 1.1 lightyears out having an orbit that brings it that close to the sun, one in millions?
So how many object are there out there that have a more circular orbit?

If you assume most of the material that is orbiting the sun, is in highly excentric orbits, then these objects spend millions of years travelling slowly near aphelion, and only a few years near the sun.
As the solar system is only one billionth of the Ort cloud, the chances of true interstellar material in the cloud entering the inner solar system is very low, unless it is captured first by hitting a dense pocket of gas or something.
When you multiple all these probabilities together you still get high densities of objects at a lightyear out, and very vonerable to the tiniest external forces.

As the Sun is a larger than average star, it is likely to capture more than it loses in interactions with other stars on close approach.

I just don't see any evidence to prove the point either way.
However, if you assume every star has regular close encounters with other stars at high speed, spraying their Ort clouds in every direction, then a sea of interstellar debris looks more likely than a spherical cloud.

I find your argument here very weak.
1. Of course the observed orbits are close to what one would expect from an interestellar visitor: the mathematics requires that they should be.
2. Out of hundreds of observed comets you come up with one that may have an interstellar trajectory. No one is saying that we may not be visited by interstellar comets. I am saying there is little evidence to suggest these are commonplace and that the Oort cloud does not exist.
3. Your statements about highly eccentric orbits is flawed. The comets that come into the inner solar system have been displaced from orbits that never had an aphelion in the inner solar system. That part of your argument is simply wrong.
4. On balance, then, I find nothing in your argument to support your contention that the Oort cloud does not exist in any meaningful way and that, on the contrary, there is solid evidence for its reality.

Originally Posted by John Galt

I find your argument here very weak.
1. Of course the observed orbits are close to what one would expect from an interestellar visitor: the mathematics requires that they should be.
2. Out of hundreds of observed comets you come up with one that may have an interstellar trajectory. No one is saying that we may not be visited by interstellar comets. I am saying there is little evidence to suggest these are commonplace and that the Oort cloud does not exist.
3. Your statements about highly eccentric orbits is flawed. The comets that come into the inner solar system have been displaced from orbits that never had an aphelion in the inner solar system. That part of your argument is simply wrong.
4. On balance, then, I find nothing in your argument to support your contention that the Oort cloud does not exist in any meaningful way and that, on the contrary, there is solid evidence for its reality.

I agree there is strong evidence the Oort cloud exists, but I am questioning that it ends.
Is it just an increase in interstellar debris density near stars?

Chemical analysis does indicate most if not all periodic comets were created as part of our solar system, hence they are permanent members of a local cloud.

However, my argument that you do not like, is based the chances of seeing a comet from a population orbit 1.1 light years out, in the inner solar system.
Ie if we see one in a 50-year period, then the must be hundreds of millions out there, based on orbital period, probability of a comet having a transverse vel of only cm per second for a hundred thousand years that far out, when circular obital velocity of 113 m/s etc.

What originally got me to question the idea, was estimates of interstellar comets being as high as 10¹³ per cubic parsec
So 1.2 10¹¹ inside the Oort cloud at any one time.
If you, then assume the Sum is moving at hundreds fo km/s through these comets the number parsing through our system over the millions of years gets large.

http://en.wikipedia.org/wiki/Interstellar_comets