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 Oort clouds.

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mm5agm

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PostSubject: Oort clouds.    Tue Aug 23, 2011 3:47 pm

How common are Oort clouds? Have we detected them around other stars and if so then how far do these exo-oort clouds extend from their stars?

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scopeman

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PostSubject: Re: Oort clouds.    Tue Aug 23, 2011 3:48 pm

I would tend to argue that Oort-type clouds would be fairly common---however we still don't quite know as much about our own to give it such a characterization as saying it will have so many planetesimals (Sedna-type mini-planets) or comets of one type or another.

It may take some time before we can say that we can recognize any or all Oort-type clouds besides our own with any means of telescope--the objects are just too small.

But, the means of characterizing our own heliopause and bowshock boundaries will go a long way to understanding where Oort type clouds extend in other "solar sytems"?

A question that I would have is: how does a binary star system affect the formation of any Oort-type cloud?

I am fairly certain that theoreticians have attempted to tackle the problem, but just as any multi-body computation entails a certain amount estimation---this problem may face a problem of understanding initial conditions.

One major stumbling block is characterizing all possible birth environments (or initial star cluster) from which star (or stars) came.
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PostSubject: Re: Oort clouds.    Tue Aug 23, 2011 3:53 pm

I don't think our technology is good enough yet to detect such things, but we have detected planetary disks (planet forming disks) around young stars.
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mm5agm

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PostSubject: Re: Oort clouds.    Tue Aug 23, 2011 3:53 pm

Sounds like interstellar travel at even a fraction of light speed could be rather hazardous. Interstellar space might not be quite the void we previously thought it to be.
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scopeman

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PostSubject: Re: Oort clouds.    Tue Aug 23, 2011 3:54 pm

mm5agm wrote:
Sounds like interstellar travel at even a fraction of light speed could be rather hazardous. Interstellar space might not be quite the void we previously thought it to be.

No, interstellar space is pretty much the void we think it to be. There could still be a lot of stuff in interstellar space in total, though, because it's just so incredibly vast.

What you need to remember is that our solar system is filled with much more stuff than interstellar space. We know this because the solar system itself is plowing through interstellar space. We have detected lots of comets bound to the solar system--including several on escape trajectories nudged to escape by Jupiter. But we haven't detected a single interstellar interloper yet.

We have detectors that can sense when tiny bits of debris collide with Earth's upper atmosphere. Practically all of this debris is interplanetary in nature--originally from our own solar system, rather than interstellar in nature.
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mm5agm

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PostSubject: Re: Oort clouds.    Tue Aug 23, 2011 3:56 pm

scopeman wrote:
No, interstellar space is pretty much the void we think it to be.

How do we know? The kuiper belt was only discovered in 1992 and extends from the orbit of Neptune (at 30 AU) to approximately 55 AU from the Sun. the Oort cloud is nearly a quarter of the distance to Proxima Centauri. If that binary system has a similar cloud of similar dimensions then thats a lot of interstellar debris.

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But we haven't detected a single interstellar interloper yet.

We are still finding dozens of new moons around the gas giants. Maybe our telescopes are inadequate to the task.

Quote :
We have detectors that can sense when tiny bits of debris collide with Earth's upper atmosphere. Practically all of this debris is interplanetary in nature--originally from our own solar system, rather than interstellar in nature.

Proto-planetary disk material from different stars is variable in constitution? Thats interesting. I knew stars had different spectra and therefore material. Ie some are richer in metals. Didn't think primordial dust would vary all that much. Ofcourse we are bobbing up and down as we move around the galaxy so the lack of interstellar material could be cyclical and change in a million years or so couldn't it?
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PostSubject: Re: Oort clouds.    Tue Aug 23, 2011 3:57 pm

mm5agm wrote:

How do we know? The kuiper belt was only discovered in 1992 and extends from the orbit of Neptune (at 30 AU) to approximately 55 AU from the Sun. the Oort cloud is nearly a quarter of the distance to Proxima Centauri. If that binary system has a similar cloud of similar dimensions then thats a lot of interstellar debris.

It might be a lot of interstellar debris (not my wording), but it is spread out over a really, really, really big volume. The density is still very, very low.
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mm5agm

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PostSubject: Re: Oort clouds.    Tue Aug 23, 2011 3:59 pm

The kuiper belt, scattered disk and oort cloud are just separate names for different parts of the remaining proto-planetary disk. The visible bodies are clumped dust piles that never made it to the planetary stage. Few seem to have even reached the level of dwarf planet. Therefore they haven't cleared their orbital paths. Doesn't that mean a lot of dust is out there? For a vehicle anywhere close to a decent percentage of lightspeed that sounds like a minefield to me. Are you saying the chances of hitting a speck of dust is negligible?
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PostSubject: Re: Oort clouds.    Tue Aug 23, 2011 4:01 pm

mm5agm wrote:


How do we know? The kuiper belt was only discovered in 1992 and extends from the orbit of Neptune (at 30 AU) to approximately 55 AU from the Sun. the Oort cloud is nearly a quarter of the distance to Proxima Centauri. If that binary system has a similar cloud of similar dimensions then thats a lot of interstellar debris.

The Kuiper belt was hard to detect because it's so far away. In contrast, we are literally swimming in what little interstellar debris there is in our neighborhood.


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We are still finding dozens of new moons around the gas giants. Maybe our telescopes are inadequate to the task.

Moons around the gas giants are far away, because the gravity of the gas giants keeps them far away. There is no magical gravitational effect which keeps interstellar debris far away. By definition, interstellar debris is not gravitationally bound to a star system. As such, it should be just as dense near Earth as it is in between the stars.

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Proto-planetary disk material from different stars is variable in constitution?

Yes, but isotope composition is not something that is determined by radio meteor detection--only velocity and time of impact. Interstellar debris would be obvious by having velocity much greater than solar escape.

Also, if there were significant amounts of interstellar debris it would have a sinusoidal yearly distribution as Earth's motion varies between going with and against the solar system's motion through the interstellar medium.
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mm5agm

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PostSubject: Re: Oort clouds.    Tue Aug 23, 2011 4:03 pm

scopeman wrote:

By definition, interstellar debris is not gravitationally bound to a star system. As such, it should be just as dense near Earth as it is in between the stars.

Stars orbit one another and they lie at interstellar distances from one another. So you are saying that the dust at those distances is too diffuse to share a common gravity and stay bound to our solar system then? So why is the Oort cloud there?

There is insufficient visible mass to hold the galaxy together isn't there?

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Interstellar debris would be obvious by having velocity much greater than solar escape.

If it had greater velocity than solar escape then wouldn't it leave our region of space and become undetectable? There are comets with highly elliptical trajectories which take them well beyond our solar system.
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PostSubject: Re: Oort clouds.    Tue Aug 23, 2011 4:06 pm

mm5agm wrote:

Stars orbit one another and they lie at interstellar distances from one another.

Only stars which are pretty close to each other orbit each other.

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If it had greater velocity than solar escape then wouldn't it leave our region of space and become undetectable?

Yes, any interstellar interlopers will leave our solar system because it's not gravitationally bound to our solar system (unless they collide with something). When they arrive, they already have greater than solar escape velocity. The Sun's gravity accelerates them even faster as they get closer, and then decelerates them when they leave. But the Sun's gravity only removes as much speed as it gave in the first place.

(Solar escape velocity is dependent on distance from the Sun. The closer you are to the Sun, the greater the velocity required to escape. If a piece of interstellar debris enters the solar system, it enters with greater than escape velocity and the Sun's gravity will accelerate it so it remains above escape velocity at all times.)


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There are comets with highly elliptical trajectories which take them well beyond our solar system.

They don't have solar escape velocity. While aphelion is much further away than the Kuiper belt, it's still close enough for the Sun to pull them back in. That's the definition of being gravitationally bound to the Sun.


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So you are saying that the dust at those distances is too diffuse to share a common gravity and stay bound to our solar system then? So why is the Oort cloud there?

Umm...you seem very confused about what it means to be gravitationally bound. There is no minimum mass required to be gravitationally bound to the Sun. All that's necessary is to be sufficiently close and to have a sufficiently low velocity relative to the Sun (below solar escape velocity). It doesn't matter whether you're a speck of dust or a gas giant planet. If you've got less than solar escape velocity, you're gravitationally bound to the Sun.

The Oort cloud consists of objects that are gravitationally bound to the Sun. If you were an observer in Alpha Centauri, you would never see one of our Oort cloud objects get closer than 3 light years no matter how long you waited, because the Sun's gravity keeps them close to the Sun. In contrast, interstellar debris is not bound by any star's gravity. So there's no particular reason for them to magically avoid the Alpha Centauri system.
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mm5agm

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PostSubject: Re: Oort clouds.    Tue Aug 23, 2011 4:08 pm

scopeman wrote:

Umm...you seem very confused about what it means to be gravitationally bound.

I am. But less so now.


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If you were an observer in Alpha Centauri, you would never see one of our Oort cloud objects get closer than 3 light years no matter how long you waited, because the Sun's gravity keeps them close to the Sun.

What about perturbations by Neptune and other trans-neptunian objects like local minor planets? Couldn't they cause objects to increase speed and leave the solar system just as they sometimes cause long period comets to fall inwards and cause mass extinctions?

Thank you for taking the time to answer my questions.
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scopeman

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PostSubject: Re: Oort clouds.    Tue Aug 23, 2011 4:09 pm

mm5agm wrote:

What about perturbations by Neptune and other trans-neptunian objects like local minor planets? Couldn't they cause objects to increase speed and leave the solar system just as they sometimes cause long period comets to fall inwards and cause mass extinctions?

Minor planets are unlikely to ever cause objects to escape the solar system because of the weak gravity. Neptune can eject an object which gets close enough, but this would be very rare due to the long distances and low speeds involved.

But the most likely ejection mechanism is easily Jupiter. It is theorized that near-ejection by Jupiter is what populated the Oort cloud in the first place. Due to the extremely low orbital speeds at aphelion, the gravity of nearby stars would be enough to slightly deflect their orbits. A slight deflection wouldn't be enough to cause the Oort cloud object to escape the Sun, but it would be enough to raise perihelion by many AUs. This would be enough to prevent this object from getting anywhere close to Jupiter (or any other planet) for billions of years.

But the slight deflections which can raise that perihelion can eventually lower it again by random chance. So, billions of years later the Oort cloud object comes close enough for us to see it--a long period comet. At this point, it's at risk of being ejected by Jupiter. Or its perihelion might get raised again and we don't see it again for billions of years.
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mm5agm

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PostSubject: Re: Oort clouds.    Tue Aug 23, 2011 4:10 pm

If stars form out of nebulae then wouldn't there be a lot of material inbetween the stars just drifting about though?
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PostSubject: Re: Oort clouds.    Tue Aug 23, 2011 4:10 pm

There is material between the stars; it's called the interstellar medium. The total amount of material is quite a lot, but it's spread over a stupendously large volume. As a result, there just isn't a whole lot of it along the path of a starship traveling between two nearby stars.

In contrast, the interstellar gas and dust can be significant over galactic distances. It's the reason why we can't see the center of the galaxy in visible wavelengths--there's too much dust in the way.
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PostSubject: Re: Oort clouds.    Tue Aug 23, 2011 4:10 pm

If you go 20 kilometers per second, instead of 10 kilometers per second, micro impacts decrease in some directions and triple in some other directions. At 3000 kilometers per second = 0.01c impacts, we think, are approximately proportional to speed in all directions. So yes, lots more impacts at very high speed, and the damage increases as the square of the speed. At 0.9999c, you get nanoseconds warning of what you are about to hit, and even a single proton or neutron is quite destructive = hard gamma, so even one particle per cubic meter = 1,000,000,000 particles per cubic kilometer is bad.
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