What exactly is holding the Oort Cloud intact?

[Gamefreak89]

I'm just wondering being only half a light year away from us what celestial body is keeping the Oort Cloud intact?

[IsaacKuo]

Nothing keeps it "intact", but the gravity of the Sun is the thing which keeps it from drifting away.

[Gamefreak89]

The gravity of the sun doesn't reach that far does it?

[IsaacKuo]

Yes, it does.

[Gamefreak89]

How? I thought that after about Pluto the sun's gravity weakens?

[IsaacKuo]

The Sun's gravity weakens from its very own surface. It gets weaker the further away you get from the Sun, period. There's nothing special about Pluto.

[Gamefreak89]

No I was using Pluto as a distance marker that's it.

[Romanus]

Take Earth. The planet's gravity never "vanishes" completely with distance; however, the farther one goes the more predominant the influence of the Sun's gravity is, until it's more important than the Earth's. The Sun is no different; past a certain distance, its gravity weakens to the point where the Milky Way's own gravity (along with that of passing stars) predominates, and prevents any stable orbits from forming. The Oort Cloud is within this limit.

[Gamefreak89]

Well that explains it lol.

[TampaDude]

It is kind of amazing that the Sun's gravity can hold objects in orbit at that distance. Astronomy is cool.

[tdvance]

well--gravity has infinite reach. It decreases according to the inverse of the square of distance and gets pretty faint, but it's still infinite (or 13.8 billion light years, since the effect of gravity propagates at the speed of light and the universe has only been around that long).

[slang]

It is kind of amazing that the Sun's gravity can hold objects in orbit at that distance. Astronomy is cool.

It puts context around the often used words: the Sun is big.

[Gamefreak89]

Well if the oort cloud is half a ligt year away from us then how long does it take for a comet to reach us and at speeds can they reach?

[IsaacKuo]

Well if the oort cloud is half a ligt year away from us then how long does it take for a comet to reach us and at speeds can they reach?

The easier question is that of speed. The speed when it reaches Earth would be almost exactly solar escape velocity, or 42km/s (equal to Earth's orbital speed multiplied by sqrt(2)). However, this is 42km/s relative to the Sun. Relative to us, this could be anywhere from 12km/s to 72km/s depending on whether it hits us coming from behind or in front or from the side. If you add in the effect of Earth's own gravity, the impact velocity would range from 16km/s to 72km/s.

The time it takes to reach us depends on how far away the comet is. Half a light year is about 32,000AU, so the semi-major axis is about 16,000AU. This results in an orbital period of 16,000^1.5 years, or 2 million years. It takes half an orbit to "fall" from 32,000AU to 1AU, so that means it takes 1 million years for the comet to fall to Earth.

However, this assumes an absolutely perfectly aimed minimum energy nudge that eliminates the comet's orbital speed (assuming a circular orbit, about 240m/s). The nudge would more plausibly leave the comet with some inward or outward velocity. It doesn't take much velocity to have a big effect over a million years!

[Glom]

Newton's Law of Universal Gravitation states that the gravitational force exerted by one body on another is given by:

F = G*M*m/r²
where F is the gravitational force, G is the universal gravitational constant (6.67e-11 /(kg.m.s²)), M and m are the two masses involved and r is the distance between them.

This formula clearly shows that the gravitational force never expires, it decreases asymptotically as distance increases but only reaches zero at infinity.

[chornedsnorkack]

Roughly which proportion of comets that were in Oort cloud 4 566 000 000 years ago are still there?

Also, how much is the Oort cloud denser than the comet field of Milky Way disc?

[EDG]

I don't think there were any comets in the Oort Cloud when the solar system formed, they all accumulated there after everything got tossed out of the protoplanetary disc, didn't they?

[PraedSt]

Roughly which proportion of comets that were in Oort cloud 4 566 000 000 years ago are still there?

A small proportion. They're weakly held, so I think most got tossed out over the years.

Also, how much is the Oort cloud denser than the comet field of Milky Way disc?

I'm not sure that the Milky Way has a comet "field".

I don't think there were any comets in the Oort Cloud when the solar system formed, they all accumulated there after everything got tossed out of the protoplanetary disc, didn't they?

More or less what I read: they initially accumulated there after getting tossed out of the inner Solar System. Tossing out of the entire System then occurred over time.

[Ilya]

I'm not sure that the Milky Way has a comet "field".
More or less what I read: they initially accumulated there after getting tossed out of the inner Solar System. Tossing out of the entire System then occurred over time.

I would say Milky Way must have a "comet field" -- in the sense of diffuse population of comets throughout entire Milky Way's volume. Assuming current theories of comet formation are correct, most (as in 90-99%) of primordial solar system comets did not end up in Oort Cloud, but were flung out of solar system altogether. They are still wandering the galaxy. If all (or even substantial portion) of planetary systems developed similarly to ours, Milky way must have comet population on the order of 10²⁴.

[Robert Tulip]

I don't think there were any comets in the Oort Cloud when the solar system formed, they all accumulated there after everything got tossed out of the protoplanetary disc, didn't they?

One suggestion is that the late heavy bombardment of 3.9 billion years ago was caused by Neptune moving from #3 to #4 gas giant, clearing the orbits inside its current distance from the Sun and sending a lot of matter to the Oort Cloud.

[neilzero]

If we assume the nebula that formed our solar system had a radius of 1/2 light year (it may have been much larger, and produced other suns, perhaps many other suns) then some of the comets likely formed near the outer edge, perhaps even sooner than most of the planetesimals which were likely similar to present asteroids. Collisions were frequent and sometimes lead to mergers. The 8 planets assembled, perhaps several more about the size of Earth. The extra planets, if any, and up to 90% of the smaller bodies ended up in the Kuiper belt and the Oort cloud. Likely a few were born close to their present orbit, but most everything got moved around for a few million years and the extra sun's, if any, left and took some of the stuff with them, as a result of near miss sling shot maneuvers. Perhaps more correctly it was our solar system that left. I don't think the last two sentences are mainstream, so perhaps it did not happen anything like that. Most of the stuff that was occasionally more than a light year from our sun, is now much farther away. Likely occasional stuff joins our Oort cloud for a few a few million years, then drifts away, so a thinner Oort cloud of temporary stuff may exist more than three light years away in directions where there has been no other close suns the last few million years. I'm guessing, so I better get some opinions before I am cast to ATM= Against The Mainstream. Neil

[neilzero]

Ilya suggested that our galaxy may have 10E24 unattached comets. Since the volume of our galaxy is about 10E15 cubic light years that would mean a billion comets per cubic light year, average. If the comet density was that high in the solar systems path, about a million years ago, we should see at least one hyperbolic comet per decade, closer than Saturn. There are some that are border line hyperbolic. Perhaps the 10E24 includes mostly comets smaller than one cubic kilometer, which we may miss if they come no closer than Saturn? Neil

[Ilya]

I checked your math, and it seems correct. But that's assuming comets move in random orbits, which is very unlikely. Matter in Milky Way tends to orbit in more or less the same direction, which greatly reduces the probability of random encounters.

[Henry Krinkle]

well--gravity has infinite reach. It decreases according to the inverse of the square of distance and gets pretty faint, but it's still infinite (or 13.8 billion light years, since the effect of gravity propagates at the speed of light and the universe has only been around that long).

So theoretically speaking if the universe consisted of one star, and all else was a vacuum, then a planet 13.8 billion light years away would be bound by that stars gravity?

[IsaacKuo]

So theoretically speaking if the universe consisted of one star, and all else was a vacuum, then a planet 13.8 billion light years away would be bound by that stars gravity?

Essentially no. It could only be bound if the relative velocity with the star were below escape velocity--which at that distance would be astronomically close to zero. Even if the relative velocity were somehow zero for a moment, it would soon be non-zero as the random photons and atoms leaving the star would change the velocity of the star.

[Jeff Root]

I checked your math, and it seems correct. But that's assuming comets
move in random orbits, which is very unlikely. Matter in Milky Way tends
to orbit in more or less the same direction, which greatly reduces the
probability of random encounters.

I don't think that's right. Although everything in the Milky Way tends to
orbit around the center in a rough plane in pretty much the same direction,
that would have little or no relevance to the relative paths of two bodies.
You can consider one body to be at rest, and the other body's motion
relative to it can be in any direction with equal probability. Comets from
the Galaxy at large should arrive equally from all directions.

-- Jeff, in Minneapolis