Oort cloud

I was just reading about the Oort cloud, but

  1. everywhere it says that its hypothetical, but many places also say that most astronomers believe the Oort cloud exists. Why?

  2. About how likely is it to exist?

  3. Many sources also say that its a cloud, unlike our asteroid belt which completely encircles the sun. Why is it thought to be a cloud instead of a belt, especially when we’ve never seen any trace of it? Or have we?

  4. Is there any evidence that it doesn’t exist?

  5. Do we have any indication of how large it is? Whether it’s very large or small, at least?

IANAA (I am not an astronomer), but I understand that the main reason why the Oort Cloud has been hypothesised is as the origin of long-period comets. Presumably they come from somewhere, and they have limited lifetimes, as they will eventually be captured by and crash into one of the larger planets (Jupiter, Saturn, etc.).

The objects in the Oort cloud are too distant and small to observe directly, which is why it is referred to as hypothetical. Astronomers are very confident it exists. Those long-period comets are coming from somewhere. Comets have a limited lifetime as comets. After repeated passes through the solar system, the volatile components (which give the comet it’s spectacular tail) are boiled away or trapped beneath a surface layer, and they end up appearing very similar to asteroids. This happens relatively quickly, compared to the age of the solar system.

Very, very likely. No other credible hypothesis for the origin of comets exists. So far, no comet has been observed on a trajectory that suggests it originated from interstellar space.

Early in the history of the solar system, the Oort cloud objects were thought to be scattered by the influence of the giant planets, primarily Jupiter. Early in it’s history, the solar system was a rough disc of material. However, a comet sized body could easily be ejected away from the plane of the ecliptic by a close encounter with Jupiter. It would only have a pass a little above or below the planet.

Recent research suggests many Oort cloud objects were actually captured from other stellar systems early in the solar system’s history. The sun formed in a collapsing molecular cloud, in close proximity to other young stars. Stars don’t form in isolation.

There are estimates, but they vary quite widely. We’re talking about several Earth-masses of material, and trillions of comet-sized bodies, over a distance of a couple light years or more.

A couple of light years? Does that mean that our Oort Cloud overlaps with the Oort clouds of neighboring systems?

How would such a trajectory differ from that of an object coming from the Oort cloud? Surely any trajectory of an object moving under the influence of the Sun’s gravitational field is going to be a conic section.

Yes, but an elliptical orbit would mean it is in orbit around the sun (and came from the Oort cloud) while a parabolic orbit would be from outside the solar system.

True, but neither a parabola nor a hyperbola is an orbit. A comet with a parabolic trajectory will leave the influence of the sun and cannot be in orbit. It is possible that a comet could be observed that has been perturbed out of orbit in the Oort cloud and has gained enough energy in the perturbation to be in the process of being ejected from orbit, so observation of a parabolic trajectory is still not perfect evidence of extra solar origin, but given we have never seen one of those either, evidence is good that they all come from some solar orbit, and retain an orbital trajectory.

Have we not observed any? I was under the impression that many, perhaps even most, comets do have parabolic or even hyperbolic trajectories and swing by the inner solar system (to be observed) only once. Periodic comets have elliptical, orbital trajectories, but not all comets are periodic.

Note, I am not saying that a-non elliptical orbit is evidence of an origin from beyond the Oort cloud. My understanding is that comets from the Oort cloud can have any sort of conic-section trajectory, but that the same would also apply to objects originating from further away. All of the comets that we observe, and that do come from the Oort cloud, have, of course, somehow been “perturbed” out of their original (presumably elliptical, but relatively close to circular) orbits within the cloud.

Actually, Wikipedia, says the following:

The (not terribly pellucid) material that follows this, however, does imply that hyperbolic trajectories are relatively rarely observed, and those that are are only ‘slightly’ hyperbolic. Perhaps the answer to my question is that if many comets were of entirely extrasolar origin, then we would expect many more of them to have hyperbolic, and even highly hyperbolic, trajectories. I am not sure if that is true (or why one would expect it), but it seems a possibility. (But I do not see why it is not also a possibility that a comet of extrasolar origin could be captured by the Sun’s gravity and become a periodic comet, in an elliptical orbit.)

I would guess this is right. It is going to come down to the distribution of comets we see. If the majority are in elliptical orbits, or generally have trajectories that can reasonably be perturbations of Oort cloud orbits, we have good evidence for the cloud. Capture of extra-solar comets is clearly reasonable, and probably still occurs on some inhuman timescale. One suspects a statistical analysis of the question is the only useful answer.

As you say, a number of comets have been observed on parabolic or hyperbolic trajectories. However:

Bolding mine. This means the comet has only achieved solar escape velocity due to performing a gravitational slingshot around the sun. An interstellar comet would be expected to have a very high initial velocity. The total number of interstellar comets is though to be very large, modelling suggests 90-99% of comet-like bodies are ejected, but the inner solar system is a very small target.

I believe that it was noticed early on that comets divided fairly naturally into “short-period” and “long-period” comets. The short-period comets tended to have orbits near the plane of the solar system, while the long-period comets had orbits at any old angle relative to the plane of the solar system. (See, for example, Comet Hale-Bopp, whose orbit is almost at right angles to the plane of the solar system.) This suggests that the short-period comets come from a belt-shaped region near the plane of the solar system (the Kuiper Belt), while the region that the long-period comets come from has to be in all directions from the sun (i.e., a cloud.)

If you only take the interactions between the Sun and the comet into account, this can’t happen. Roughly speaking, the reason for this is that the angular momentum and the energy of the comet are constant, and these two quantities uniquely determine the shape of the orbit. The energy is really the important one; if the comet has enough energy, it can get arbitrarily far away from the sun and so can be said to “escape” the solar system. This means that a comet coming in from interstellar space would, by definition, start out with enough energy to be outside the solar system, and so would have enough energy to “get back out” of the solar system.

Now, if the interstellar comet interacts with a planet on the way in, then all bets are off. Essentially, the comet can give up some of its energy to Jupiter (say), and then it won’t have enough energy left to get back out of the solar system. The net result is that the comet is captured. But the Sun needs the assistance of a third body in order to capture a body that has enough energy to be outside of the solar system; it can’t do it on its own.

I may be misconstruing you here, but do you have the impression that a “cloud” would be in a grouping, mostly all in one direction from the Sun?

I think the Oort cloud also completely encircles the Sun, but makes for a three-dimensional cloud with the Sun at its center, contrasted with the asteroid belt which is on a two-dimensional ellipse (also the Kuiper belt is like this I think).

The other piece of evidence is the number (and distribution) of these comets; if there are enough that we see them falling in toward the sun very regularly, then there must be a lot of interesting things in the “cloud” causing these perturbations. Something triggers an instability in a protocomet’s orbit. Is it interaction with another object in the cloud? Accumulated “kicks” if it has some level of harmonic interaction with Jupiter or other large planets (somewhat) nearby?

Can you expand on this? I’m not seeing it for all cases. It does seem intuitive to me that a comet coming in at high speed relative to the sun will deflect a little and then just keep going. But what about an interstellar comet that is heading in the same basic direction as the sun, just at a slightly faster or slower speed? A comet like that would have a very small difference in velocity relative to the sun and it seems to me that certain set of conditions should make it possible to have the comet wind up in orbit of the sun.

First of all, even if they start off with very small relative speed, the comet is going to speed up as it approaches, due to gravity.

Second, it might help to remember that gravity (like, so far as we can tell, all the other fundamental forces) is time-symmetric. So if it were possible for an originally-free comet to get captured into a closed orbit, it would also be possible for a comet initially in a closed orbit to end up escaping.


Adding to what Chronos wrote above:

You seem to be saying that the fact that the Sun and the comet are both moving is important for the Sun to capture a comet. However (as you seem to recognize), the relative velocity is what’s important. If you look at everything in a reference frame that’s riding along with the Sun, it’ll look like the Sun is at rest while the comet is moving around it. The motion of the Sun is (to a very good approximation) unimportant.

But if you accept this fact, and you can convince yourself that a star “at rest” can’t capture comets, then it follows that any star can’t capture comets, regardless of how it moves relative to the comet’s initial trajectory. Either the comet can get arbitrarily far away from the star, or it can’t; the truth or falsehood of this statement isn’t affected which reference frame you’re in. So if a star at rest can’t capture comets, then a moving star can’t capture comets either.

What Chronos says in regards to the comet speeding up as it approaches does make sense. It is similar to a thought experiment I’ve already done in regards to asteroids and the Earth (namely, that there’s no way to have a “gentle collision” because even an asteroid that starts off with a small velocity difference to the Earth will be accelerated to a high speed as it approaches).

So I guess I can take it as a given that an extrasolar asteroid approaching the sun will, for any velocity/position combination, have too much energy to wind up in an orbit. I guess I’ll have to leave any deeper understanding for a time when I can teach myself more about the orbital mechanics.

Possibly the simplest way to look at it: There’s a threshold of energy where, if the star-comet system has more than that amount of energy, the comet is free, and if it has less, the comet is bound. But the total energy of the system is constant. So if the comet is ever above that threshold, it’s always above it, and if it’s ever below that threshold, it’s always below it.