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Comets form where stars form. And they are then ejected by the orbital changes of giant planets. But why are they ejected to form specifically a local Oort cloud instead of going interstellar and becoming vagabonds? The fact that comets come from all directions doesn't discriminate between them coming from an Oort cloud or from interstellar sources.

The main argument against the idea of one star capturing a comet from another star, seems to be that the relative velocity is too high for that. But consider the huge age of the Sun which is now on its 18th or so orbit through this crowded galactic star field disc. And which by chance or co-formation might have now, or at some earlier time, roughly the same velocity as some other millions of other stars. And what if there are many (many) more comets formed and/or ejected from young stars than currently supposed?

Is it really unlikely that the Oort "cloud of comets" instead largely represents an interstellar source?

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Since most comets are on a predictable orbit that has them circling the sun (several times), then they cannot be from outside the solar system.

Any comet that originates outside the solar system, will pick up enough velocity approaching the sun to be able to leave the solar system. Therefore, we would only see the comet once. It is unlikely that the comet approaches Jupiter (Saturn, Neptune or Uranus) where the trajectory shifts enough so that it will remain in the solar system.

Since the solar system is believe to be formed from the gravitational attraction of interstellar gas and debris, it is very unlikely that comets will be ejected unless their orbits are perturbed.

So, there is a low probability of a comet being ejected from one system, another low probability that the comet will encounter another system (after millions and billions of years) and yet another low probability that the comet gets captured in the system.

Added:
If the object was pushed out to 1 LY and it then returned as a comet, it was still under the influence the sun's gravity and would not be considered as extra-solar. The object would need to come from another system to be called extra-solar. Since the ejected objects are in a nearby orbit to the solar system, they will either return, or travel outward. Think about a satellite with a period of 12 hours. A slight push will put it in orbit 2 meters higher, and will increase the period significantly.

There is a lot of space at Neptune's orbital distance, and the intersection with Neptune's gravity is quite small (ratio of area of influence (0.1g) / surface area = ${8.6\times 10^{-11}}$). The reason the Jupiter captures asteroids is because they are in the same plane and are moving in the same direction. This increases the probability of changing the orbit significantly. It could also take several hundred passes before the orbit changes significantly.

The area of influence is the cross section area where the gravity is greater than 0.1g. The surface area is the spherical surface area of the mean distance to the planet.

Jupiter - ${5.325\times 10^{-8}}$ ~= 0.0000053%
Saturn - ${4.703\times 10^{-9}}$ ~= 0.00000047%
Uranus - ${1.791\times 10^{-10}}$ ~= 0.000000018%
Neptune - ${8.605\times 10^{-11}}$ ~= 0.0000000086%

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  • $\begingroup$ Why couldn't comets with orbits which are currently stable and/or predictable, once have been captured from other stars? If I understand the Oort cloud hypothesis correctly, billions and billions of comets were ejected to almost a light year from here, early in the formation of the Solar system. Billions of years ago. How could then 18 tours around the galaxy NOT have made a large part of them interstellar? What if there's 1000 non-captured interstellar comets passing by at Neptune for every comet captured? $\endgroup$ – LocalFluff Dec 28 '14 at 2:25
  • $\begingroup$ @LCD3 0.000001% is not so much on a cosmological scale. It's only an 8 on the logarithmic scale. If there are a factor of 18 ejected comets per star, and a factor of 9 stars in the Suns orbit around the Milky Way, then maybe a factor of 8 is negligible. You might be right, you just don't convince me in this case. $\endgroup$ – LocalFluff Dec 28 '14 at 11:42
  • $\begingroup$ @LocalFluff Jupiter is half of what I guessed at (0.000001%). $\endgroup$ – LDC3 Dec 28 '14 at 16:29
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And they are then ejected by the orbital changes of giant planets. But why are they ejected to form specifically a local Oort cloud instead of going interstellar and becoming vagabonds?

Per multiple models, most comets did become vagabonds. Estimates vary, but the lowest I've seen is 65% being ejected, and some estimate that well over 90% were ejected. Moreover, comets in the Oort cloud are ejected as well over time. The long period comets and Halley-style comets that we see are but a small fraction of the perturbed comets in the Oort cloud. Most perturbations result in escape rather than diving deep into the solar system.

If other stars are like our Sun, interstellar space should be awash with escaped comets.

The fact that comets come from all directions doesn't discriminate between them coming from an Oort cloud or from interstellar sources.

You are ignoring velocity and eccentricity, which do distinguish between comets that originated from within the gravitational bounds of the solar system versus comets from interstellar sources. This is the key reason Jan Oort hypothesized the Oort cloud. All comets observed to date are consistent with having an intra-solar system origin and are inconsistent with having an extra-solar system origin. An interstellar comet should have a large excess velocity. That is not what has been observed. The few comets that apparently are on a hyperbolic trajectory are deemed to have originated within the gravitational bounds of the solar system, with the hyperbolic orbit a result of non-gravitational forces and gravitational interactions with planets.

If interstellar space is awash with ejected comets, estimates on how often we should see an interstellar comet range from once per 25 years (which has not been observed) to once every 450 years (which is consistent with not seeing any). Moreover, there are reasons to think that interstellar space is not awash with comets. If interstellar space was awash with comets, we should see a lot more gamma ray bursts than are observed, and we shouldn't see any white dwarfs depleted of hydrogen (which are observed; 16% of the white dwarfs are DB white dwarfs). This suggests to some that the solar system is an oddity, and that most stars don't produce Oort clouds.

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