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I admit without any fact that the Oort cloud (comet reservoir) should exist, and it seems to be accepted by far by the large majority of astronomers.

But it is still a theory without any direct observation. Except the fact that we guess that comets come from this cloud (a cloud of comets).

So what are the facts or indirect observations that may validate this comet cloud theory?

Collateral to this question, is it easy to observe these indirect facts, and is there any special way to observe these Oort comets to validate the theory?

Is it acceptable to say there is no cloud and just a few solar and most inter-solar objects?

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The orbits of long period comets have such large semi-major axes that they have to come from a reservoir as far away as the Oort Cloud. –  Andrew Nov 29 '11 at 22:32
    
And what about all these comets that fall directly to the sun ? Not all comet got real (interpreted) orbit.And what about captured interstellar object... May be this question can't be answer and is subject to open debates.(not in goal of astronomy.stackexchange) –  Emmanuel Devaux Nov 30 '11 at 14:42

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up vote 11 down vote accepted

There are several points of evidence that the Oort Cloud exists, though it is indeed still a hypothesis and lacks direct observation.

The first is indirectly observational, as proposed by Ernst Öpik back in 1932 as the source of long-period comets. This was revised by Jan Oort in 1950. All you need to determine an orbit is three observations of the object, separated in time. The greater the separation in time and the more observations, the more certainty we have in its orbit. Comets with periods longer than Pluto's must, by definition, have come from beyond Pluto. Pluto's orbit basically loosely defines the extent of the Kuiper Belt (30-50 AU).

So there needs to be a source for these bound objects, and interstellar ones don't cut it because if they're interstellar, then they should not be on bound orbits.

The second is theoretical: Solar system formation models predict that the formation of the giant planets would have scattered small icy objects into the outer solar system. While some would be given enough energy to completely escape the solar system, others would be scattered out to the hypothetical Oort Cloud.

Third, we've seen Kuiper Belts around other star systems, and it's likely that the Oort Cloud is a continuation of the Kuiper Belt, so this may be evidence for Oort Clouds as well.

So if we need a source for long-period comets and the orbits work out to this cloud beyond the Kuiper Belt, dynamical models predict that the bodies would exist there, and we see similar dynamical structures around other stars, then that's fairly compelling evidence it exists.

But, you are correct that, at present, it is not technologically possible to view comets that are members of the Oort Cloud that are still in the Oort Cloud. Viewing a chunk of ice 1/4 of the way to the nearest star is simply not possible ... yet.

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"Third, we've seen Oort Clouds around other star systems." seams to be a good way to accept the theory. Do you have a reference of these observation thanks. –  Emmanuel Devaux Nov 30 '11 at 21:26
    
Alright, looks like I was remembering wrong (universetoday.com/11283/…). It's Kuiper Belt disks we see around other stars. I've revised my answer. –  Stuart Robbins Dec 1 '11 at 4:41

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