Another newbie question:
What's exactly the difference between the Kuiper belt & the Oort cloud ?

Another non-expert reply:

The Kuiper belt is a very sparse ring of "objects" orbiting the sun, just like the planets, starting at the distance of Pluto (some consider Pluto the nearest Kuiper belt object) and extending very far out from there. Not a lot is known about the "objects". there are some the size of Puto or (a little) larger and many smaller. They are probably mostly icy. I don't know how much rocky material is thought to exist out there. But they are very far appart from each other. The belt can not be seen as the rings of Saturn are. An astronomer is lucky to see one Kuiper belt object (KBO) at a time with his telescope.

The Oort cloud is farther out than the Kuiper belt and surrounds the entire solar system. it is in the shape of a sphere, rather than a ring, or belt. No objects in the Oort cloud have been seen. They are too far away. The "cloud" is thought to be there because many comets approaching the sun seem to come from that region of space, so there is thought to be a large population of comet-like objects out there.

A few more notes:

(1) The Kuiper Belt is a damned important newly discovered feature in the Solar System -- it's thought to contain about 100 times the total mass of material in the Main Asteroid Belt! (It's no wonder that a poll of planetary scientists ranked its discovery, along with the detection of the first extrasolar planets, as by far the most important planetary discoveries of the 1990s.)

(2) Some calculations indicate that around 3-6% of the objects in it may have been ejected from the inner Solar System, and thus be rocky -- but the overwhelming majority of them are primarily icy (although, like Pluto, many of them have a large minority fraction of rock mixed in).

(3) By a strange paradox, the comets in the Oort Cloud -- a staggeringly greater distance from the Sun than the Kuiper Belt -- are though to have formed in the vicinity of Jupiter and Saturn. They were flung into that vastly distant realm by close flybys of those two giant planets (whereas huge numbers of other comets in the vicinity either crashed into and added to the giant planets, got flung into the inner Solar System and eventually crashed into the inner planets or the Sun itself, or got flung totally and permanently away from the Sun's graviational influence. The Oort Cloud comets -- TRILLIONS of km from the Sun -- then, over the eons, tended to have their orbits somewhat circularized, and spread out into a spherical cloud around the Sun, by the gravitational tuggings of occasional passing starts, since they are actually in interstellar space. Some of them, no doubt, were tugged completely away from the Sun by such encounters, and some of the Oort Cloud's current denizens in turn are no doubt immigrants from the Oort Clouds of other stars. At other times, such stellar perturbations make Oort Cloud comets return to the Solar Sysyem proper. (The recently discovered "Sedna" is of interest because, judging from its strange orbit, it is by far the biggest Oort Cloud object yet identified, and there's a chance it may even have come from another stellar system. )

By contrast, Kuiper Belt objects are pretty much though to have originally formed at their greater distance from the Sun, out beyond the orbit of Neptune -- although they may have formed closer to the Sun at a time when Neptune itself was closer, and then been "herded" farther out to their present distance as Neptune itself slowly spiralled out from the Sun. But they -- uinlike the Oort comets -- pretty definitely represent samples of the material from the farthest outskirts of the original solar nebula out of which the Solar System formed. For this reason, scientists are greatly interested in learning more about the compositional differences of Oort and Kuiper comets.

Bruce, from what I've read, Sedna is neither a Kuiper Belt object nor an Oort Cloud object, which is part of its fascination. It orbits between about 76 and 902 AU, which puts its perihelion beyond the Kuiper Belt, and its aphelion well short of the Oort Cloud. Maybe I'm just not up on my Sedna news. Is there a consensus on how it got into its odd orbit?

<reposting something badly placed in an enceladus thread>
My arm waving understanding is that any star approaches within closer than 2 light years is very rare or statistically less than 50% chance over the age of the solar system. I'm not sure on the number but I think it's something rather surprisingly large.

And Oort cloud comets have typical aphelions of a light month or so. Part of the problem is larger orbits interact with the radial tides and vertical gravity field of the galaxy and progressively get peturbed and are eventually lost. That was an essential arguement against the proposed Nemesis sub-stellar object that hypothetically caused periodic extinctions... Such an object in an orbit with a ?27? (is that the number) million year period would last a few hundred million years and be lost to interstellar space.

Sedna... it was being discussed a bit ago in some forum... may not be an Oort cloud object.. it's in way too short period an orbit for an inner-Oort cloud object (as predicted.. NONE are known yet) yet has a perihelion way too far out for solar system planetary peturbations to have pushed it out that far. That's a big reason they are looking at late or just post nebular phase close stellar encounters from another star in the cluster or association the sun formed in.... far more likely than any encounter over the age of the solar system with a random "field" star.

Is there really a KUIPER-belt ?
The 'hypothetical' ring-shaped reservoir of icy rocks is believed to exist beyond the orbit of SEDNA ( Kuiper Belt Object ?) but I believe we don't have photo of the belt ( Hubble ? )... It's probably very difficult to take a good photo as this is the case with the asteroid belt between Mars & Jupiter ( any good photos of that one ?)
Philip

The density of matter within things such as the Asteroid belt, Kuiper belt or Oort cloud is so very very low that you can't see the 'belt' or 'cloud' - only discreet members of it.

Doug

Doug:

Almost right; you can also see all of them at once, in the form of the Gegenschein and Zodiacal Light:

http://www.as.wvu.edu/~jel/skywatch/skw9810h.html

Of course, you're *not* seeing the discrete objects themselves, just the gazillions of little reflections of sunlight, which after a while *do* add up to a faint glow.

Bob Shaw

Actually, as I clicked post I thought "oops - forgot about the whole zodiacal light effect" - but having never seen it, I have no idea how spectacular or easy to see it is

Doug

Doug:

Nor have I; allegedly, the tropics are the best location for visibility, presumably because everything in the plane of the ecliptic gets higher in the sky than for us in the far north (or deep south).

Bob Shaw

Gegenschein... That must be an amazing sight ?

It's really good density is too low so our UMSF can cross the belt