[X] My Assistant

[Steffen]

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

[Guest_BruceMoomaw_*]

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.