Is the Kuiper Belt a disc or a sphere? If an object is over one of the Sun’s poles, but farther away than Neptune, can it be said to be in the Kuiper Belt?
A disk. More of a donut to the average eye, actually, but certainly not a sphere. Pretty much all stellar systems end up being disc-like rather than spherical. The reason comes from the earliest beginnings of the star as a cloud of gas.
When the cloud of gas collapses and crushes down under gravity to form a star, it has some residual spin about an axis. Stuff spinning in the plane of that rotation will collapse more slowly - it is being flung outwards in a way, kept in orbit by its angular momentum. But in the axis perpendicular to that plane, it has no momentum, so gravity just tugs it down until pretty much everything is in a common plane - a disk of matter usually called an accretion disk. Eventually, enough matter collects in the center of the disk for ignition to occur and the star turns on, leaving theprotoplanetary disk of gas and dust that, over millions of years, turns into planets and asteroids and moons in the huge mess we see today.
This process occurs, we believe, for all stars. The answer to your question in some sense had to be “a disk.”
Nope. Such an object would be extremely unusual if it were relatively close to the sun. If you’ve found one, you should probably write to the Astrophysical Journal.
Oh, I haven’t, I just wondered what would happen if something drifted into that vicinity.
Thanks for that. I’ve read a couple explanations of why solar systems form into discs, but that’s the first time I’ve understood it.
On the other hand, the Oort cloud is thought to be roughly spherical. The reason being, the comet nuclei originally formed much closer to the sun, and were ejected from the inner solar system by close encounters with the giant planets. A comet nucleus could be put into a polar orbit by passing under or above a giant planet relative to the plane of the ecliptic.