What exactly orbits IN the Oort cloud?
Comets. The thing about the Oort cloud is this: when a comet is seen for the first time, it is almost always on an orbit that is very nearly parabolic. That is odd, because if it came from anywhere near the inner solar system, its orbit would be elliptical; if it came from interstellar space, its orbit would be hyperbolic. But instead, it’s precisely balanced just in between elliptical and hyperbolic, meaning that it comes from very, very far out, but its velocity out there is quite exactly matched to the velocity of the Sun. Hence the Oort cloud—a cloud of comets that are way, way out there, but still part of the Solar System, and still travelling around the Galaxy with the Sun, ever since the Sun was formed 4.7 billion years ago.
