These new planetoids / Kyper Belt objects, how do we know that they’re actually in orbit around the sun and not simply passing through?
WAG: By knowing their position, speed, and direction of travel; they will know that they are in orbit.
The sketch of the tentative orbit that I saw showx it to be tilted considerably from the plane of the 8 planets, Pluto excepted. As I recall the plane of Pluto’s orbit is also quite far from the normal orbital plane.
Could they not be passing through and be being perturbed by the sun’s gravity?
Yes, anything that close to the sun would be ‘perturbed’ by the sun’s gravity. However. anything that is just passing through would need to have ‘escape velocity’ or it will be unable to continue on its course and will keep orbiting around the sun.
Once the position of any object in a known gravity well has been observed at three different times, its approximate course can be calculated, which would be enough to tell if its path is elliptical (in orbit, a permanent part of our solar system,) or hyperbolic (a traveller ‘passing through’)
Does that help? In any event, rest assured that those bright people in the white lab coats know pretty much what they’re doing.
To elaborate a little on what chrisk said, whether or not an object is actually orbiting the sun is a function of how much kinetic energy it has. Above a certain amount, it can escape the pull of the Sun; below that amount, it can’t. By observing the new planet (or any object) enough times, we can figure out how much kinetic energy it has, and thus whether or not it could ever escape the Solar System.
The only way we generally have to determine the kinetic energy is to first find its actual position and speed. To do this we can just plot its apparent position and lateral speed; by parallax, we can find the the actual position and plot a conic path through it; if it is an ellipse, it’s in a captured orbit, if it is a hyperbola or parabola, it’s in an escape orbit.
This sounds simple; in reality, it is very complicated, especially for objects that are at Pluto’s orbit and beyond, because their apparent motion is so small with respect to the Earth. (By apparent I mean correcting for the movement of the Earth; their relative motion, with respect to Earth-based observers is considerable but confusing, as Copernicus found out.) And of course, objects that are in a low energy orbit can be easily perturbed by massive bodies like Saturn and Jupiter, making their actual orbit somewhat more complicated than a simple ellipse.
Stranger
First off, they’re definitely in orbit. Even if they’re just swinging once past the Sun thereafter to be lost to the Great Beyond, that’s still a hyperbolic orbit. The real question is whether they’re in a closed orbit.
In principle, three observations of an object’s position in the sky are sufficient to determine the orbit entirely: That is to say, you can know exactly where an object is, at any time you care to name. In practice, there’s always some error in the measured position of an object, so it usually takes more observations. But given enough observations, you can determine it. This is even more true if you have some source of information other than just the direction in the sky, such as Doppler shifts or brightness. The math required in calculating the orbit from these observations is a bit tedious, but fairly straightforward.