Have astronomers ever detected any objects that were passing through our solar system, or “nearby”?
What I mean is, by observing the position and velocity of an object, one can determine whether or not it is in a “closed” orbit. An object with sufficient velocity could be seen approaching, passing by, and leaving our solar neighborhood, never to return. I’m thinking of planet-sized objects or less.
Would there be a name for such an object? Asteroid, planet or comet seem to imply by definition that the object is in a closed orbit. Would there be any way to determine if some good-sized object flew through our system in the distant past? Would there be any reliable way of searching for such a thing?
I got this idea from seeing those pictures of galaxies colliding. That is, they pass through each other, scattering stars all over the place. If two star-planet systems passed through each other like that, you might imagine the planets could get scattered away from their stars. If such a “rogue planet” ever headed our way, it seems like it could mess things up for us.
If you observe the position of an object for a few days (or possibly few weeks) you can calculate the orbit and see if the orbit is elliptical (open) or hyperbolic (closed). There have been some comets with hyperbolic orbits, but that does not necessarily mean they came from outside the solar system. It could have originated in our solar system but somehow thrown into a hyperbolic orbit, possibly by gravitational interaction with another comet or planet. I don’t think you can tell for sure that something came from outside the solar system.
Many comets have been observed to follow parabolic or hyperbolic trajectories in which, once they round they sun, they are ejected from the solar system, never to return (See Figure 1).
While many comets end up with an escape trajectory as a result of the gravitational influence of the planets, there still doesn’t seem to be a proven example of a comet that initially came into the solar system on such a path. This dearth probably reflects more the difficulty of tracking such small, dim objects out past the orbit of Jupiter, than a real absence of truly intestellar visitors.
See also Ask the Space Scientist on hyperbolic comets.