Ask A Scientist© Astronomy Archive

Question:  What do astronomers use to figure out if it is a star/planet or just an
asteroid or comet?
vinod n thanki 

Answer:  If you just look at an object in the sky once, it could be
hard to tell anything much about them (although astronomers
do have techniques called "spectroscopy" that tell something
about what an object is made of). But if you watch the object
for a while, you can see where it is going, trace out its orbit,
see what influence it has on its neighbors, etc, which tells
you a lot about it: stars hardly move at all; planets move in
pretty ordinary close-to-circular orbits about the sun, while
other things can be seen to orbit around them (which then tells
us how massive those planets are) and asteroids tend to be
clustered in the asteroid belt or else to have kind of elongated
orbits, and comets have really wild orbits, and in particular
tend to get close to the sun where they flare up and get bright
every once in a while.
asmith