Discovering exoplanets..................................

one of the methods for discovering exoplanets is to look for the dimming of stars as planets orbiting around stars block a portion of the light we can see. So how do we know when its an actual planet orbiting the star that's blocking a portion of the light and not merly something in Hubble's field of vision that is in between the observed stars and the telescope

The signature of an orbiting planet would be very recognisable. For a start, it would be regular,as the planet passes repeatedly in front of its star. That means that the planets we are most likely to identify are very short-period so they pass in front of their star frequently. The light curve would also be pretty clear. The amount of light blocked would initially rise quite quickly, as the planet moved across the stellar disk from the dimmer edge, where the star's atmosphere would absorb quite a lot of light, but the stabilise as the planet moved more towards the centre of the stellar disk. That sort of light curve indicates something getting in the way, and regular repetition indicates something in orbit roound the star.

Because the dimming happens cyclically (so the observations are repeated several times) with a characteristic curve.

http://1.bp.blogspot.com/_FgKwPBKWZpo/TB…

They don't use Hubble for this work - they use Kepler.