An observed dip in the transit lightcurve does not automatically infer the detection of an exoplanet. There are three common types of false exoplanet candidates which astronomers encouver all the time.
This NASA/ESA Hubble Telescope near-infrared image of newborn binary stars (image center) reveals a long thin nebula pointing toward a faint companion object. Credit: Susan Terebey (Extrasolar Research Corp.), and NASA/ESA
Grazing binary systems
Usually binary stars cause big dips in the lightcurve. In some cases though the closer of the two binary stars grazes the limb of the other star (as seen from earth) and a small dip, similar to that of an exoplanet lightcruve, is observed. Some differences can be spotted though. For one, the lightcurve from grazing binary stars will have a distinct V shape whilst an exoplanet crossing the surface of the star will have more of a U shape with a flat central bottom caused my the whole exoplanet occulting the host star. Also, if the grazing eclipsing binary system has a circular orbit, two conjuctions will happen, each star alternating with one star infront of the other. Thus a full orbital lightcurve will contain two eclipses of different lightcurve depths. There is an excpetion however, and that is if the eclipsing binaries are main sequence stars of similar mass (i.e same radius and luminosity). To determine if it is in fact an exoplanet or not, the lightcurve shape has to be examined or radial velocity followup observations will have to be done.
Blended eclipsing binary systems
This is more of a problem with exoplanet surveys using small (< 1 m) telescopes where stars are more likely to not be spacially resolved. A shallow dip in the lightcurve resembling an exoplanet transit can occur when a deeply eclipsing binary star happens to lign up with another isolated star along the same line of sight. The isolated star acts to diminsh the effects of a big flux change which would have otherwise occured had the observations of the deep eclipsing binary not been along the line of site of the isolated star. For larger telescopes this is not so much of a problem as the stars are more likely to be spacially resolved with their light hitting different pixels on the detector.
Credit: NASA/Wendy Stenzel.
Brown dwarf and white dwarf stars
The transiting lightcurve depends only on the size of the transiting object. Brown dwarfs and white dwarfs have similar sizes to the giant gas planets and thus can be mistaken as planets despite the fact that the giant gas planets are much less massive. To get around this a radial velocity curve of the host star will have to be obtained in order to estimate the minimum mass. The potential planet is given said to be a exoplanet candidate until such an observation is done.
