Starspots can occupy the same area as an exoplanet (or larger or smaller) and dim starlight as they rotate in and out of view. However, a number of things distinguish them from a planet transit. Spots do not have sharp edges or circular shapes, but they do appear with each stellar rotation and have a period that can mimic a planet. Spots, however, move in both longitude and latitude as the star rotates, causing their period to change slightly after a few rotations. Stars with spots also tend to have larger amplitude variations in their light, which are usually greater than a planet transit depth and lead to a sine-like light curve.
Stellar companions differ from exoplanet transits in a number of ways as well. Planets do not shine in the optical-light wavelengths, so they cross the star as black disks. Binary companions are self-luminous and cause not only a different shape of eclipse (V compared to a U shape) but also a color change in the light. This happens because they block some light and emit some as well, changing the total light color we see during their eclipse.
Exoplanet transits are usually U-shaped due to both their blackness and the host stars’ limb darkening — where the center of the star is brighter than the edge. They also keep the same period in most cases (exceptions are exoplanets in highly elliptical orbits). We measure the planet’s size by the depth of its transit, given that the star’s radius is known.
Thus, while it is often the case that astronomers can distinguish a planet transit from starspots or eclipsing binary companions, it is not perfect. Smaller planets, more variable stars, and less than optimal light curve data can be confusing and fool us at times.
NASA Ames Research Center
Moffett Field, California