Detecting an exoplanet

If a planet passes directly between a star and an observer's line of sight, it blocks out a tiny portion of the star's light, thus reducing its apparent brightness. Sensitive instruments can detect this periodic dip in brightness. From the period and depth of the transits, the orbit and size of the planetary companions can be calculated. Smaller planets will produce a smaller effect, and vice versa. A terrestrial planet in an Earth-like orbit, for example, would produce a very small dip in stellar brightness that would last just a few hours. This is one of several ways in which astronomers can find exoplanets.