A planetary transit is an astronomical event in which a planet passes directly between a star and an observer such that the planet appears to pass in front of the star. As the planet crosses in front of a star it blocks some of the star’s light, which makes the star appear to dim – which we see as a dip in the light curve.
Light curves are plotted with time on the x axis and relative brightness on the y axis.
There are 3 important characteristics of transit light curves:
Different sized planets will block different amounts of light as they transit, which is reflected in the depth of the dip in the light curve. In the NASA animation below we can see how two different planets transiting the same star create different light curves and that the larger planet’s light curve has a deeper transit.
The transit depth alone is not enough to determine the size of a planet. We also need the size of the star which the planet is transiting. The image below compares the normalized light curves of two different sized stars transited by planets of the same size. Note how the smaller star has a much large dip.
Specifically, the transit depth is equal to the ratio of the planet's radius (Rp) and the star's radius (Rs) squared.
Note that the surface temperature of the star does not affect the eclipse depth as with a hotter star the planet will block more light, but there is more total light produced. So the eclipse depth remains the same — depending only upon the square of the ratio of areas.