# Finding an Extrasolar Planet's Radius

by crocvox
Tags: extrasolar planets
 P: 701 When a planet transits in front of the stellar disc, it obscures a part of it, which we observe as a reduction in brightness. That reduction is directly proportional to the relative areas of the planetary and stellar discs. So let's say you observe an (improbable) 50% reduction in brightness during transit. It'd mean the area of the obscuring disc is 50% of the star's. Once you've got the area of a circle it's trivial to get the radius. So, $$\frac{L_o}{L}=\frac{A_s-A_p}{A_s}$$ where L is the star's luminosity Lo is the star's luminosity when obscured As is the stellar disc area Ap is the planetary disc area Of course, you need to know the As. For main sequence stars it can be inferred from the luminosity, as these stars obey luminosity-mass and mass-radius relations as explained here: http://www2.astro.psu.edu/users/rbc/a534/lec18.pdf. Otherwise a measurement is in order.