Radius of planet from Transits

[Jhero]

Hi, is there a way to find the radius of a planet by only knowing that the maximum depth in brightness of a transiting planet over a star is 0.7% ?

 

[russ_watters]

Assuming you are talking about exoplanets, you could, if you know the size and brightness of the star (which you probably do). As the distance from Earth to the star is much larger than from the planet to the star, the distance between the planet and star does not affect the reduction in brightness.

 

[natski]

This is basically just working out the fractional projected area of the planet relative to the star. So the dip in brightness should be something like (Rp/Rs)^2 where Rp is the planet radius and Rs is the star's radius. The absolute britghness of the star is not necessary if you know the dip is 0.7% but you will need to know it's radius and this will probably come from brightness measurements anyway!