How Is The Density Of An Exoplanet Calculated?

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Welcome, my name is Phil, and in this video I explain how the density of some exoplanets can be calculated.
The calculate the density of an exoplanet we first need to find the volume and then the mass of the exoplanet. We can find the volume by assuming the planet is spherical and by determining the radius of the planet. The radius of the planet can be found by measuring the dip in brightness of the star as a planet passes in front of the star. Here, the dip in brightness relates to the ratio of the area of the star and the planet. For example, large planets passing in front of a small star will cause a large drop in brightness of the star. On the other hand a small planet orbiting a large star will hardly change the stars brightness, which is why planets like Earth are hard to detect.
The radial velocity method can be used to calculate the mass. Here, the wobble of the the star can be measured using the doppler shift in light of the star. Assuming the orbit is circular, Kepler's 3rd law can then be used to find the semi-major axis and orbital velocity of the exoplanet, which can then be used to find the mass.

[@moizkhokhar815]

why is it divide and not minus though
shouldn't it be that you subtract the area of the planet from the star

[@AstroPhil2000]

Do you mean to get the radius? It is the ratio of the area of the planet and star, which relates to the amount of light blocked out from the star. The flux, or brightness of the star, is always normalized so that it is at 1 before the transit. During the transit the value it drops by will be a percentage drop in brightness.