What must an astronomer know accurately to calculate the radius of a transiting planet using transit depth data?
Answer
An accurate measurement of the star's radius
The depth of the light dimming observed during a transit—how far the light drops—is intrinsically linked to the ratio of the areas of the two bodies involved: the transiting planet and the host star. Specifically, the calculation of the planet's radius depends on this measured depth. However, since the depth represents a relative measurement, scientists must have an independent and accurate measurement of the star's own radius. Once the star's size is known, the depth data can be used to scale that relationship and determine the absolute size, or radius, of the orbiting planetary body.
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