# BEST - Background

Since the first detection of an exoplanet around the star 51 Pegasi in 1995, more than 700 exoplanets have since been discovered around solar-type stars. Most were found by the radial velocity method. Only a limited number of planets have been detected by the method of photometric transits. This method is able to detect extrasolar planets when an orbiting planet is dimming the light of the central star. The dimming of the stellar light is proportional to the ratio of the planetary to the stellar radius:

For Jupiter-sized planets, the reduction of the light level during transits is of the order of 1%. Intensity variations of this order can be observed by relative photometry in ground-based observations. Indeed, the transit of the planet orbiting the star HD 209458 (which had previously been detected by radial velocity measurements) was observed successfully several times by different teams. The detection of smaller planets, however, requires improved photometric accuracy, e.g. a transiting Earth-like planet decreases the intensity of a Sun-like star by 0,01%. This will be detectable just for space based observations.

Thus, knowing the parameters stellar radius and the stellar mass, allows us to determine the planetary radius and the orbital distance a with help of the Kepler-law:

With the transit duration, orbital period, stellar radius, planetary radius and the orbital distance it is easy to calculate the angular distance from center k:

Knowing the distance from center, the stellar radius and the duration of ingress/egress we can get the inclination:

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