Extrasolar transit. Animation of an extrasolar Jupiter_like gas giant planet transiting its parent star. A transit occurs when an observer sees an astronomical object pass in front of another one. This method is used to detect planets orbiting other stars, as the light received from the star decreases as the planet passes across it. The variation in the light allows calculation of the size of the planet and even the composition of its atmosphere. Starspots the equivalent of sunspots on the Sun are the dark areas on the surface of the star.