Extrasolar planet detection. Diagram showing the principle of detecting an extrasolar planet by the planet's transit of its star. A transit is the passage of a smaller celestial body in front of a larger one. Here, the planet is too close to the star to be seen, but its presence may be inferred by the regular (periodic) dimming of the star as the planet passes in front of it (left). This is known as the photometric transit method, and only works if the planet passes directly between its star and Earth. Although the probability of this alignment is below 1 percent, several extrasolar planets orbiting other stars have been detected by this method.