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Direct imaging uses infrared wavelengths to observe planets. This works because at infrared wavelengths a star like the Sun is only 100 times brighter than Jupiter, compared to a billion (109) times brighter at visual wavelengths. This method works for planets that are very far from their stars, so an orbit might take hundreds or thousands of years for a planet discovered by this method. Astronomers have to verify that the planet and the star move together through space to prove that the planet orbits the star since observing an entire orbit will take so long. This method does not allow astronomers to measure the mass of a planet directly, but they can use the spectrum and brightness to get information about its surface temperature and diameter. At least 29 planets have been discovered by this method (as of 7 December 2011).