Life in the Universe
How do astronomers find exoplanets?
One common method used to find exoplanets is the Doppler method, which uses the Doppler shift of light. As a planet orbits its star, it moves the center of gravity of the star system back and forth. By measuring that motion in the spectra of stars, it is possible to deduce whether or not a planet is orbiting that star, how massive it is, and what its orbital distance is.
Another method for finding exoplanets is called the transit method, which looks for the shadow of a planet moving across our line of sight to its star. When planets are found this way, astronomers can learn much more about the exoplanet than with only the Doppler method. Parameters including size, temperature, chemical composition, and atmospheric density are measurable.
Most astronomers feel that the best way to find exoplanets would be to take a direct image of them. Unfortunately, that method is very difficult with current astronomical instrumentation because the host stars outshine the planets by such a huge factor that it would be harder than finding a firefly in the beam of a searchlight. Scientists have worked diligently, though, to develop ways that allow us to compensate for the effects of such a huge contrast level. These efforts paid off when the first definitive images of exoplanets orbiting a star were successfully made in 2008, using infrared telescope technology.