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Stars are many times brighter than their planets. Any light from a planet orbiting a distant star will be drowned out in the overwhelming glare of the star, making direct observation of an extrasolar planet extremely difficult. It is necessary to block out as much of the light from the star as possible so that the faint light from the planet may be detected. There are two main ways to exclude light from the star.
- Nulling interferometry. Traditional interferometry is used in astronomy to combine images produced by two telescopes to provide a greater resolution than only one telescope could obtain. It works on the principle of constructive interference. Nulling interferometry works instead on the principle of destructive interference to cancel out the light obtained directly from a star by two or more telescopes and hence increase the visibility of a planet which may otherwise be drowned out in the glare of its parent star.
- Coronagraphs are commonly used to observe the corona of our Sun by simply placing a disc over the image of the Sun so that objects close to the Sun may be observed in the absence of solar glare. ( This same principle may be extended, using more advanced coronagraphs, to blocking out the light from a distant star to observe the light from a planet close to it.
The chance of directly imaging a planet may be improved by carefully choosing which star systems to observe. A dim star with a planet whose orbital radius is very large would be the easiest candidate for direct imaging. Very young stars, for example, are less bright than their main sequence counterparts and any newly-formed planets orbiting them may be hot enough to emit some light. Alternatively, planets orbiting (relatively dim) white dwarves may be good candidates, particularly as any planets orbiting a star which becomes a white dwarf would (assuming they survive the death of the star!) migrate out to an orbit with a larger radius as the star loses mass. These two factors would make a planet orbiting a young star more visible relative to its parent star.