Planets, such as the ones in our Solar System tend to emit very little light of their own, particularly at optical wavelengths. Depending on their albedo they reflect varying degrees of light from their parent star. As you might imagine, they are extremely faint when compared with their star, making a direct detection of one similar to an analogy of detecting a lit match held by someone in the middle of a floodlit sports stadium.
Direct detection should in principle become easier, the further away an exoplanet is from its star, but unfortunately, the greater the distance, the less light the object receives and re-emits.
Sometimes though, it’s possible to detect a distant planet from its thermal emission. Using our knowledge of black-bodies and Wien’s Law, we can determine that a planet which has a temperature of 300 kelvin (such as our Earth) emits most of its light in the infra-red part of the spectrum (see Figure 1).
For nearby star systems, it is then possible to use a coronagraph; a circular plate that blocks light in a telescope to reveal faint objects around the brighter star. Not all of these objects are confirmed exoplanets; the existence of a group of objects, known as brown dwarfs, confuses the picture, since these objects are also of low-mass, faint and detected most easily in the infra-red.
While direct imaging can be used to confirm exoplanets detected initially by other methods, a list of objects that have been discovered by this method is here.
Go back to the main detection methods page.