Amongst the different detection techniques, direct imagining is the only technique which allows for the direct measurement of the exoplanet itself. Despite the basic principle behind directly imaging a planet, it is a notoriously difficult technique in practice. This is due to the the enormous luminosity contrast between the exoplanet and the host star and the small angular separation between the two bodies.
At optical wavelengths, where the black body curve of the host star peaks, the brightness contrast is on the order of ~109. Towards longer wavelengths the situation improves as the black body curve of the planet peaks, whilst that of the host star diminishes, resulting in a more favourable contrast ratio of ~106. For this reason, direct imaging observations are always done at near-IR to mid-IR wavelengths. The diffraction limit of a telescope is the minimum angular resolution before the the two objects observed can no longer be separated. The angular resolution, θ can be expressed as
where D is the diameter of the telescope and λ the wavelength at which the observations are done. To be able to directly image planets which orbit close to the host star, it is most favourable to use a telescope with a large mirror diameter and also observe at longer wavelengths.
The technique is most sensitive to high mass (), young () exoplanet systems on wide orbits ( AU) and therefore complements the other techniques (especially the transit method and RV method) which are more likely to detect planets which orbit close to their host star. Unlike the transit and phase curve measurements (described in the next section), which are the only other methods capable of detecting an exoplanet atmosphere, the direct imaging method allows for photometric and spectroscopic observations across a range of wavelengths to be obtained directly. The wide orbits means the exoplanet atmospheres are not subjected to strong irradiation from the host star (see thermal inversions LINK) nor influenced much by stellar activity (see LINK). Directly imaged planets therefore occupy a lot of the same parameter space as BDs allowing comparisons to be made (see section on the exoplanet-BD connection, LINK). An added benefit of wide orbits is that the observations are not time critical, a natural consequence of Kepler’s law.