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<-   Direct imaging   ->
images/gq_lupi.jpg
The young star GQ Lupi and its faint planetary companion.
Copyright : ESO/VLT

Direct detection consists of acquiring a partial or a full image of the planetary system, where the planet appears as a point. This is by far the richer and the most promising method. Indeed, it allows us to derive numerous properties from the exoplanets.

Reflected flux - Thermal flux

There are two different regimes in which we can receive the light from a planet :

  • The light from the star reflected by the planet
  • The thermal emission of the planet, heated by the star.

A crucial parameter is the flux ratio of planet/star.
Concerning the reflected flux, this ratio is (for all the wavelengths) : (A/4)*(R_(pl)/a)^2*phi(t) where A is the albedo (reflecting power) of the planet and phi(t) is a "phase factor", which indicates the relative portion of the planet's surface visible by the observer (as for the phases of the Moon or Venus). The albedo depends on the wavelength (depending on the type of planet and its physical conditions).

The thermal flux depends on the temperature of the planet. If we neglect the internal heat sources, this temperature is provided by the incident flux of the star that heats the planet. It is given by T_(pl)= racine(R_(et) /(2*a)) *(1-A)^slash(1;4) *Swhere S is a factor that characterizes the greenhouse effect. The latter measures the proportion of the planet's radiation absorbed by its atmosphere. Therefore, the thermal flux ratio planet/star depends a lot on the wavelength :
(R_(pl)/a)^2 *(1/(1-exp(-h*c/(lambda * k*T))))

We notice that, both in the thermal and the reflected regimes, the flux ratio of planet/star is extremely low. In addition, the planet is very close to its star, as seen by the observer, so that the observer is dazzled by the star to the detriment of the planet.

The answer to this disadvantage consists of strongly reducing the stellar flux without reducing that of the planet. There are two techniques for doing this : through coronography and through the interferometric extinction of the star.

Coronography

To simplify matters, this technique consists of hiding the star with a patch (coronographic mask) in the plane of the image (without hiding the planet).

Interferometric extinction

We can also lower the stellar flux using an interferometer made up of at least 2 mirrors. We make the stellar flux "negatively" interfere through one of the mirrors with the flux passing through the other mirror. This destructive interference turns off the star. We can arrange the configuration of the interferometer in order to keep the flux from the planet. Indeed, since the light rays from the planet come from a different direction than that of the star, the path followed by these rays is not the same.