The CoRoT satellite will serve two scientific programs for which it is necessary to :
- Observe the oscillations of a star
- Detect the transits of distant small planets
These two phenomena require the same observation techniques : which are the two primary techniques currently used to detect stellar oscillations and planetary transits :
The spectroscopic technique allows for the measure of the variations of relative velocity.
The spectroscopic technique can be used from the ground, but it only makes it possible to detect the oscillations of a small number of stars and to discover giant extrasolar planets.
The photometric technique allows for the measure of luminosity variations.
The photometric technique allows us to make these observations on a large number of stars. This is the technique used by CoRoT. It consists of counting the photons emitted by the star and received by the telescope, but it requires observation from space.
The planetary transits, like stellar oscillations, appear as variations from the star's luminosity or as variations from its relative velocity from the Earth, which have to be measured with a high accuracy.
It is also necessary to observe continously, and for a very long time, so we must avoid breaks due to clouds, and free ourselves from the alternation day and night, and the rotation of the Earth around the Sun. That's why we have to go into space.
Let us observe the opposite simulation :
The star shows a pulse, its radius varies (yellow disk) and the velocities on its surface are alternatively in our direction (blue) or in the opposite direction (red). This leads to periodic variations in its brightness (yellow line) and in the velocity of its atmosphere (iridescent line). This velocity is measured thanks to the Doppler effect, by the variation of the wavelength of the spectral lines emitted by the atmosphere (at the bottom on the right).