Terme de Glossaire

The observation of the transit of a planet by photometry (mesurement of the light intensity) aims to detect the presence of the planet  in question by the variation in the luminous flux of its star when it passes between the star and the observer. Telescopes that operate by means of photometry do not cover the entire light spectrum, but rather cover certain precise wavelengths (visible, near infrared, etc.), which make it possible to give different indications on the planet. According to the wavelength, photometry can give indications about the presence and composition of an atmosphere, for example (its gas or metal content or whether it is hazy or not).

This technique is therefore ideal to find new planets, or study smaller bodies such as asteroids or comets. In both cases, it is possible to establish a good estimate of their size.

The more a planet is observed during its transits at different wavelengths, the more its luminous spectrum will be precisely known, and the more researchers will be able to learn about it. The MOST satellite, for example, observes the variation in the luminous flux at wavelengths of around  0.5 microns, like the human eye. The Hubble satellite makes it possible to record the spectrum between 1 and 1.6 microns, in the infrared, while Spitzer make possible a photometric observation at 3.6 and 4.5 microns, still in the infrared.

Photometry simply takes into account the light intensity of a star and should not be confused with spectroscopy, which separates light into different wavelengths. Depending on the absorption or the emission of light by the material, it enables us to work out the chemical composition of the object observed.