Imaging

The first image of an exoplanet was taken by the team of French astronomer Gaël Chauvin in 2004, using the NACO instrument of the Very Large Telescope (VLT) at the European Southern Observatory (ESO) in Chile. This first image shows a young gas giant planet orbiting brown dwarf ?  2M1207.

The first image of an exoplanet: 2M1207 b
(Chauvin et al. 2004)

To be detected by imaging, a planet must be bright and far from the star. Unlike indirect methods, imaging does not require watching for several rotation periods ? of the planet. However, to confirm detection via direct imaging, it is necessary to verify that the light source is linked to the star. We therefore take at least two images several months apart (according to the amplitude of the star’s own motion) to see whether the movement of the planet indeed corresponds to an object in a Keplerian orbit.

The planet/star flux ratio is extremely weak. As, when seen by the observer, the planet is also very close to its star, the observer is “dazzled” by the star and has difficulty seeing the planet.

The solution to this drawback is to significantly lessen the flux from the star without reducing that from the planet. There are two techniques for doing this: the coronagraph and nulling interferometry of the star.

  • Coronagraph:
    Simply speaking, this technique consists in hiding the star by means of a coronagraph mask in the plane of the image (without hiding the planet).
  • Nulling interferometry:
    The flux from a star can also be reduced in an interferometer ? consisting of at least two mirrors. The star flux from one of the mirrors can be made to “negatively” interfere with that from another mirror. This destructive interference annuls the star. The interferometer ? ’s configuration can be arranged so that the flux from the planet is not annulled. As the light rays from the planet come from a direction slightly different from that of the star, the path travelled by these rays is not the same.

This technology works well but has not yet reached the maturity needed to detect planets like Jupiter “close” to their stars. Active research is under way to be able to get closer. This will notably be possible with the ELT in about 2030. The JWST cannot reach lower inner angles but will be able to detect planets less massive than Jupiter which is the current limit…

Steaua tânără GQ Lupi împreună cu exoplaneta sa, detectată prin imagine directă.
(ESO/VLT)

This technique has so far 15/10/2026 led to the discovery of

objects.

Advantages:

  • Can obtain the spectrum of the planet, which gives a measurement of its temperature and its composition.
  • Can be used to study planets far from a star.

Drawbacks:

  • Gives no information on the mass and radius of the planet.
  • Is not able to study planets close to their star.