PHOTOGRAPHING EXTRASOLAR PLANETS

Amongst all the existing techniques for discovering exoplanets, the most immediate and understandable is to photograph them directly. In theory it is enough to go to a telescope powerful enough to solve the star from the planet and photograph them. But this is not so simple.

In reality, the planets are several orders of magnitude less luminous than the star and, even with the most powerful telescopes, you are not able to observe them. 

In recent years, thanks to the Hubble Space Telescope and the development of adaptive optics for terrestrial telescopes, more and more exoplanets have been photographed directly. To date there are over 300 objects observed with this technique.

The easiest exoplanets to observe are very large planets orbiting at a great distance from their star. The greater the distance, the easier it is to solve the planet from the star.

Observations are then generally performed in the infrared. This has two advantages: the star is less luminous in the infrared than in the visible, while the planets (especially the newly formed gas giants) emit residual radiation in these wavelengths, which increases the probability of identifying them.

For the moment, with current technologies, we are able to directly photograph only gas giants with a minimum mass similar to that of Jupiter and in orbits with an orbital radius greater than 10 AU. For the moment it is impossible to directly observe Earth-sized planets in orbit close to the star.

This 2020 image shows the two planets orbiting TYC8998-760-1. This is a 17-million-year-old star that has the same radius and mass as the Sun. The two planets indicated by the arrows are the first two exoplanets photographed directly in orbit around a star similar to the Sun.

Credit: ESO/Bohn et al.

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