First Direct Detection of an Exoplanet using Interferometry!

For the first time, astronomers observed an extra-solar planet named HR 8799e using the optical interferometry method. This technical achievement was realized from the European Paranal Observatory in Chile with the four 8-meter VLT telescopes (Very Large Telescope).

Artist's impression of the transit of an exoplanet in front of his star. Credit: ESO / L. Calçada

★ A technological feat

On March 27, 2019, a team of dozens of astronomers from ESO announce this historic result, which has been waiting for several years. By exploiting the data of the star HR 8799 acquired on August 28, 2018 with the GRAVITY instrument, they managed to detect the presence of an exoplanet and to record its light spectrum, thanks to a very recent optical technique called interferometry. This one aims to push back the limits of resolution and contrast of the current telescopes by combining their light simultaneously.

Thanks to this complex method, astronomers have been able to collect the light of this exoplanet located only 0.390 arc seconds from its star, by analogy, it is equivalent to distinguish your smartphone from a distance of 80 km!

Before explaining in more detail the principle of this method, it is important to emphasize that scientists already knew the existence of this planet, whose discovery had been made in 2010. Moreover, the star in question, located 130 light-years from Earth, has at least 3 additional exoplanets. Which had been detected a few years earlier, in 2008.

★ Interferometry: increase telescope resolution by simulating a large virtual mirror

Optical interferometry consists of combining the coherent light of several telescopes observing the same object of the sky. This superposition of light no longer produces a conventional image but an interference image composed of alternating light and dark fringes.

Principle of Interferometry Credit : ESO

The interest of this complex method lies in the fact that the resolving power is much higher than if a single telescope had been used. In the specific case of the exoplanet HR 8799, the astronomers used the 4 telescopes of the VLT with an individual diameter of 8.2 meters, which means that the resolution obtained in interferometry is equivalent to the resolution that a telescope would have. a diameter equal to the largest distance between the most distant telescopes, in this case 130 meters!

This gigantic gain of interferometry makes it possible to separate two objects from the sky, which are usually unobservable with a single telescope.

★ The interferometric data reveal the properties of the exoplanet HR8799 e

In particular, astronomers were able to extract from their interferometric observations valuable data on the characteristics of the exoplanet HR8799 e.

Thus, its radius would correspond to about 1.17 times that of the planet Jupiter (81,795 km) and its mass would be estimated between 6 and 17 times that of Jupiter.

Spectrum of the planet HR8799 obtained with the GRAVITY instrument of the VLT. Source : https://www.aanda.org/articles/aa/full_html/2019/03/aa35253-19/F3.html

The spectrum obtained with the GRAVITY instrument made it possible to deduce a surface temperature of the planet of 1200 degrees Kelvin as well as a composition of its atmosphere rich in carbon monoxide. All of his data allow us to confirm the model of a "brown dwarf" or "super-Jupiter" exoplanet, similar to the planet Jupiter of our own solar system.

★ New discoveries to come ...

Although the planet HR 8799e is far from being an exoplanet sheltering life, it allows for the first time in the world to demonstrate the incredible potential of interferometry in astronomy applied to the search for extrasolar planets. It confirms that the theoretical promises of this recent technique are experimentally verified and opens a new path towards detection methods of potentially habitable worlds outside our solar system!

Note: The angular separation between the planet HR8799 e and its star was 390 milliseconds of arc, whereas in theory, the interferometry would make it possible to reach separations of 100 mas, that is to say to detect planets closer to their star. It is not excluded that it will one day lead to the discovery of rocky planets ...