Since August 2019, we knew of its existence, the period of its orbit, and even its mass, but it was still escaping our gaze.
The presence of β Pictoris c had been betrayed after more than 10 years of observations of its eponymous star thanks to an indirect method called radial velocities: the small displacements of the star indeed show that a giant planet weighing the equivalent of nine Jupiter is very close to it, at only 2.7 astronomical units [1]. So close that it had not yet been possible to observe it directly, since classical telescopes were unable to distinguish it from the radiation of its star.
But using the GRAVITY interferometer, whose accuracy is 100 times higher than that of conventional telescopes, the international research team [2]2 was able to capture for the first time the light emitted by β Pictoris c.
This feat would not have been possible without the very precise calculations of the position of the exoplanet around its star. Indeed, GRAVITY is very sensitive, but its field of view is very small. It was thus necessary that the estimation of the position of β Pictoris c be precise to be able to point the telescopes at the right place.
This is the first time that a direct observation of an exoplanet is based on data provided by the radial velocity method.
Scientists have therefore been able to combine the information offered by these two techniques: one gives mass and the other luminosity. These data are of primary interest because the exact relationship between the mass and luminosity of planets is linked to the mechanisms of their formation.
Thus, the research team was able to confirm that the young planet β Pictoris c is cooling down and still expelling part of the thermal energy accumulated during its formation.
These observations obtained on a young planet will allow a better understanding of how giant planets are formed. The scientists also hope to be able to answer a new question raised by their study: why is β Pictoris c six times less luminous than its sister planet β Pictoris b, when their masses are very close?
Bibliographie
Unveiling the beta Pictoris system, coupling high contrast imaging, interferometric, and radial velocity data. A-M Lagrange et al. Astronomy & Astrophysics, le 2 octobre 2020. DOI:10.1051/0004-6361/202038823
Direct confirmation of the radial-velocity planet β Pic c. M. Nowak et al. Astronomy & Astrophysics, le 2 octobre 2020. DOI:10.1051/0004-6361/202039039
[1] One astronomical unit is equivalent to the distance Earth-Sun, about 150 million kilometers. While its big sister β Pictoris b is seen evolving at the same distance as Saturn around the Sun, β Pictoris c evolves on an orbit equivalent to that of the asteroid belt in the solar system, between Mars and Jupiter
[2] In France, researchers from the Laboratoire d’études spatiales et d’instrumentation en astrophysique (Observatoire de Paris - PSL/CNRS/Sorbonne Université/Université de Paris), the Institut de planétologie et d’astrophysique de Grenoble (CNRS/Université Grenoble Alpes) participated in this work, the Astrophysics Laboratory of Marseille (CNRS/CNES/Aix-Marseille University), the Astrophysics Research Center of Lyon (CNRS/ENS de Lyon/Université Claude Bernard Lyon 1), the laboratory Galaxies, étoiles, physique, instrumentation (Observatoire de Paris - PSL/CNRS), of the Institut de mécanique céleste et de calcul des éphémérides (Observatoire de Paris - PSL/CNRS/Sorbonne Université/Université de Lille) and of the Laboratoire franco-chilien d’astronomie (CNRS/Universidad de Concepcion/Pontificia universidad catolica de Chile/Universidad de Chile).
