Mass of the exoplanet Beta Pictoris determined by astrometric measurements

The mass of the young exoplanet discovered in 2008 around Beta Pictoris has just been measured quite precisely thanks to careful observation of the motions of Beta Pictoris by the Gaia telescope and its distant predecessor Hipparcos.

Ignas Snellen and Anthony Brown (Leiden University) had the idea of ​​using astrometric measurements (position measurements) of the star Beta Pictoris to infer the mass of its exoplanet discovered ten years ago. The star and its planet are more or less the same age, they are very young, only twenty million years old. Beta Pictoris, which is located at 19.44 pc (63.4 y), became famous on the one hand for its vast protoplanetary disk, and on the other for the observation direct of one of these planets, beta Pic b. The latter is a giant planet of the Jupiter type. But the study of this planet is made difficult with classical methods because of the high temperature of the star, its large variations in brightness and the presence of the dust disk. For example, measuring the variations in the radial velocity of beta Pic (its speed along the line of sight) is very complicated. Measuring the radial velocity and its periodic variations are commonly used to estimate the mass of exoplanets that slightly modify the motion of their star.

Snellen and Brown then turned to very long-term position measurements to detect periodic variations that could indicate the characteristics of the planet. Nothing to do with astrometric measurements Gaia offered to the astronomical community last spring. With these data, astronomers were able to trace the evolution of the different contributions to the motion of beta Pic: its motion (induced by its rotation in the galaxy), its parallax (induced by the motion of the Earth around the Sun), and finally its residual oscillatory motion which is nothing other than the effect produced by the planet and which corresponds to variations in radial velocity seen on another plane.

Astronomers calculate what the observed trajectory would be if there were no planet, then compare it to the trajectory and can then infer the planet’s mass. But to get there, researchers have to follow the star’s trajectory for a very long time, typically several years. During the 22 months of Gaia data, Beta Pictoris was measured thirty times. That was positive, but still not good enough for Snellen and Brown, who are now publishing their work in Astronomy of nature.

The astronomers then delved into the telescope’s archives Hipparchus which had performed the same type of measurements as Gaia in the early 1990s: between 1990 and 1993, the European telescope had measured beta Pictoris 111 times, providing valuable positional data to be combined with the positions provided by Gaia.

According to Snellen and Brown the mass of the planet beta Pic b is therefore 11±2 times the mass of Jupiter. It is not only very young, but also very massive.

This is the first time that the mass of a very young planet has been determined using astrometric measurements, and the contribution of measurements older than 25 years combined with recent measurements has been decisive.

Of course, this is just the beginning of a long sequence of studies of exoplanets using Gaia’s astrometric measurements. Specialists estimate that the data from the European telescope could allow us to estimate the masses of several hundred very young exoplanets and thus improve our understanding of their formation.

Source

The mass of the young planet Beta Pictoris b through the astrometric motion of its host star

Ignas Snellen and Anthony Brown

Astronomy of Nature (August 20, 2018)

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