Abstract
Context. With an orbital distance comparable to that of Saturn in the solar system, β Pictoris b is the closest (semi-major axis ≃9 au) exoplanet that has been imaged to orbit a star. Thus it offers unique opportunities for detailed studies of its orbital, physical, and atmospheric properties, and of disk-planet interactions. With the exception of the discovery observations in 2003 with NaCo at the Very Large Telescope (VLT), all following astrometric measurements relative to β Pictoris have been obtained in the southwestern part of the orbit, which severely limits the determination of the planet’s orbital parameters. Aims. We aimed at further constraining β Pictoris b orbital properties using more data, and, in particular, data taken in the northeastern part of the orbit. Methods. We used SPHERE at the VLT to precisely monitor the orbital motion of beta β Pictoris b since first light of the instrument in 2014. Results. We were able to monitor the planet until November 2016, when its angular separation became too small (125 mas, i.e., 1.6 au) and prevented further detection. We redetected β Pictoris b on the northeast side of the disk at a separation of 139 mas and a PA of 30° in September 2018. The planetary orbit is now well constrained. With a semi-major axis (sma) of a = 9.0 ± 0.5 au (1σ), it definitely excludes previously reported possible long orbital periods, and excludes β Pictoris b as the origin of photometric variations that took place in 1981. We also refine the eccentricity and inclination of the planet. From an instrumental point of view, these data demonstrate that it is possible to detect, if they exist, young massive Jupiters that orbit at less than 2 au from a star that is 20 pc away.
Highlights
With its imaged debris disk of dust, its falling, evaporating exocomets (Kiefer et al 2014, and references therein), and an imaged giant planet (Lagrange et al 2010), the ∼20 Myr old β Pictoris is a unique proxy for the study of the early stages of planetary system formation and evolution, when giant planets are formed, Earthmass planets may still be forming, and most of the protoplanetary gas has disappeared from the disk
We note that the orbital period distribution is compatible with a period greater than 22 yr found by Snellen & Brown (2018) within only 2σ error bars
The Spectro-Polarimetric High-Contrast Exoplanet Research instrument (SPHERE) data confirm the conclusions reached in Lagrange et al (2012b): the planet projection is between the main disk and the warp
Summary
With its imaged debris disk of dust (see Smith & Terrile 1984, for the discovery image), its falling, evaporating exocomets (Kiefer et al 2014, and references therein), and an imaged giant planet (Lagrange et al 2010), the ∼20 Myr old β Pictoris is a unique proxy for the study of the early stages of planetary system formation and evolution, when giant planets are formed, Earthmass planets may still be forming, and most of the protoplanetary gas has disappeared from the disk. Careful monitoring of the position of the planet relative to the star (referred to as astrometric measurements) with the Nasmyth Adaptive Optics System (NAOS) Near-Infrared Imager and Spectrograph (CONICA) (NaCo) at the Very Large Telescope (VLT) constrained its orbital properties. Snellen & Brown (2018) used Gaia and Hipparcos measurements to constrain the planet period to ≥22 yr, and reported a mass of 11±2 MJup, which is compatible with the constraints derived from radial velocity data alone (Lagrange et al 2012a) or by the combined analysis of direct imaging and radial velocity data (Bonnefoy et al 2014). The orbital plane is close to equatorial, and the disk of β Pictoris is seen almost edge-on This geometrical configuration prevented following the planet in direct imaging when it was projected too close (typically less than 120 mas) to the star.
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