Context. Aims. The goal of this paper is to demonstrate the outstanding quality of the second data release of the Gaia mission and its power for constraining many different aspects of the dynamics of the satellites of the Milky Way. We focus here on determining the proper motions of 75 Galactic globular clusters, nine dwarf spheroidal galaxies, one ultra-faint system, and the Large and Small Magellanic Clouds. Methods. Using data extracted from the Gaia archive, we derived the proper motions and parallaxes for these systems, as well as their uncertainties. We demonstrate that the errors, statistical and systematic, are relatively well understood. We integrated the orbits of these objects in three different Galactic potentials, and characterised their properties. We present the derived proper motions, space velocities, and characteristic orbital parameters in various tables to facilitate their use by the astronomical community. Results. Our limited and straightforward analyses have allowed us for example to (i) determine absolute and very precise proper motions for globular clusters; (ii) detect clear rotation signatures in the proper motions of at least five globular clusters; (iii) show that the satellites of the Milky Way are all on high-inclination orbits, but that they do not share a single plane of motion; (iv) derive a lower limit for the mass of the Milky Way of 9.1-2.6+6.2 × 1011 M⊙ based on the assumption that the Leo I dwarf spheroidal is bound; (v) derive a rotation curve for the Large Magellanic Cloud based solely on proper motions that is competitive with line-of-sight velocity curves, now using many orders of magnitude more sources; and (vi) unveil the dynamical effect of the bar on the motions of stars in the Large Magellanic Cloud. Conclusions. All these results highlight the incredible power of the Gaia astrometric mission, and in particular of its second data release.

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Current theories of planetary evolution predict that infant giant planets have large radii and very low densities before they slowly contract to reach their final size after about several hundred million years 1,2 . These theoretical expectations remain untested so far as the detection and characterization of very young planets is extremely challenging due to the intense stellar activity of their host stars 3,4 . Only the recent discoveries of young planetary transiting systems allow initial constraints to be placed on evolutionary models [5][6][7] . With an estimated age of 20 million years, V1298 Tau is one of the youngest solar-type stars known to host transiting planets; it harbours a system composed of four planets, two Neptune-sized, one Saturn-sized and one Jupiter-sized 8,9 .Here we report a multi-instrument radial velocity campaign of V1298 Tau, which allowed us to determine the masses of two of the planets in the system. We find that the two outermost giant planets, V1298 Tau b and e (0.64 ± 0.19 and 1.16 ± 0.30 Jupiter masses, respectively), seem to contradict our knowledge of early-stages planetary evolution. According to models, they should reach their mass-radius combination only hundreds of millions of years after formation. This result suggests that giant planets can contract much more quickly than usually assumed.V1298 Tau is a relatively bright (V = 10.1), very young K1 star with a mass of 1.170 ± 0.060 M ⊙ (where M ⊙ is the solar mass), a radius of 1.278 ± 0.070 R ⊙ (where R ⊙ is the solar radius), an effective temperature of 5,050 ± 100 K and solar metallicity (Table 1 and Extended Data Fig. 1). It is the physical companion of the G2 star HD 284154. The pair belongs to the Group 29 stellar association 10 and has an age of 20 ± 10 Myr (Extended Data Figs. 1 and 2). V1298 Tau was observed by Kepler's 'Second Light' K2 mission 11 . The analysis of the K2 data revealed the presence of four transiting planets in the system 9 . The three inner planets (b, c and d) were determined to have orbital periods of 24.1396 ± 0.0018, 8.24958 ± 0.00072 and 12.4032 ± 0.0015 days, and radii of 0.916 +0.052 −0.047 , 0.499 +0.032 −0.029 and 0.572 +0.040 −0.035 R Jup (where R Jup is the Jupiter radius). The fourth planet, e, was identified with only a single transit event, with a radius of 0.780 +0.075 −0.064 R Jup and orbital period estimated to be between 40 and 120 days. A previous study constrained the mass of V1298 Tau b to be less than 2.2 M Jup (ref. 12 ) (where M Jup is the Jupiter mass).To measure the planetary masses, we performed an intensive spectroscopic campaign, collecting more than 260 radial velocity (RV) measurements of V1298 Tau using the high-resolution spectrographs HARPS-N, CARMENES, SES and HERMES between April 2019 and April 2020. To monitor its stellar activity variations, we performed contemporaneous V-band photometry using the Las Cumbres Observatory Global Telescope (LCOGT) network 13 .V1298 Tau is a very active star that induces large RV activity variations. To extract the planetary sig...

The last decade of direct imaging (DI) searches for sub-stellar companions has uncovered a widely diverse sample that challenges the current formation models, while highlighting the intrinsically low occurrence rate of wide companions, especially at the lower end of the mass distribution. These results clearly show how blind surveys, crucial to constrain the underlying planet and sub-stellar companion population, are not an efficient way to increase the sample of DI companions. It is therefore becoming clear that efficient target selection methods are essential to ensure a larger number of detections. We present the results of the COPAINS Survey conducted with SPHERE/VLT, searching for sub-stellar companions to stars showing significant proper motion differences (Δμ) between different astrometric catalogues. We observed twenty-five stars and detected ten companions, including four new brown dwarfs: HIP 21152 B, HIP 29724 B, HD 60584 B and HIP 63734 B. Our results clearly demonstrates how astrometric signatures, in the past only giving access to stellar companions, can now thanks to Gaia reveal companions well in the sub-stellar regime. We also introduce FORECAST (Finley Optimised REtrieval of Companions of Accelerating STars), a tool which allows to check the agreement between position and mass of the detected companions with the measured Δμ. Given the agreement between the values of the masses of the new sub-stellar companions from the photometry with the model-independent ones obtained with FORECAST, the results of COPAINS represent a significant increase of the number of potential benchmarks for brown dwarf and planet formation and evolution theories.