At a distance of 1.295 parsecs, 1 the red-dwarf Proxima Centauri (α Centauri C, GL 551, HIP 70890, or simply Proxima) is the Sun's closest stellar neighbour and one of the best studied low-mass stars. It has an effective temperature of only ∼ 3050 K, a luminosity of ∼0.1 per cent solar, a measured radius of 0.14 R ⊙ 2 and a mass of about 12 per cent the mass of the Sun. Although Proxima is considered a moderately active star, its rotation period is ∼ 83 days, 3 and its quiescent activity levels and X-ray luminosity 4 are comparable to the Sun's. New observations reveal the presence of a small planet orbiting Proxima with a minimum mass of 1.3 Earth masses and an orbital period of ∼11.2 days. Its orbital semi-major axis is ∼ 0.05 AU, with an equilibrium temperature in the range where water could be liquid on its surface. 5 The results presented here consist of the analysis of previously obtained Doppler measurements (pre-2016 data), and the confirmation of a signal in a specifically designed follow-up campaign in 2016. The Doppler data comes from two precision radial velocity instruments, both at the European Southern Observatory (ESO): the High Accuracy Radial velocity Planet Searcher (HARPS) and the Ultraviolet and Visual Echelle Spectrograph (UVES). HARPS is a high-resolution stabilized echelle spectrometer installed at the ESO 3.6m telescope (La Silla observatory, Chile), and is calibrated in wavelength using hollow cathode lamps. HARPS has demonstrated radial velocity measurements at ∼1 ms −1 precision over time-scales of years, 6 including on low-mass stars. 7 All HARPS spectra were extracted and calibrated with the standard ESO Data Reduction Software, and radial velocities were measured using a least-squares template matching technique. 7 HARPS data is separated into two datasets. The first set includes all data obtained before 2016 by several programmes (HARPS pre-2016 work, and its value is then used to assess the false-alarm probability (or FAP) of the detection. 14 A FAP below 1% is considered suggestive of periodic variability, and anything below 0.1% is considered to be a significant detection. In the Bayesian framework, signals are first searched using a specialized sampling method 16 that enables exploration of multiple local maxima of the posterior density (the result of this process are the gray lines in Figure 1), and significances are then assessed by obtaining the ratios of evidences of models. If the evidence ratio exceeds some threshold (e.g. B 1 /B 0 > 10 3 ), then the model in the numerator (with one planet) is favoured against the model in the denominator (no planet).A well isolated peak at ∼11.2 days was recovered when analyzing all the night averages in the pre-2016 datasets (Figure 1, panel a). Despite the significance of the signal, the analysis of pre-2016 subsets produced slightly different periods depending on the noise assumptions and which subsets were considered. Confirmation or refutation of this signal at 11.2 days was the main driver for proposing the HARPS PRD campaign. T...

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As planets form and grow within gaseous protoplanetary disks, the mutual gravitational interaction between the disk and planet leads to the exchange of angular momentum, and migration of the planet. We review current understanding of disk-planet interactions, focussing in particular on physical processes that determine the speed and direction of migration. We describe the evolution of low mass planets embedded in protoplanetary disks, and examine the influence of Lindblad and corotation torques as a function of the disk properties. The role of the disk in causing the evolution of eccentricities and inclinations is also discussed. We describe the rapid migration of intermediate mass planets that may occur as a runaway process, and examine the transition to gap formation and slower migration driven by the viscous evolution of the disk for massive planets. The roles and influence of disk self-gravity and magnetohydrodynamic turbulence are discussed in detail, as a function of the planet mass, as is the evolution of multiple planet systems. Finally, we address the question of how well global models of planetary formation that include migration are able to match observations of extrasolar planets.

We analyse the stability and nonlinear dynamical evolution of power-law accretion disc models. These have midplane densities that follow radial power-laws, and have either temperature or entropy distributions that are strict power-law functions of cylindrical radius, R. We employ two different hydrodynamic codes to perform high resolution 2D-axisymmetric and 3D simulations that examine the long-term evolution of the disc models as a function of the power-law indices of the temperature or entropy, the thermal relaxation time of the fluid, and the disc viscosity. We present an accompanying stability analysis of the problem, based on asymptotic methods, that we use to interpret the results of the simulations. We find that axisymmetric disc models whose temperature or entropy profiles cause the equilibrium angular velocity to vary with height are unstable to the growth of modes with wavenumber ratios |k R /k Z | 1 when the thermodynamic response of the fluid is isothermal, or the thermal evolution time is comparable to or shorter than the local dynamical time scale. These discs are subject to the Goldreich-Schubert-Fricke (GSF) or 'vertical shear' linear instability. Development of the instability involves excitation of vertical breathing and corrugation modes in the disc, with the corrugation modes in particular being a feature of the nonlinear saturated state. Instability is found to operate when the dimensionless disc kinematic viscosity ν < 10 −6 , corresponding to Reynolds numbers Re = Hc s /ν > 2500. In three dimensions the instability generates a quasiturbulent flow, and the associated Reynolds stress produces a fluctuating effective viscosity coefficient whose mean value reaches α ∼ 6 × 10 −4 by the end of the simulation. The evolution and saturation of the vertical shear instability in astrophysical disc models which include realistic treatments of the thermal physics has yet to be examined. Should it occur on either global or local scales, however, our results suggest that it will have significant consequences for their internal dynamics, transport properties, and observational appearance.

We report a new and complete model of the β Pictoris disk, which succeeds in accounting for both the surface brightness distribution, warp characteristics, the outer "butterfly" asymmetry as observed by HST/STIS in scattered light, as well as the infrared emission. Our model includes the presence of a disk of planetesimals extending out to 120-150 AU, perturbed gravitationally by a giant planet on an inclined orbit, following the approach of Mouillet et al. (1997b). At any time, the planetesimal disk is assumed to be the source of a distribution of grains produced through collisional evolution, with the same initial orbital parameter distribution. The steady state spatial grain distribution is found incorporating the effects of radiation pressure which can cause the distribution of the smallest particles to become very distended. With realistic assumptions about the grains' chemical properties, the modeling confirms the previously evident need for an additional population of hot grains close to the star, to account for the 12 µm fluxes at short distances from the star. It also indicates that this population cannot explain the outer 12 µm flux distribution when the effects of gravity and radiation pressure determine the distribution. Very small grains, produced by collisions among aggregates, are tentatively proposed to account for this 12 µm outer emission.