In the core-accretion model, the nominal runaway gas-accretion phase brings most planets to multiple Jupiter masses. However, known giant planets are predominantly Jupiter mass bodies. Obtaining longer timescales for gas accretion may require using realistic equations of states, or accounting for the dynamics of the circumplanetary disk (CPD) in the low-viscosity regime, or both. Here we explore the second way by using global, three-dimensional isothermal hydrodynamical simulations with eight levels of nested grids around the planet. In our simulations, the vertical inflow from the circumstellar disk (CSD) to the CPD determines the shape of the CPD and its accretion rate. Even without a prescribed viscosity, Jupiter's mass-doubling time is ∼10 4 yr, assuming the planet at 5.2 AU and a Minimum Mass Solar Nebula. However, we show that this high accretion rate is due to resolution-dependent numerical viscosity. Furthermore, we consider the scenario of a layered CSD, viscous only in its surface layer, and an inviscid CPD. We identify two planet-accretion mechanisms that are independent of the viscosity in the CPD: (1) the polar inflow-defined as a part of the vertical inflow with a centrifugal radius smaller than two Jupiter radii and (2) the torque exerted by the star on the CPD. In the limit of zero effective viscosity, these two mechanisms would produce an accretion rate 40 times smaller than in the simulation.

Planetary systems are born in the disks of gas, dust and rocky fragments that surround newly formed stars. Solid content assembles into ever-larger rocky fragments that eventually become planetary embryos. These then continue their growth by accreting leftover material in the disc. Concurrently, tidal effects in the disc cause a radial drift in the embryo orbits, a process known as migration [1][2][3][4] . Fast inward migration is predicted by theory for embryos smaller than three to five Earth masses [5][6][7] . With only inward migration, these embryos can only rarely become giant planets located at Earth's distance from the Sun and beyond 8,9 , in contrast with observations 10 . Here we report that asymmetries in the temperature rise associated with accreting infalling material 11, 12 produce a force (which gives rise to an effect that we call "heating torque") that counteracts inward migration. This provides a channel for the formation of giant planets 8 and also explains the strong planet-metallicity correlation found between the incidence of giant planets and the heavy-element abundance of the host stars 13, 14 .We solve the equations governing the disc hydrodynamics in combination with the equations of radiative transfer. Planets have an angular momentum that increases with their orbital radius. In the case of a nearly circular orbit, the rate of change of angular momentum, or torque, gives the migration rate. Our calculations are performed in three dimensions, yielding a reliable value for the net torque, from which the direction and rate of migration are inferred.Our fiducial computation is one in which a rocky core with 3 Earth masses is located at a distance comparable to that of Jupiter from the Sun and is being bombarded by solid material at a rate that doubles its mass in 100 thousand years. We assume that the gravitational energy of the infalling solid material is transformed entirely into heat and ultimately radiated by the planet 11 . A second computation is performed with the same set up, but without the planet's radiation, in order to distinguish the effects of the heating torque from other torques. We find that the heating torque (defined as the torque difference between cases with accretion turned respectively on and off) has a positive sign (figure 1), which enables it to counteract the effect of the standard, negative torque.The latter includes all torque components of the non-heating case, and is always negative for small 2 mass embryos (typically smaller than 5 M ⊕ , where M ⊕ is the Earth's mass). Thus, the effect of the heating torque is to either slow down the inward migration, cancel it, or reverse its direction.The most important factors governing the strength of the heating torque and thus, the direction of migration, are the accretion rate of the embryo, its mass and the opacity of the disc. For our fiducial values of opacity, disc structure and embryo mass, we find that outward migration occurs for accretion rates corresponding to a mass doubling time less than approximately 60 t...

Context. Dozens of protoplanetary disks have been imaged in scattered light during the last decade. Aims. The variety of brightness, extension, and morphology from this census motivates a taxonomical study of protoplanetary disks in polarimetric light to constrain their evolution and establish the current framework of this type of observation. Methods. We classified 58 disks with available polarimetric observations into six major categories (Ring, Spiral, Giant, Rim, Faint, and Small disks) based on their appearance in scattered light. We re-calculated the stellar and disk properties from the newly available GAIA DR2 and related these properties with the disk categories. Results. More than half of our sample shows disk substructures. For the remaining sources, the absence of detected features is due to their faintness, their small size, or the disk geometry. Faint disks are typically found around young stars and typically host no cavity. There is a possible dichotomy in the near-infrared (NIR) excess of sources with spiral-disks (high) and ring-disks (low). Like spirals, shadows are associated with a high NIR excess. If we account for the pre-main sequence evolutionary timescale of stars with different mass, spiral arms are likely associated to old disks. We also found a loose, shallow declining trend for the disk dust mass with time. Conclusions. Protoplanetary disks may form substructures like rings very early in their evolution but their detectability in scattered light is limited to relatively old sources ( 5 Myr) where the recurrently detected disk cavities cause the outer disk to be illuminate. The shallow decrease of disk mass with time might be due to a selection effect, where disks observed thus far in scattered light are typically massive, bright transition disks with longer lifetimes than most disks. Our study points toward spirals and shadows being generated by planets of a fraction of a jupiter mass to a few jupiter masses in size that leave their (observed) imprint on both the inner disk near the star and the outer disk cavity.