Previous models of the combined growth and migration of protoplanets needed large ad hoc reduction factors for the type I migration rate as found in the isothermal approximation. In order to eliminate these factors, a simple semi-analytical model is presented that incorporates recent results on the migration of low mass planets in non-isothermal disks. It allows for outward migration. The model is used to conduct planetary populations synthesis calculations. Two points with zero torque are found in the disks. Planets migrate both in- and outward towards these convergence zones. They could be important for accelerating planetary growth by concentrating matter in one point. We also find that the updated type I migration models allow the formation of both close-in low mass planets, but also of giant planets at large semimajor axes. The problem of too rapid migration is significantly mitigated.

In order to understand general planet characteristics and constrain formation models it is necessary to scan over the widest possible parameter range of discovered systems. Due to detection biases, the domain of very-low mass planets had remained poorly explored. Only with improving measurement precision it has been possible to enter in the sub-Neptune mass range. The HARPS planet search program has been particularly efficient in detecting such ice giants and super earths. The present talk will summarize the obtained results and the characteristics of the low-mass population of exoplanets.

The Smooth Particle Hydrodynamics (SPH) impact code (Benz & Asphaug 1994) has been developed for the simulation of impacts and collisions involving brittle solids in the strength-and gravity-dominated regime. In the latter regime, the gravitational overburden is used to increase the fracture threshold. In this paper, we extend our numerical approach to include the effect of porosity at a sub-resolution scale by adapting the so-called P -α model (Herrman 1969). Using our extended 3D SPH impact code, we investigated collisions between porous bodies to examine the sensitivity of collisional outcomes to the degree of porosity. Two applications that illustrate the capabilities of our approach are shown: 1) the modeling of a Deep Impact-like impact and 2) the computation of the amount of momentum transferred to an asteroid following the impact of a high velocity projectile.

Massive but invisible black holes (BH) are often assumed to lurk in the center of many galaxies but definitive proof of their existence has not yet been established. However, in the surrounding stellar cluster stars are unavoidably being destroyed by tidal disruptions and stellar collisions liberating of order 1M⊙ in each event. The subsequent accretion of this gas by the BH could bring it back to “life” and reveal its presence.

We review recent work on gravitational instabilities in circumstellar disks and discuss implications for the possibility of forming binary companions within these disks. Linear stability analysis indicates that self-gravitating modes with azimuthal wavenumber m = 1 will grow strongly in star/disk systems with the physical properties observed in astrophysical systems. Smooth particle hydrodynamic simulations indicate that these self-gravitating instabilities (with m = 1 and higher) will grow into the nonlinear regime. In many cases, a spiral arm will collapse into a gravitationally bound “knot” of gas; these knots typically contain 1% of the mass of the disk and travel on elliptical orbits about the central star.