Physics and Chemistry of Planet Formation

Planets are thought to grow from a solid core that accretes condensed material from the disc (planetesimals or pebbles), as well as gas. The internal structure of the planet (its composition and internal energy) is what determines how the planet cools and contracts. The layer of the protoplanet that plays the most important role in the cooling is the atmosphere: since it is made of gas, it contracts substantially when cooling, allowing for more gas from the disc to be accreted onto the planet.

As the protoplanet becomes more massive, the more it interacts with the disc of solids. This interaction planet-planetesimal (if the solids are planetesimals) is very complex. On one hand, the more massive the planet, the stronger its gravitational pull: more planetesimals are expected to fall into the planet. On the other, bigger protoplanets excite planetesimals (the effect depends on the planetesimal size), which causes them to move at faster speeds, making them more difficult to be accreted by the planet.

Some of the aspects we study in the group regarding gas and solid accretion are presented in the following papers:

• Fortier, Andrea; Alibert, Yann; Carron, Frédéric; Benz, Willy; Dittkrist, Kai-Martin (2013):
  Planet formation models: the interplay with the planetesimal disc. A&A, 549, A44
• Venturini, Julia; Alibert, Yann; Benz, Willy; Ikoma, Masahiro (2015):
  Critical core mass for enriched envelopes: the role of H2O condensation. A&A, 576, A114

Huge numbers of discovered (exo-)planet and observed proto-planetary discs manifest that planetary systems are the outgrowth of protoplanetary discs. Although only a small fraction of the dust and gas in a protoplanetary disc remains in the end in the form of planets around their host star and the rest of the material is dispersed, the details of the dispersion and evolution of the disc have key roles in defining the final planetary system architecture.

During the disc evolution, the disc density and temperature profiles change by various processes such as: viscous accretion, thermal irradiation, stellar radiation, and photo-evaporation. The temperature and density profiles affect the final planetary system in the two following ways:

1. Altering the chemical processes: This leads to changing the position of ice-lines and also the gas ionization fraction. The ice-line locations influence the solid reservoir for the planetary growth and composition. The ionization fraction in the disc determines the location of a dead zone, where the turbulent viscosity is less active than the rest of the disc and is a favorable position for trapping and growing planets.
2. Migration of the planetary objects: the final positions of the planets are the result of their long term tidal interaction with the disc. As the planets grow and the disc properties change, the migration rate and even direction of the planetary migration may vary. Both of chemical and physical properties of the disc determine the final structure and composition of the planets.

Collisions and impact processes play a fundamental role in the initial accretion and subsequent evolution of planets, moons and the small body populations (asteroids and comets).  They take place in vastly different regimes and lead to a large spectrum of outcomes.

The last major impacts occurring at the final stages of planet formation determine the properties of planets and moons to a large degree. Outstanding examples are the origin of the Earth-Moon and Pluto-Charon systems, the unusual composition of Mercury, or the crustal dichotomy of Mars. For small bodies such as asteroids and comets, the last global scale impact or disruption event determines their shapes, surface morphologies, densities and strengths.

As a complement to experimental and theoretical approaches, numerical modeling has become an important component to study collisions and impact processes. Our group has a long tradition of developing shock physics codes based on the Smooth Particle Hydrodynamics (SPH) method. Our state-of-the-art numerical tools are specially suited to study the regimes of collisions among small bodies where the complex effects of material strength, friction, porosity as well as self-gravitation determine the outcome concurrently. The study of large scale “giant” impacts, for instance in the context of the moon formation, is another focus area.

Some recent highlights are described in the papers below:

• Reufer, Andreas; Meier, Matthias M. M.; Benz, Willy; Wieler, Rainer (2012):
  A hit-and-run giant impact scenario. Icarus, 221, 296
• Jutzi, Martin; Asphaug, Erik; Gillet, Philippe; Barrat, Jean-Alix; Benz, Willy (2013):
  The structure of the asteroid 4 Vesta as revealed by models of planet-scale collisions. Nature, 494, 207
• Jutzi, Martin & Asphaug, Erik (2015):
  The shape and structure of cometary nuclei as a result of low-velocity accretion. Science, 348, 1355

The dust of protoplanetary discs is made of surviving grains from the interstellar medium (ISM, mainly SiC grains) and condensates formed during the cooling of the stellar nebula. Consequently, the chemical composition of the disc is a mixture of very refractory grains that did not sublimate during stellar formation, and gaseous material that condensed. The contribution of pre-stellar grains is currently unknown, and it is often assumed that they play no role in the resulting disc chemistry.There are different way to account for chemistry in protoplanetary discs. Kinetics are the most interesting as they enable us to determine and follow with time all processes that play a role, but need the knowledge of chemical networks that are difficult to achieve. If it is possible to realize almost complete networks
for simple molecules such as volatile molecules in general in protoplanetary discs (including non equilibrium effects, see, e.g., Walsh et al. 2010; Heinzeller et al. 2011; Walsh et al. 2012), long and complex molecules, such as refractory materials, are difficult to account for. Due to lack of data concerning reaction rates and network, it is usually assumed for such species that they form in thermodynamical equilibrium, following the so called "condensation sequence" which tells us at which pressures and temperatures a specie is stable in solid state, and traces the slow cooling of the stellar nebula (Lodders 2003, Ebel 2006). This assumes that the stellar nebula was initially hot (>3000K) so that every solid matter of the molecular cloud has sublimated, "resetting" the chemistry, and that the subsequent cooling has been slow enough to be able to reach equilibrium.

Such assumption is used for the computation of molecular abundances of refractory species, since only thermodynamical data are usually available (see, e.g., the NIST Chemistry WebBook). In a search for consistency, volatile molecules are also computed in equilibrium, although not quite in the same fashion.