Module Two: Planet Formation Pathways and Planet Compositions
The surprising variety of exoplanetary systems highlighted the diversity of formation pathways. We will build on our EOS Team’s comprehensive studies of disk evolution and planet accretion. We will test observational evidence for different planet formation pathways and model how these pathways impact the bulk composition and properties of the emerging habitable worlds.
Methods
Protoplanetary Disk Observations
Planet Formation and Disk Evolution Models
HOW DO DISK GAPS INFLUENCE THE CHEMICAL AND PHYSICAL
PROPERTIES OF PLANETESIMALS?
Project 2.1 | Lead: Fred Ciesla
Disk structures such as gaps and rings, of the type we will investigate in Projects 1.1, 1.2, and 1.4, are common features in protoplanetary disks of all ages. These features are associated with local pressure minima or maxima within the gas, and thus limit the radial drift of pebbles, concentrating them and initiating subsequent planetesimal formation. Naturally, we must ask: what are the physical and chemical consequences of having distinct zones in a protoplanetary disk where planetary materials are processed and locked away?
In Project 2.1 we are modeling the impact of disk gaps on the migration of icy grains and pebbles through protoplanetary disks.
CONSTRAINING THE PEBBLE MASS FLUX
Project 2.2 | Lead: Ilaria Pascucci
Planet formation via pebble accretion has recently emerged as a plausible pathway to explain different classes of planets, including the numerous close-in super-earths detected by Kepler. The linear scaling between the most common planet mass and the mass of their host star is consistent with the pebble isolation mass setting the planet mass. However, the outcome of pebble-accretion strongly depends on the influx of icy pebbles (from the outer disk at tens of AU) to the inner disk, i.e., to within the water snowline.
In Project 2.2 we will combine observations probing the outer and inner disk with modeling carried out in Project 2.1 to constrain the pebble mass flux.
HOW DO PLANET FORMATION PATHWAYS IMPACT THE BULK COMPOSITIONS OF ROCKY PLANETS?
Project 2.3 | Lead: Gijs Mulders
The bulk compostion of rocky planets depends on the region in the disk from which planetesimals and pebbles are accreted (see Projects 2.1 and 2.2). The terrestrial planets in the Solar System formed locally by accretion of planetesimals, with only small amounts of volatile-rich material delivered from asteroidal and cometary material farther out (Project 1.3). In contrast, the exoplanets discovered by Kepler have revealed additional formation pathways for rocky planets. In particular, the large concentration of mass in planets close to their stars, compared to what is observed in protoplanetary disks, indicates that inward migration of planetary building blocks played a crucial role in exoplanet formation. Two main pathways have been proposed: type-I migration of protoplanets and the drift and accretion of pebbles. These new pathways strongly affect the planets’ bulk compositions, but they do so in different ways. The type-I migration of protoplanets formed outside the snow line leads to volatile-rich rocky planets, while drifting pebbles that devolatilize at snow lines lead to volatile-poor planets. A planet’s bulk composition therefore depends on the planet formation pathway, but it is not clear to what extent each pathway contributes to the observed rocky exoplanets.
In Project 2.3 we will use numerical models to study the composition of forming rocky exoplanets in the context of state-of-the-art planet migration, pebble accretion, and planetesimal accretion models.