Heidelberg Joint Astronomical Colloquium

Speaker Zoë Leinhardt
Title Forming protoplanets from planetesimals

Nearly 500 extrasolar planets have been discovered to date. The observed planetary systems are very different from our solar system and surprisingly diverse. The large number of planets detected suggests that planet formation is common around main sequence stars. The major problem facing the scientific community with regards to these discoveries is that observations cannot trace the history of planet formation. Instead observations provide snapshots of either the early stages of a protoplanetary gas disk orbiting a young star or the late stages after planetary systems have formed. But the evolution from a young star to a planetary system has not been observed. Thus, the challenge is to connect the early and late stages of planet formation.

Planets form from the collisional growth of planetary building blocks, planetesimals. In recent numerical work we have found that the resistance of planetesimals to collisional erosion changes dramatically during planet formation. Young planetesimals are weak aggregates that are easily disrupted due to efficient momentum coupling during low-velocity collisions in the early phases of their collisional evolution. However, as impact speeds increase the same weak planetesimals become dramatically stronger because the shock from a supersonic impact loses energy to deformation and phase changes. Our work identifies a paradox for the early stages of planet formation. Objects in the km-size range are weak and susceptible to collisional disruption. However, this disruption may actually produce large amounts of debris that can be accreted by remaining undisrupted planetesimals allowing growth. As we work to disentangle these sorts of conundrums we can expect to put forward hypotheses for collisional remnants in our solar system – for example, the dwarf planet Haumea and its collisional family.

In this talk I will review the current understanding of planetesimal evolution and discuss how future numerical simulations may connect observational snapshots to provide a complete history of planet formation.