News Overview

With simulations that resolve finer details than ever before, scientists Brooke Polak (Hei­del­berg Uni­ver­si­ty) and STRUC­TURES member Hubert Klahr (MPIA, Hei­del­berg Uni­ver­si­ty) have modelled a key phase in the formation of planets in our solar system: the way that centimetre-size pebbles aggregate into so-called planetesimals tens to hundreds kilometres in size. The simulation reproduces the initial size distribution of planetesimals that can be checked against observations of present-day asteroids. It also predicts the prevalence of close binary planetesimals in our solar system.

Planet formation around a star proceeds in several stages. Initially, cosmic dust in the swirling protoplanetary disk around a new star clumps together due to electrostatic (van der Waals) forces, forming so-called pebbles a few centimetres in size. The pebbles in turn join together to form planetesimals: space rocks between tens and hundreds of kilometres in diameter. Finally, collisions among these planetesimals form even larger, gravitationally-bound, solid cosmic objects: planetary embryos, which can continue accreting planetesimals and pebbles until they become planets. Simulating this progression from centimetre-size pebbles to planetesimals, however, is challenging due to the disparate scales involved. The simulations by Polak and Klahr follow an innovative approach by using a kinetic description in which small groups of pebbles in a collapsing cloud in a protoplanetary disk are treated like a gas that can undergo certain phase transitions, and assigning a pressure to this “pebble gas.” In their new work, Polak and Klahr look at several versions of collapsing protoplanetary disk regions, each with at a different distance from the Sun, starting with a distance as close as Mercury's orbit and ending with a collapsing region as far away as Neptune.

• Max Planck Institute for Astronomy (MPIA) - Press Release
• Hubert Klahr: Planet and Star Formation Theory Group