|
The standard model for planet formation involves the aggregation of dust
particles in a circumstellar accretion disc. It is surmised that the gas in the
accretion disc must be turbulent in order to explain the relatively short lifetime
of the disc. This makes it necessary to understand the dynamics of dust
particles in a turbulent gas. I will review some basic properties of turbulent
flows, and describe some recent results on the relative velocities and
collision rates particles. I shall also describe how the properties of the gas in
the circumstellar accretion disc can be estimated from a steady-state theory
with very few input parameters. The dust particles are bound by van der Waals
and other weak electrostatic forces. I will describe some estimates for the collision
speed at which aggregates of dust particles will be fragmented, and show that
the relative speeds of colliding dust aggregates make them very vulnerable to
disruption. This is a strong indication that an alternative theory must
be sought.
I shall describe an alternative hypothesis for the formation of planets, developed in collaboration with Bernhard Mehlig, which we term Concurrent Collapse. According to our hypothesis, when a cloud of interstellar gas collapses to form a star, it fragments, giving rise to smaller objects which are gravitationally bound to the star. These 'juvenile planets' are initially formed in non-circular orbits and have an elemental composition which is representative of the star. The juvenile planets can interact with the accretion disc in such a way that their orbit and their composition can be dramatically changed. Collisions between juvenile planets are also possible. Our hypothesis avoids having to resolve the difficulties faced by the dust aggregation model. It also provides satisfying explanations for the existence of exoplanets with eccentric orbits, for the occurrence of FU Orionis outbursts and for the melting of 'chondrules' found in meterorites. |
|