  The timing of formation for the first planetesimals determines the mode of
planetary accretion and their geophysical and compositional evolution.
Astronomical observations of circumstellar discs and Solar System geochronology
provide evidence for planetesimal formation during molecular cloud collapse,
much earlier than previously estimated. Here, we present distinct observational
evidence from white dwarf planetary systems for planetesimal formation
occurring during the first few hundred thousand years after cloud collapse in
exoplanetary systems. A significant fraction of white dwarfs have accreted
planetary material rich in iron core or mantle material. In order for the
exo-asteroids accreted by white dwarfs to form iron cores, substantial heating
is required. By simulating planetesimal evolution and collisional evolution we
show that the most likely heat source is short-lived radioactive nuclides such
as Al-2 (half life of approximately 0.7 Myr). Core-rich materials in the
atmospheres of white dwarfs, therefore, provide independent evidence for rapid
planetesimal formation, concurrent with star formation.
