To understand how, when, and where planets form in a star system, we need to first look at planetesimals, the building blocks of the planets. Age dates of meteorites provide evidence that planetesimal formation began very early in the history of the solar system and continued for some time, with iron meteorites dated to one million years (1 Myr) after the formation of the first solids (Calcium-Aluminum-Rich Inclusions, known as CAIs) and carbonaceous chondrites dated at 3-5 Myr after CAIs. For gas giant planets such as Jupiter, from which we have no meteorites, it is critical to understand whether planetesimals form sufficiently early to allow time for the accretion of larger protoplanets before dissipation of the nebular (gas) disk, which usually only lasts a few million years.

In distant star systems, for which we do not know when the first solids formed, it is challenging to understand precisely when planetesimals would have started to form. Three possibilities are 1) during the collapse of the disk (Class 0/I disks), 2) while the protostar is still forming from its molecular cloud, and 3) after the disk has fully separated from the star-forming environment (a Class II disk). Most traditional models assume that planetesimals start to form at the beginning of the Class II phase.

Using the Atacama Large Millimeter/submillimeter Array (ALMA) telescope, Amy Bonsor from the University of Cambridge and coauthors obtained direct evidence that planetesimal formation began early in many exoplanetary systems. The team observed white dwarf stars and found that many of these stars have accreted fragments of metal cores or silicate material, which must have been derived from differentiated planetary bodies. The authors suggest that these fragments must have accreted at or before the collapse of the molecular cloud (Class 0/I). This work provides a new picture of planet formation occurring much earlier than traditional models would have suggested. READ MORE