Vol. 670
10. Planets and planetary systems

The Curie line in protoplanetary disks and the formation of Mercury-like planets

Protoplanetary disks imaged in the optical or millimeter range show rich ring-like structures, which have been attributed to a variety of processes, including embedded planets, snow lines, the pile-up of material at pressure maxima, or other local instabilities. Snow lines, in particular, are favored locations for the triggering of planetesimal formation. Bogdan et al. here propose the new concept of a Curie line in protoplanetary disks. Laboratory measurements, in which chondritic material is heated to 1400 K in a hydrogen atmosphere and analyzed with Mössbauer spectroscopy and magnetometry, indicate the formation of metallic iron from silicates near temperatures of 1200 K or lower. This implies that inward drifting material in protoplanetary disks must be a source of metallic iron. The presence of a disk magnetic field then makes the magnetic aggregation of dust aggregates possible, a mechanism previously proposed for the selective growth of iron-rich bodies through streaming instabilities. However, coincidentally, the temperature of metallic iron formation from silicates is similar to the Curie temperature (1041 K), above which iron loses its permanent magnetic properties and becomes paramagnetic. Therefore, inside the Curie line, the large magnetic clusters will dissolve but generate a large density of small particles, possibly causing a "traffic jam" (analogous to the process thought to occur at ice snow lines), which might also be beneficial for planetesimal formation by gravitational instability. The Curie line may thus appear as a dividing line between two possible formation processes for iron-rich, Mercury-like planets.