Trinary system protoplanetary disks reveal clues about planet formation

Ars Technica : Planet formation is complicated in a single-star system and is even more so in systems with two or more stars. It is thought that in multistar systems, each star will have its own protoplanetary disk of dust, and a larger disk will surround the system. GG Tau A is a trinary system 460 light-years away, and recent observations have revealed some details about how such systems work. Emmanuel Di Folco of the University of Bordeaux in France and his colleagues used the Atacama Large Millimeter/Submillimeter Array (ALMA) to study the system. The primary star in the system, GG Tau Aa, has a disk, while the pair of secondary stars GG Tau Ab1 and GG Tau Ab2 (which orbit each other 35 AU from GG Tau Aa) were not seen to have disks, although previous IR observations suggested they might. ALMA also showed evidence of a larger disk surrounding the system. More importantly, ALMA revealed a flow of material from the outer disk to the disk around GG Tau Aa. Without that transfer of material, the internal disk would likely have fallen apart due to the stars’ gravity, long before any planets could have formed. ALMA did not reveal any planets around the three stars, but the images did suggest that a planet may be forming in the surrounding disk. The presence of a protoplanet there could explain why the disk is relatively narrow, with 80% of its mass in a region just 90 AU wide.