ALMA confirms predictions on the interaction between protoplanetary disks and planets

ALMA OBSERVATORY/DICYT New observations made with the Atacama Large Millimeter/submillimeter Array (ALMA) of the disk that surrounds a young star, less massive than the Sun, confirm theories about the interaction between recently formed planets and disks. A team of astronomers led by Héctor Cánovas from Universidad de Valparaíso and the Millennium ALMA Disk Nucleus (MAD) observed the dust ring possibly sculpted by planets in formation around the star Sz 91, at a distance roughly 650 light years from Earth.

The results obtained show the first disk around a star that is less massive than ours - it has only half of the mass of our Sun - which simultaneously presents a migration of dust particles from the outermost zones and evident signs of interaction between young planets with the disk in the innermost zone.

Planets are born in dust and gas disks that surround young stars and feed them with matter, leaving a “footprint” of this interaction in the structure of the disk. The theoretical models that study this interaction predict that the giant planets carve the protoplanetary disk, creating a “hole” in the innermost part of the disk, and preventing mm-sized dust particles (like grains of sand on a beach) from continuing their journey towards the central star. At the same time, dust particles in the outermost parts of the disk (the farthest from the star) are moving inward by the combined action of gravity and aerodynamic forces (gas-drag).

The combination of all these effects is expected to create an accumulation of dust at the edge of the hole. As a consequence, a sharp ring maybe seen in the emission of disks that host recently formed giant planets. This is what ALMA observed.

“The sharp image from ALMA shows a ring around the young star. And it is a surprisingly large ring, over three times the size of Neptune's orbit (a radius of approximately 110 astronomical units (AU)” explains Héctor Cánovas.

The image from ALMA only shows the ring, as the radio telescope detects the cold dust particles that make it up, and not the planets and the star, as these are primarily made up of hot gas.

“Based on the current paradigm of planet-disk interactions, only giant planets orbiting the innermost parts of the disk can explain the presence of a ring with such a large radius,” indicates Antonio Hales, ALMA astronomer and member of the research team.

The accumulation of dust particles in a narrow annular structure, as is the case with Sz91, can favor the formation of more planets, because the high density of dust particles in the ring would provide the ideal conditions for the dust particles to agglutinate and grow in size until they form small planetary nuclei.

“The results of this investigation show that Sz91 is a highly important protoplanetary disk for the study of planetary formation, planet-disk interactions, and the evolution of these disks around stars of lower mass, as Sz91 shows evidence of all these processes simultaneously,” concludes Matthias Schreiber, coauthor of the study.