Rings of dust have already been identified in many protoplanetary systems from their infrared and submillimeter emission. The origin of these rings is debated. They might have formed from dust “pile-up,” dust settling, gravitational instabilities, or even from variations in the optical properties of the dust. Alternatively, the rings could result dynamically from the orbital motions of planets that have already developed or that are well on their way. Planets will induce waves in the dusty discs which, as they dissipate, can produce gaps or rings. The key to solving the problem is recognizing that different-sized dust grains behave differently, with small grains being strongly coupled to the gas and so track the gas mass, whereas larger grains (millimeter-sized or larger) tend to follow pressure gradients and concentrate near gap edges. CfA astronomers Sean Andrews and David Wilner were members of a team of scientists who used the ALMA facility to image the dust around the young star Elias 24 with a resolution of about 28 au (one astronomical unit is about the average distance of the Earth from the Sun). The astronomers find evidence for gaps and rings and, assuming these are produced by an orbiting planet, they model the system allowing both the planet’s mass and location and the dust’s density distribution to evolve. Their best model explains the observations quite well: after about forty-four thousand years the inferred planet has a mass 70% of Jupiter’s mass and is located 61.7 au from the star. The result reinforces the conclusion that both gaps and rings are prevalent in a wide variety of young circumstellar disks, and signal the presence of orbiting planets. Reference: “Rings and Gaps in the Disc Around Elias 24 Revealed by ALMA” by G Dipierro, L Ricci, L Pérez, G Lodato, R D Alexander, G Laibe, S Andrews, J M Carpenter, C J Chandler, J A Greaves, C Hall, T Henning, W Kwon, H Linz, L Mundy, A Sargent, M Tazzari, L Testi and D Wilner, 23 January 2018, MNRAS.DOI: 10.1093/mnras/sty181
