Planet Formation in Stellar Infancy
The disk around the young star IRS63 (it is only about five hundred thousand years old) as imaged by ALMA at millimeter wavelengths. Its two rings and two gaps are labeled with R and G respectively; the scale length in astronomical units is shown. This is the youngest stellar disk ever imaged and its non-distinct edges suggest that at this early stage planets (but perhaps something else) have only just begun to shape the disk into the sharply defined rings and gaps seen in older systems.
As the slightly spinning material in a pre-stellar condensation collapses to form a star, angular momentum conservation shapes it into a circumstellar disk. Planets form from the gas and dust in these disks, whose structure and evolution are thus keys to unraveling the planet-building process. There are currently two favored scenarios, a core accretion model in which planets assemble through the aggregation of dust grains and a gravitational instability model in which clumps develop during the initial stages of the disk evolution and grow into planetesimals.
Astronomers have been able to image disks in over one hundred young stars so far and infer their presence in many more. Thirty-five of the imaged disks are only about one million years old and have recently begun to disperse the natal cloud of material from which they were born. Imaged at submillimeter wavelengths where the cool dust radiates efficiently, these disks reveal bright rings and empty gaps which are thought to be due to the presence of young planets that shepherd the dust into rings and clear out the gaps. These discoveries imply, however, that planet formation must have begun at even earlier stages -- but no younger stars with disk have been discovered, until now.
CfA astronomer Ian Stephens was a member of a team that used the ALMA millimeter facility to image the object IRS63 whose disk had been identified by the Submillimeter Array but with insufficient resolution to see rings. The infrared emission from this system indicates it is younger than about five hundred thousand years. The star is relatively nearby, only about five hundred light-years distant; ALMA images can resolve structures as small as five astronomical units (one AU is the average Earth-Sun distance), and they reveal two rings and two gaps, the first detection of such protoplanetary disks in such a young star. Notably, the rings and gaps appear to be much less mature than those around older stars in the sense that the rings are much less well-defined and the gaps are not yet cleared of dust. The inner ring is situated about twenty-seven AU from the star, and the outer ring is about fifty-one AU away.
The scientists caution that concentric circumstellar annular structures do not by themselves require the presence of planets. A variety of other physical processes might produce similar structures including disk winds or the asymmetric accretion of material from an outer envelope. However even if planets have not yet formed, or if the dust grains have not yet coalesced into large masses, these dust rings around such an early stage star could serve as ideal zones for future planetesimal development. The new paper, appearing in the latest Nature, is an important step in constraining the earliest stages of planet formation.
Reference(s): "Four Annular Structures in a Protostellar Disk Less Than 500,000 Years Old,” Dominique M. Segura-Cox, Anika Schmiedeke, Jaime E. Pineda, Ian W. Stephens, Manuel Fernández-López, Leslie W. Looney, Paola Caselli, Zhi-Yun Li, Lee G. Mundy, Woojin Kwon & Robert J. Harris, Nature 586, 228, 2020.