A diverse range of planetary nurseries
Circumstellar disks made up of gas and dust are the birthplaces of planets. New observations indicate that the structures of such disks are highly diverse, and that planets form at much faster rates than current theories predict.
Almost 3000 planets have been discovered beyond the reaches of our Solar System, but little is known about the process of planet formation as such. Current models suggest that planets evolve within ‘protoplanetary’ disks associated with young stars. Over time, the dust and gas in such a disk is thought to condense into defined rings, separated by gaps. This material then progressively coalesces into larger bodies by sweeping up and accreting the dust within each ring. Nearby circumstellar disks that exhibit rings and gaps are thus taken as evidence for the ongoing evolution of exoplanets. So far, very few such systems have been investigated, and it has remained unclear whether such features are in fact hallmarks of planet formations or simply anomalous products of specific local conditions. An international team of astronomers, including LMU physicist Til Birnstiel, has now carried out a comprehensive study of 20 of these putative planetary nurseries.
The study is based on newly acquired high-resolution maps obtained by ALMA (the Atacama Large Millimeter/Submillimeter Array), an interlinked set of radiotelescopes located in the Atacama Desert in the Chilean Andes. ALMA’s antennas are sensitive to the electromagnetic radiation that is emitted by dust particles, which falls in the submillimeter wavelength range. This makes it possible to map the density distribution of these particles in protoplanetary disks around nearby young stars. ALMA’s latest observations reveal the diversity of environments in which planets are born. The degree of resolution attained is unprecedented, and the maps show unexpectedly complex distributions of matter, and demonstrate that specific substructures – such as concentric gaps and thin rings – occur in almost all of the disks examined. Furthermore, the relative sizes of these features, as well as the distances between the disks and their respective parent stars, vary widely. According to the authors of the new study, these observations imply that planets form much more rapidly than current theories allow. The new findings are reported in several papers that appear in a special focus issue of Astrophysical Journal Letters.
Astrophysical Journal Letters 2018