Behind the Cover: August 2021
Each month, Physics Today editors explore the research and design choices that inspired the latest cover of the magazine.
The research: A protoplanetary disk is a rotating blob of dense gas, dust, rocks, and ice that surrounds a young star. The disk forms out of a molecular cloud of mostly hydrogen. When the cloud reaches a critical size, mass, or density, it collapses under its own gravity over about 100 000 years. As the collapsing cloud—known as a solar nebula—becomes denser, it flows in the direction of the nebula’s net angular momentum and flattens out into a pancake: the protoplanetary disk. The solids the disk inherits from the molecular cloud are initially small, perhaps a few microns across. Within a million years they grow by at least 10 orders of magnitude in a process thought to require the presence of localized density perturbations, or substructures in the disk. Those perturbations trap pebble-size particles and trigger a growth instability that converts them into much larger solids, including planets.
The cover: An artist’s impression depicts a protoplanetary disk surrounding a star. Material from the disk flows about the star’s magnetic field lines and is deposited on the star’s surface. When it hits the star, the material lights up in a flare that can be observed directly and as echoes once the light reaches and illuminates the inner disk. The time between the material’s flare and the resulting echo yields the radius of the inner disk. For details on new observations of substructures in the disk and how they enrich our understanding of planet formation, see the Physics Today article by Sean Andrews on page 36 . (Image courtesy of NASA/JPL-Caltech.)
Data adapted from refs. 6, 7, 14, and 15 in Andrews’s article .
The design: For the August cover, protoplanetary disks were an obvious choice because of their varied and stunning appearance and the ready availability of high-quality images. But the art team wanted to ensure that the cover complemented rather than detracted from the collection of experimental images featured in the article, some of which are shown above. The art and editorial teams decided that an artist’s rendering of a protoplanetary disk would be distinct and likely offer more visual interest. They found their favorites, and the tempestuous impression that made the cover was deemed the most relevant to Andrews’s article.