![]() After that point, most of the gas and dust from the initial circumstellar disk has cleared. The protoplanetary disk phase lasts for several million years. Nearly all stars that are younger than a few million years are surrounded by disks that most likely harbor a zoo of new planetary systems. The baby protoplanets orbit within the disk and continue accumulating material, carving out gaps in the disk in a game of planetary Pac-Man. As seeds start to collide and stick together, they grow larger and larger until they have enough gravity to start attracting more material through a process known as accretion. Rock, metal and ice condense out of the disk to form planetary seeds. These young, massive Frisbees are “protoplanetary” because we think this is where planets are actively forming. For a star like the sun, that amounts to a disk with roughly 100 times the mass of Jupiter. When stars are still very young (only a few million years old), their circumstellar disks are relatively huge, often with about 1 to 10 percent of the mass of the central star in a typical system. Astronomers call these collections of dust and gas “circumstellar” (meaning “around stars”) disks. At the same time, the initial rotation of the collapsing core and the conservation of angular momentum naturally form a disk surrounding the newly born star. ![]() When these spots, or “cores,” become dense enough, they start to collapse under their own gravity to make stars. The typical density of empty space is only one atom per cubic centimeter, but the thickest areas of molecular clouds can reach densities 10,000 to one million times this norm. Stars form out of vast regions of gas and dust called molecular clouds. ALMA has taken several hundred planetary baby pictures, helping us to build a new view of how such systems form and revealing troves of planets we never could have detected otherwise. Meanwhile the study of circumstellar disks and planet formation has exploded. Since that early image, the capabilities of ALMA have continued to expand, and the array now has new dishes, higher resolution and more wavelength coverage. Now we have an unprecedented opportunity to see how the material in the disk evolved and interacted with the newly formed planets. Other scientists have recently discovered two planets in the system: one about the mass of Jupiter and the other about the mass of Saturn, both orbiting fairly close to their star. We think this band is the rubble left over after planets formed around AU Mic, a young M dwarf star about 32 light-years away. The other two bright spots marked the edges of a disk of debris circling the central star, akin to the Kuiper Belt that orbits our sun. The central spot was actually the star, which we now know is flaring, sending bursts of high-energy particles out into space. What we were glimpsing was a solar system growing up. I can still remember the anticipation, the butterfly feeling in my stomach as I waited for the data download and, when it was finally ready, the awe when the image appeared on my computer screen-a long, thin blob of light with three bright spots: one in the center and two on either side at the edges. ![]() Then we waited patiently (or, more often, impatiently) for our observations to be scheduled and completed. Modern astronomy is often done at a distance: rather than spending long nights at the remote mountain observatory, all we had to do was submit a computer script that told the telescope what to observe and when. ![]() It took another year for the data to be delivered. ![]() Dust and rubble might not sound that exciting, but they are the raw materials planets are made of, and this observatory was giving us a chance to see the process in action. The subject of our observations was something scientists had never seen in such detail before ALMA was built. I leaped at the opportunity to join one of its first projects-a study of a disk of dust and rubble around a nearby star called AU Mic. With this extreme resolution, ALMA can see deeper and farther in millimeter- and submillimeter-wavelength light than any previous telescope. This groundbreaking facility uses dozens of radio antennas working in concert to create images as detailed as those made by a single telescope 16 kilometers wide. The week I started graduate school, the first science projects were announced for the new Atacama Large Millimeter/submillimeter Array (ALMA) telescope in Chile. ![]()
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