Newborn stars don’t have enough dust to build planets. What are the missing ingredients?

Abstract

Building a planet is a mysterious process. New worlds emerge from the disk of leftover dust and gas that swirls around an infant star, but it’s still not clear how planets form or how quickly they grow. Astronomers have typically had difficulty peering into the dusty planet nurseries formed from newborn stars. But in the past few years, they’re starting to get glimpses—and important clues as to exactly how planets form. Image credit: Shutterstock/Neo Edmund. Now researchers have added another puzzling discovery to the mix. On average, these planet-forming disks appear to contain far less material than the planets they spawn, a recent study in Astronomy & Astrophysics suggests (1). “We expected the disks to be more massive than the planets that formed in them,” says astronomer Carlo Manara of the Garching, Germany-based European Southern Observatory, part of the team behind the study. “But the disks are smaller—less massive—than what is needed.” The cocoon of dust and gas around a newborn star mostly contains hydrogen molecules and helium along with traces of carbon, oxygen, and other elements. Once the star’s rotation has pulled this cocoon into a disk, planets have only about 3 million years to form before stellar winds blow off most of the hydrogen gas, leaving only dust behind. Somehow, “gas giant” planets such as Jupiter or Saturn must be created before that gas disappears. These dusty nurseries have long hidden nascent worlds from curious astronomers. But over the past few years, the Atacama Large Millimeter/submillimeter Array (ALMA), a group of radio telescopes on the Chajnantor plateau in northern Chile, has probed the disks’ millimeter-sized dust particles and measured their abundance with incredible precision. In 2014, ALMA captured images of a protoplanetary disk in unprecedented detail, and a few hundred more disks have been observed since then. Meanwhile, …