Gaps in disks of dust around young stars point to unseen planets sweeping them clear. Theorists had not expected planets to form so quickly.
S. Andrews, et al./ALMA (ESO/NAOJ/NRAO); S. Dagnello/NRAO/AUI/NSF

Planets probably lurk in the gaps of these stellar disks. But how did they form so fast?

HL Tau, a mere stripling of a star no more than 1 million years old, was swaddled in a surprise. Four years ago, the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile revealed gaps in a bright disk of dust around HL Tau—apparently swept clean by unseen planets that had formed millions of years earlier than astronomers thought possible. But now, an ALMA survey of 20 disks around nearby young stars suggests the precocious planets around HL Tau are no anomalies—a result that will keep theorists busy for years.

“It’s spectacular,” says Joshua Winn of Princeton University. “We will never think about disks in the same way.”

Many theorists believe planets form by core accretion, which begins as dust particles collide and stick together. The clumps grow into pebbles, rocks, and, finally, large “cores” the size of small planets that can vacuum up remaining dust and gas through the pull of their gravity. The process is expected to be slow, taking millions of years to play out.

The new survey results, published in 10 papers last week in The Astrophysical Journal Letters, suggest a much tighter timescale. ALMA, an array of 64 movable dishes high in the Atacama Desert of northern Chile, captures the glow of dust particles at millimeter wavelengths, between the infrared and microwave bands. Once a year, the array is expanded to its widest extent, 15 kilometers across, to get the highest resolution. The survey team won a coveted “large program,” which gave it 65 hours of high-resolution observing time, enough to find and study disks around 20 young stars.

The team was rewarded with a clear view of gaps, even around stars as young as 300,000 years old. Equally puzzling, many gaps lay far from their stars—much farther than Neptune’s orbit—where a planet’s slow orbital motion would make it even tougher to sweep up dust and gas and form planets by core accretion.

An alternative model relying on unstable ripples or clumps in the disk that collapse under their own gravity can make planets faster, especially large ones in distant orbits. But Marco Tazzari, of the Institute of Astronomy at the University of Cambridge in the United Kingdom, notes that the survey found few spiral arms—signs of disk instabilities—in the disks. “There are many structures we cannot account for,” he says.

The new observations did reveal dense bands of material in the disks, which could alleviate one challenge for the core accretion model. Centimeter-size grains should experience drag from the surrounding gas and quickly fall in toward the star, depleting the disk of planet-forming material. But the dense bands could be acting as traps, Tazzari says, stopping grains from migrating inward and preserving them for planet growth.

The interpretations all assume unseen planets really are responsible for the gaps. But Roman Rafikov at Cambridge says they could have been created by pressure changes at snow lines, where gases such as water vapor freeze onto grains, or by magnetic fields in the disk, which can bunch up charged particles in bands. “What we see at work could be several mechanisms working simultaneously,” he says.

To untangle these issues, astronomers need additional observations. ALMA and certain radio telescopes could spy on gas in the disks, which makes up 99% of their mass. That would allow astronomers to see whether the same bands and gaps exist in the gas and understand how gas and dust interact.

“The smoking gun,” however, “would be finding the planets,” Rafikov says. Large optical telescopes have captured images of a handful of exoplanets in distant orbits around young stars, but only once the deed is done and the disk has already dispersed.

Still, Tazzari says, results like the ALMA survey are transforming the field. “It’s making planet formation an observational field, not just a theoretical one.”