SCIENCE sciencemag.org 21 DECEMBER 2018 • VOL 362 ISSUE 6421 1337IMAGE: S. ANDREWS
ET AL
./ALMA (ESO/NAOJ/NRAO); S. DAGNELLO/NRAO/AUI/NSF
H
L 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 pos-
sible. 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 par-
ticles 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 ex-
pected 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 time-
scale. ALMA, an array of 64 movable dishes
high in the Atacama Desert of northernChile, captures the glow of dust particles
at millimeter wavelengths, between the in-
frared 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 or-
bits. But Marco Tazzari, of the Institute of
Astronomy at the University of Cambridge
in the United Kingdom, notes that the sur-
vey 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 accre-
tion model. Centimeter-size grains should
experience drag from the surrounding gas
and quickly fall in toward the star, deplet-ing 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 mag-
netic 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 mak-
ing planet formation an observational field,
not just a theoretical one.” jIN DEPTH
By Daniel CleryASTRONOMYHints of young planets puzzle theorists
Astronomical survey finds telltale gaps in dusty disks around newborn stars
Gaps in disks of dust around young stars point to unseen planets sweeping them clear. Theorists had not expected planets to form so quickly.Published by AAASon December 20, 2018^http://science.sciencemag.org/Downloaded from