8 ASTRONOMY • FEBRUARY 2022
QUA N T U M GR AVIT Y
Some 1,300 light-years away
in the constellation Orion
the Hunter is the triple-star system
GW Orionis. This trio of young stars is
only about 1 million years old and still
surrounded by a disk of dust and gas
left over from their formation. This so-
called protoplanetary disk is organized
into three concentric rings, none of
which are aligned with any of the
orbits of the three stars. Additionally,
the innermost ring is out of alignment
with the outer two rings; it also tilts
and wobbles as it orbits. And there’s a
large gap between the inner and middle
rings, indicating the material there has
been cleared out.
A paper published Sept. 17
in Monthly Notices of the Royal
Astronomical Society suggests the
reason for that last oddity is a so-called
circumtriple planet (or planets) forming
within the disk, orbiting all three starsat once and clearing out material to
form the gap.MAKING SPACE
Gaps in protoplanetary disks often
indicate where planets are forming
within them. As a planet pulls in
nearby gas and dust to grow, it clears
out its surroundings. But in this case,
researchers weren’t sure whether the
disk’s behavior should be attributed
to a f ledgling planet or the three stars
whirling in a complex dance at its
center.
They found, based on 3D modeling,
that the stars’ gravity can’t produce
enough torque to create the observed
gap. Instead, the researchers say, it is
likely due to at least one Jupiter-sizedplanet forming there. If confirmed,
it would be the first exoplanet found
orbiting three stars. (Although planets
have been found in nearly 30 triple
systems to date, none of these other
worlds orbit all three stars.)
The supposed planet orbits about
100 astronomical units (AU) from the
center, where 1 AU is the average Earth-
Sun distance. The stars themselves
are much closer: Two orbit each other
separated by just 1 AU, while the third
orbits both of these some 8 AU from
the system’s center.
Because this research only provides
indirect evidence for the planet, the
next step is actually spotting the
strange world. Unfortunately, the
authors conclude, that’s tricky in
a system this complex. Still, more
observations are coming down the pike,
which may yet reveal a glimpse of a
truly unique young world. — ALISON KLESMANAN EXOPLANET ORBITING
THREE STARS?
Astronomers may have found the first
planet that orbits a trio of suns.
COMPLEX RINGS. The image at left, taken
with the ALMA radio array, shows rings and
gaps in the massive disk around GW Orionis.
At right, a SPHERE image shows how the
disk’s inner region is warped and twisted;
the dark spot at center is the shadow of the
inner ring. ALMA (ESO/NAOJ/NRAO), ESO/EXETER/KRAUS ET AL.