Astronomers have captured something extraordinary: the first-ever direct photo of a baby planet growing inside a dusty ring around a young star.
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The WISPIT 2 system as seen by the Magellan Telescope in
Chile and the Large Binocular Telescope in Arizona. The protoplanet WISPIT 2b
appears as a purple dot in a dust-free gap between a bright, white dust ring
around the star and a fainter, outer ring, orbiting at about 56 times the
average distance between the Earth and the sun. The other potential planet,
CC1, appears as the red object inside the dust free cavity and is estimated to
be about 15 Earth-sun distances from its host star. Credit: Laird Close,
University of Arizona |
Using cutting-edge adaptive optics, the team detected the
glowing hydrogen gas streaming onto the infant world, essentially catching it
mid-birth.
First Detection of a Growing Exoplanet
Astronomers have, for the first time, identified a young
planet still in the process of growing beyond our solar system. The infant
world sits within a cleared-out gap inside a multi-ringed disk of dust and gas
that surrounds its host star.
The discovery was made by a team led by University of
Arizona astronomer Laird Close and Richelle van Capelleveen, a graduate student
at Leiden Observatory in the Netherlands. To spot this elusive planet, the
researchers used some of the most advanced adaptive optics instruments in the
world, including the University of Arizona’s MagAO-X system at the Magellan
Telescope in Chile, the Large Binocular Telescope in Arizona, and the European
Southern Observatory’s Very Large Telescope in Chile. Their findings appear in
The Astrophysical Journal Letters.
Astronomers have long studied dozens of these planet-forming
disks around young stars. Many show ring-like gaps that suggest hidden worlds
may be sweeping through them, clearing paths much like a snowplow pushes aside
snow. However, only about three young protoplanets had ever been directly
observed before, and all were found close to their stars, in the space between
the star itself and the edge of its surrounding disk. Until this breakthrough,
no protoplanets had been confirmed inside the dark, well-defined gaps that have
intrigued scientists for years.
A Longstanding Scientific Mystery Resolved
“Dozens of theory papers have been written about these
observed disk gaps being caused by protoplanets, but no one’s ever found a
definitive one until today,” said Close, professor of astronomy at the
University of Arizona. He calls the discovery a “big deal,” because the absence
of planet discoveries in places where they should be has prompted many in the
scientific community to invoke alternative explanations for the ring-and-gap
pattern found in many protoplanetary disks.
“It’s been a point of tension, actually, in the literature
and in astronomy in general, that we have these really dark gaps, but we cannot
detect the faint exoplanets in them,” he said. “Many have doubted that
protoplanets can make these gaps, but now we know that in fact, they can.”
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The University of Arizona-built MagAO-X instrument in the
clean room at the Magellan Telescope in Chile. Credit: Jared Males, University
of Arizona |
Lessons From Our Own Solar System’s Birth
4.5 billion years ago, our solar system began as just such a
disk. As dust coalesced into clumps, sucking up gas around them, the first
protoplanets began to form. How exactly this process unfolded, however, is
still largely a mystery. To find answers, astronomers have looked to other
planetary systems that are still in their infancy, known as planet-forming
disks, or protoplanetary disks.
Close’s team took advantage of an adaptive optics system,
one of the most formidable of its kind in the world, developed and built by
Close, Jared Males, and their students. Males is an associate astronomer at
Steward Observatory and the principal investigator of MagAO-X. MagAO-X, which
stands for “Magellan Adaptive Optics System eXtreme,” dramatically improves the
sharpness and resolution of telescope images by compensating for atmospheric
turbulence, the phenomenon that causes stars to flicker and blur, and is
dreaded by astronomers.
Probing Hidden Planets With Hydrogen Light
Suspecting there should be invisible planets hiding in the
gaps of protoplanetary disks, Close’s team surveyed all the disks with gaps and
probed them for a specific emission of visible light known as hydrogen alpha or
H-alpha.
“As planets form and grow, they suck in hydrogen gas from
their surroundings, and as that gas crashes down on them like a giant waterfall
coming from outer space and hits the surface, it creates extremely hot plasma,
which in turn, emits this particular H-alpha light signature,” Close explained.
“MagAO-X is specially designed to look for hydrogen gas falling onto young
protoplanets, and that’s how we can detect them.”
Discovery in the WISPIT-2 System
The team used the 6.5-meter Magellan Telescope and MagAO-X
to probe WISPIT-2, a disk van Capelleveen recently discovered using the VLT.
Viewed in H-alpha light, Close’s group struck gold. A dot of light appeared
inside the gap between two rings of the protoplanetary disk around the star. In
addition, the team observed a second candidate planet inside the “cavity”
between the star and the inner edge of the dust and gas disk.
“Once we turned on the adaptive optics system, the planet jumped right out at us,” said Close, who called this one of the more important discoveries in his career. “After combining two hours’ worth of images, it just popped out.”
Early Echoes of Jupiter and Saturn
According to Close, the planet, designated WISPIT 2b, is a
very rare example of a protoplanet in the process of accreting material onto
itself. Its host star, WISPIT 2 is similar to the sun and about the same mass.
The inner planet candidate, dubbed CC1, contains about nine Jupiter masses,
whereas the outer planet, WISPIT 2b, weighs in at about five Jupiter masses.
These masses were inferred, in part, from the thermal infrared light observed
by the University of Arizona’s 8.4-meter Large Binocular Telescope on Mount
Graham in Southeastern Arizona with the help of U of A astronomy graduate
student Gabriel Weible.
“It’s a bit like what our own Jupiter and Saturn would have
looked like when they were 5,000 times younger than they are now,” Weible said.
“The planets in the WISPIT-2 system appear to be about 10 times more massive
than our own gas giants and more spread out. But the overall appearance is
likely not so different from what a nearby ‘alien astronomer’ could have seen
in a ‘baby picture’ of our solar system taken 4.5 billion years ago.”
Mapping Orbits and Ringed Structures
“Our MagAO-X adaptive optics system is optimized like no
other to work well at the H-alpha wavelength, so you can separate the bright
starlight from the faint protoplanet,” Close said. “Around WISPIT 2 you likely
have two planets and four rings and four gaps. It’s an amazing system.”
CC1 might orbit at about 14-15 astronomical units – with one
AU equaling the average distance between the sun and Earth, which would place
it halfway between Saturn and Uranus, if it was part of our solar system,
according to Close. WISPIT-2b, the planet carving out the gap, is farther out
at about 56 AU, which in our own solar system, would put it well past the orbit
of Neptune, around the outer edge of the Kuiper Belt.
Confirming the Discovery in Infrared
A second paper published in parallel and led by van
Capelleveen and the University of Galway details the detection of the planet in
the infrared light spectrum and the discovery of the multi-ringed system with
the 8-meter VLT telescope’s SPHERE adaptive optics system.
“To see planets in the fleeting time of their youth,
astronomers have to find young disk systems, which are rare,” van Capelleveen
said, “because that’s the one time that they really are brighter and so
detectable. If the WISPIT-2 system was the age of our solar system and we used
the same technology to look at it, we’d see nothing. Everything would be too
cold and too dark.”