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| This is an artist's illustration of a massive, newly forming exoplanet called AB Aurigae b. |
NASA's Hubble Space Telescope has directly photographed evidence of a
Jupiter-like protoplanet forming through what researchers describe as an
"intense and violent process." This discovery supports a long-debated theory
for how planets like Jupiter form, called "disk instability."
The new world under construction is embedded in a protoplanetary disk of
dust and gas with distinct spiral structure swirling around, surrounding a
young star that's estimated to be around 2 million years old. That's about
the age of our solar system when planet formation was underway. (The solar
system's age is currently 4.6 billion years.)
"Nature is clever; it can produce planets in a range of different ways,"
said Thayne Currie of the Subaru Telescope and Eureka Scientific, lead
researcher on the study.
All planets are made from material that originated in a circumstellar disk.
The dominant theory for Jovian planet formation is called "core accretion,"
a bottom-up approach where planets embedded in the disk grow from small
objects—with sizes ranging from dust grains to boulders—colliding and
sticking together as they orbit a star. This core then slowly accumulates
gas from the disk. In contrast, the disk instability approach is a top-down
model where as a massive disk around a star cools, gravity causes the disk
to rapidly break up into one or more planet-mass fragments.
The newly forming planet, called AB Aurigae b, is probably about nine times
more massive than Jupiter and orbits its host star at a whopping distance of
8.6 billion miles—over two times farther than Pluto is from our Sun. At that
distance it would take a very long time, if ever, for a Jupiter-sized planet
to form by core accretion. This leads researchers to conclude that the disk
instability has enabled this planet to form at such a great distance. And,
it is in a striking contrast to expectations of planet formation by the
widely accepted core accretion model.
The new analysis combines data from two Hubble instruments: the Space
Telescope Imaging Spectrograph and the Near Infrared Camera and Multi-Object
Spectrograph. These data were compared to those from a state-of-the-art
planet imaging instrument called SCExAO on Japan's 8.2-meter Subaru
Telescope located at the summit of Mauna Kea, Hawaii. The wealth of data
from space and ground-based telescopes proved critical, because
distinguishing between infant planets and complex disk features unrelated to
planets is very difficult.
"Interpreting this system is extremely challenging," Currie said. "This is
one of the reasons why we needed Hubble for this project—a clean image to
better separate the light from the disk and any planet."
Nature itself also provided a helping hand: the vast disk of dust and gas
swirling around the star AB Aurigae is tilted nearly face-on to our view
from Earth.
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| Image of the star AB Aurigae taken by the Subaru Telescope showing the spiral arms in the disk and the newly-discovered protoplanet AB Aur b. The bright central star has been masked, and its location is indicated by the star mark (☆). The size of Neptune's orbit in the Solar System is shown to provide scale. Credit: T. Currie/Subaru Telescope |
Currie emphasized that Hubble's longevity played a particular role in
helping researchers measure the protoplanet's orbit. He was originally very
skeptical that AB Aurigae b was a planet. The archival data from Hubble,
combined with imaging from Subaru, proved to be a turning point in changing
his mind.
"We could not detect this motion on the order of a year or two years,"
Currie said. "Hubble provided a time baseline, combined with Subaru data, of
13 years, which was sufficient to be able to detect orbital motion."
"This result leverages ground and space observations and we get to go back
in time with Hubble archival observations," Olivier Guyon of the University
of Arizona, Tucson, and Subaru Telescope, Hawaii added. "AB Aurigae b has
now been looked at in multiple wavelengths, and a consistent picture has
emerged—one that's very solid."
The team's results are published in the April 4 issue of Nature Astronomy.
"This new discovery is strong evidence that some gas giant planets can form
by the disk instability mechanism," Alan Boss of the Carnegie Institution of
Science in Washington, D.C. emphasized. "In the end, gravity is all that
counts, as the leftovers of the star-formation process will end up being
pulled together by gravity to form planets, one way or the other."
Understanding the early days of the formation of Jupiter-like planets
provides astronomers with more context into the history of our own solar
system. This discovery paves the way for future studies of the chemical
make-up of protoplanetary disks like AB Aurigae, including with NASA's James
Webb Space Telescope.
Reference:
Thayne Currie, Images of embedded Jovian planet formation at a wide separation
around AB Aurigae, Nature Astronomy (2022).
DOI: 10.1038/s41550-022-01634-x.
Tags:
Space & Astrophysics