Hubble Finds a Planet Forming in an Unconventional Way – Watts Up With That?

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NASA’s Hubble Space Telescope has directly imaged evidence for the formation of a Jupiter-like protoplanet through what researchers describe as an “intense and violent process.” This discovery supports a long-debated theory about how planets like Jupiter form, called “disk instability.”

“Interpreting this system is extremely challenging. This is one of the reasons we needed Hubble for this project: a clean image to better separate light from the disk and any planets.”

Thayne Currie, principal investigator of the study

The new world under construction is embedded in a protoplanetary disk of dust and gas with a distinctive spiral structure that revolves around a young star estimated to be around 2 million years old. That’s about the age of our solar system when planet formation was underway. (The age of the solar system is currently 4.6 billion years.)

“Nature is smart; it can produce planets in a variety of different shapes,” said Thayne Currie of the Subaru Telescope and Eureka Scientific, principal investigator of the study.

All planets are made of material that originated in a circumstellar disk. The dominant theory for Jovian planet formation is called “core accretion,” a bottom-up approach in which disk-embedded planets grow from small objects, ranging in size from dust grains to boulders. , which collide and come together while orbiting a star. This core then slowly accumulates gas from the disk. In contrast, the disk instability approach is a top-down model in which as a massive disk around a star cools, gravity causes the disk to rapidly break apart into one or more planet-mass fragments. .

The newly formed planet, called AB Aurigae b, is probably about nine times more massive than Jupiter and orbits its host star at a distance of 8.6 billion miles, more than twice as far as Pluto is from our Sun. At that distance, it would take long, if ever, for a Jupiter-sized planet to form by accretion of the core. This leads the researchers to conclude that disk instability has allowed this planet to form at such a great distance. And it contrasts sharply with expectations of planet formation under the widely accepted core accretion model.

The new analysis combines data from two Hubble instruments: the Space Telescope Imaging Spectrograph and Near-Infrared Camera and Multi-Object Spectrograph. These data were compared to that from a state-of-the-art planet imaging instrument called SCExAO on Japan’s Subaru 8.2-meter Telescope located on the summit of Mauna Kea, Hawaii. The large amount of data from space and ground-based telescopes was essential, because it is very difficult to distinguish between young planets and complex features in the disk that are not related to planets.

The researchers were able to directly image the forming exoplanet AB Aurigae b over a 13-year period using Hubble’s Space Telescope Imaging Spectrograph (STIS) and its Near-Infrared Camera and Multiple-Object Spectrograph (NICMOS). At upper right, the Hubble NICMOS image captured in 2007 shows AB Aurigae b in a southerly position compared to its host star, which is covered by the instrument’s coronagraph. The image captured in 2021 by STIS shows that the protoplanet has moved counterclockwise over time. Credits: Science: NASA, ESA, Thayne Currie (Subaru Telescope, Eureka Scientific Inc.); Image processing: Thayne Currie (Subaru Telescope, Eureka Scientific Inc.), Alyssa Pagan (STScI)

“Interpreting this system is extremely challenging,” Currie said. “This is one of the reasons we needed Hubble for this project: a clean image to better separate light from the disk and from any planets.”

Nature itself helped us, too: The vast disk of dust and gas that revolves around the star AB Aurigae is tilted almost squarely in front of our view from Earth.

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 archive data from Hubble, combined with images from Subaru, proved to be a turning point in changing their minds.

“We couldn’t detect this movement on the order of a year or two,” Currie said. “Hubble provided a time baseline, combined with data from Subaru, 13 years old, that was enough to be able to detect orbital motion.”

“This result leverages ground-based and space-based observations and we can go back in time with Hubble’s archival observations,” added Olivier Guyon of the University of Arizona, Tucson, and the Subaru Telescope, Hawaii. “AB Aurigae b has now been analyzed at multiple wavelengths and a consistent picture has emerged, one that is very robust.”

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,” said Alan Boss of the Carnegie Institution of Science in Washington, DC. “In the end, gravity is all that counts, as the leftovers from the star-forming process will end up being pulled by gravity to form planets, one way or another.”

Understanding the early days of the formation of Jupiter-like planets provides astronomers with more context about the history of our own solar system. This discovery paves the way for future studies of the chemical composition of protoplanetary disks like AB Aurigae, including with NASA’s James Webb Space Telescope.

The Hubble Space Telescope is an international cooperative project between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, DC

Illustration credit: NASA, ESA, Joseph Olmsted (STScI)

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