Hubble discovers a protoplanet that could overturn models of planet formation

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

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

“Nature is intelligent, it can produce planets in many different ways,” said Thayne Currie of the Subaru Telescope and Eureka Scientific, the study’s lead author.

All planets are made of material that originated in a stellar disk. The dominant theory of Jovian planet formation is called “core accretion,” a bottom-up approach in which planets embedded in the disk grow out of tiny bodies—ranging in size from dust grains to rocks—and collide and stick together as they orbit a star. This gaseous core then slowly accumulates from the disc. By 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 disintegrate into one or more fragments of the planet’s mass.

The newly formed planet, called AB Aurigae b, is probably about nine times larger than Jupiter and orbits its host star at a whopping 8.6 billion miles away — more than twice the distance of Pluto from our sun. At this distance, it would take a very long time, if that happens, for a planet the size of Jupiter to form by primary accretion. This leads the researchers to conclude that the instability of the disk enabled this planet to form at such a great distance. It contrasts starkly with predictions of planet formation by the widely accepted core accretion model.

The new analysis combines data from two Hubble instruments: the Space Telescope Imaging Spectrometer and the Near Infrared Camera and Multi-Object Spectrograph. This data was compared with that obtained from the latest planetary 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 has proven to be crucial, because distinguishing minor planets from complex disk properties that have nothing to do with planets is very difficult.

“Explanation of this system is very difficult,” Corey said. “That’s one of the reasons we need Hubble for this project – a clean image to better separate light from the disk and any planet.”

Nature itself lent a helping hand, too: the massive disk of dust and gas orbiting the star AB Aurigae tilted almost head-on to our view from Earth.

Curie emphasized that Hubble’s longevity played a special role in helping researchers measure the orbit of the protoplanet. He was originally very skeptical that AB Aurigae b was a planet. Archival data from Hubble, combined with imaging from Subaru, proved a turning point in his change of mind.

“We haven’t been able to detect this movement in a year or two,” Corey said. “Hubble provided a time baseline, along with the Subaru data, for 13 years, which was enough to be able to detect orbital motion.”

“This result enhances ground-based and space-based observations, and we’ll go back in time with archival Hubble observations,” added Olivier Guyon of the University of Arizona, Tucson, and Subaru Telescope, Hawaii. “AB Aurigae b was now looked at at multiple wavelengths, and a consistent picture emerged – a very solid picture.”

The team’s results were published in the April 4 issue of natural astronomy.

“This new discovery is strong evidence that some gas giant planets can form through the mechanism of disk instability,” said Alan Buss of the Carnegie Institution for Science in Washington, DC. “Ultimately, gravity is all that matters, as the remnants of the star-forming process will end up coming together by gravity to form planets, one way or another.”

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