The researchers used new and archival data from the Hubble Space Telescope and the Subaru Telescope to confirm that these protoplanets formed through an intense and violent process called disk instability. Disc instability is a top-down approach, and is very different from the dominant primary accretion model. In this scenario, the large disk around the star cools, and gravity causes the disk to rapidly break apart into one or more pieces of the planet's mass. AB Aurigae b is estimated to have a mass about nine times the mass of Jupiter and orbits its parent star at a distance of twice Pluto from our sun. 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 about 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 orbiting around it, surrounding a young star estimated to be about two million years old. It is about the age of our solar system when planet formation was taking place. (The current solar system is 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
All planets are made of material derived from the disk of the star. The dominant theory of Jovian planet formation is called “core accretion,” a bottom-up approach in which planets embedded in disks grow from tiny objects—ranging in size from dust grains to rock—and collide and stick together as they orbit a star. . This gas core then slowly accumulates from the disk. In contrast, the disk instability approach is a top-down model in which as the large disk around the 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 as massive as Jupiter and orbits its parent star at a distance of 8.6 billion miles – more than twice the distance of Pluto from our sun. At this distance, it would have taken a very long time, if it were, for a planet the size of Jupiter to form by primary accretion. This led the researchers to conclude that the disk's instability allowed the planet to form at great distances. This is in stark contrast to the widely accepted predictions of planet formation by core accretion models.
This new analysis combines data from two Hubble instruments: the Space Telescope's Imaging Spectrometer and the Near Infrared Camera and Multi-Object Spectrograph. This data was compared with that obtained from a new planetary imaging instrument called SCExAO on Japan's 8.2-meter Subaru telescope located atop Mauna Kea, Hawaii. The wealth of data from space and ground-based telescopes has proven to be very important, because it is very difficult to distinguish minor planets from complex disk properties that have nothing to do with planets.
"Explanation of this system is very difficult," said Corey. "That's one of the reasons we needed Hubble for this project — clean images to better separate light from any disk and planet."
Nature itself is also helping: the giant disk of dust and gas orbiting the star AB Aurigae is tilted almost to our view from Earth. Curie stressed that Hubble's longevity played a special role in helping researchers measure the orbits of protoplanets. He was initially very skeptical that AB Aurigae b was a planet. Archival data from Hubble, combined with imagery 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 provides a time base, together with Subaru data, of 13 years, which is sufficient to detect orbital motion."
"These results enhance both ground-based and space-based observations, and we're going back in time with the Hubble archive observations," added Olivier Guyon of the University of Arizona, Tucson, and the Subaru Telescope, Hawaii. “AB Aurigae b is now viewed at multiple wavelengths, and a consistent image emerges – a very solid image.”
"This new discovery is strong evidence that some gas giant planets could have formed through disk instability mechanisms," said Alan Buss of the Carnegie Institution for Science in Washington, DC. "Ultimately, gravity is all that matters, because the remnants of the star formation process will end up together by gravity to form planets, one way or another."
Understanding the early days of the formation of Jupiter-like planets gives astronomers 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.
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