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Exoplanet sighting shows how gas giants can form far from their stars

Astronomers have observed a Jupiter-like planet forming around a young star in a way that we have never seen before

Space 4 April 2022

Nature Astronomy, doi:10.1038/s41550-022-01634-x Fig. 4 | SCExAO/CHARIS images of AB Aur at different wavelengths and observing modes. Left: polarized intensity wavelength-collapsed image obtained one day later. A pure scattered-light disk feature would have been detected at the position of AB Aur b (green circle). Instead this region shows no concentrated emission, indicating that AB Aur b is not detected. Right: emission at the approximate position of AB Aur b from VAMPIRES H? data using RDI/KLIP for PSF subtraction. From left to right, the intensity scaling is [0, 0.0925] mJy, [0, 0.055] mJy and [?0.007, 0.007] mJy, normalized to the source?s apparent FWHM. The x and y axes are in units of arcseconds east (along the x axis) and north (along the y axis).

Images of the star AB Aurigae at different wavelengths taken by the Subaru Telescope

SCExAO/CHARIS/Thayne Currie

A Jupiter-like planet has been observed forming around a young star, providing the first direct evidence for a hypothesis about how giant planets might form a long way from their planets.

Planets generally develop from a disc of dust and gas, called the protoplanetary disc, around a young star. Gas giants like Jupiter, with an orbit 5.2 times wider than Earth’s, are thought to have formed when solid particles in the disc collide and slowly snowball into a planet, in a process called core accretion.

Further away from the star, the disc isn’t dense enough for core accretion, so it is thought that giant planets with wider orbits must have formed in a different way, called gravitational instability. Because of the distance from the star, gas and dust cools and contracts into clumps that collapse under their own gravity to form the core of a planet.

Thayne Currie at the National Astronomical Observatory of Japan and his colleagues first spotted signs of a planet in its formation stages – also known as a protoplanet – orbiting a young 2-million-year-old star, AB Aurigae, in 2016 with the Subaru Telescope in Hawaii. The researchers continued observing the star and the protoplanet until 2021.


They determined that the protoplanet, known as AB Aur b, is around nine times heavier than Jupiter and orbits its host star at 93 times the distance from Earth to the sun. They also saw spirals of gas and dust collapsing quickly into the protoplanet, just as models of planet formation by gravitational instability predict. “It almost looks like a simulation,” says Currie.

AB Aur b is only the second protoplanet to have been directly imaged, and the youngest. The team hopes to gather more images at different wavelengths to better understand gas giant formation at its earliest stages.

“Nature is very clever: it doesn’t produce facsimiles of the solar system; it produces a wide range of different planetary systems,” says Currie. “This is one of the more bizarre types of system.”

“This is a very exciting discovery that once again reminds us how little we actually know about the first stages of planetary formation and evolution,” says Alejandro Suárez Mascareño at the Institute of Astrophysics of the Canary Islands in Spain. “While this result still can’t say which is the dominant mechanism for the formation of planets, it shows that, at least at large orbital distances, disc instability seems to be possible.”

Journal reference: Nature Astronomy, DOI: 10.1038/s41550-022-01634-x

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