Astronomers using the SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) instrument on ESO’s Very Large Telescope (VLT) have observed a protoplanetary disk around AB Aurigae, a very young star 531 light-years away in the constellation of Auriga, and spotted a prominent spiral structure with a ‘twist’ that marks the site where a planet may be forming.
This image from VLT’s SPHERE instrument shows the protoplanetary disk around AB Aurigae. Image credit: ESO / Boccaletti et al.
“Thousands of exoplanets have been identified so far, but little is known about how they form,” said lead author Dr. Anthony Boccaletti, an astronomer with the Observatoire de Paris.
“We need to observe very young systems to really capture the moment of planet formation. But until now astronomers had been unable to take sufficiently sharp and deep images of these young disks to find the ‘twist’ that marks the spot where a baby planet may be coming to existence.”
Observations of the AB Aurigae system made a few years ago with the Atacama Large Millimeter/submillimeter Array (ALMA) provided the first hints of ongoing planet formation.
In the ALMA images, astronomers spotted two spiral arms of dust and gas within in the inner region of the AB Aurigae disk.
Then, in 2019 and early 2020, Dr. Boccaletti and his colleagues from France, Taiwan, the United States and Belgium used the SPHERE instrument to perform imaging of AB Aurigae in polarized and unpolarized near-infrared light in order to study the morphology of its disk and search for signs of planet formation.
The astronomers confirmed the presence of the spiral arms first detected by ALMA.
“Spirals of this type signal the presence of baby planets, which ‘kick’ the gas, creating disturbances in the disk in the form of a wave, somewhat like the wake of a boat on a lake,” said co-author Dr. Emmanuel Di Folco, an astronomer in the Astrophysics Laboratory of Bordeaux.
“As the planet rotates around the central star, this wave gets shaped into a spiral arm.”
The protoplanetary disk around AB Aurigae: the image on the right is a zoomed-in version of the area indicated by a red square on the image on the left; it shows the inner region of the disk, including the very-bright-yellow ‘twist’ (circled in white) that scientists believe marks the spot where a planet is forming; this twist lies at about the same distance from AB Aurigae Neptune from the Sun; the blue circle represents the size of the orbit of Neptune. Image credit: ESO / Boccaletti et al.
The researchers also spotted another remarkable feature — the very bright yellow ‘twist’ in the inner region of the AB Aurigae disk.
The ‘twist’ lies at about the same distance from the star as Neptune from the Sun and is one of these disturbance sites where the astronomers believe a planet is being made.
“The twist is expected from some theoretical models of planet formation,” said co-author Dr. Anne Dutrey, also from the Astrophysics Laboratory of Bordeaux.
“It corresponds to the connection of two spirals — one winding inwards of the planet’s orbit, the other expanding outwards — which join at the planet location.”
“They allow gas and dust from the disk to accrete onto the forming planet and make it grow.”
The team’s work was published in the journal Astronomy & Astrophysics.
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A. Boccaletti et al. 2020. Possible evidence of ongoing planet formation in AB Aurigae. A showcase of the SPHERE/ALMA synergy. A&A 637, L5; doi: 10.1051/0004-6361/202038008
