The Twist Marks the Spot: ESO Telescope Sees Signs of Planet Birth

(ASIAA Science Highlight posted on 21st. May, 2020)

New observations made with the European Southern Observatory’s Very Large Telescope (ESO’s VLT) have revealed today the telltale signs of a star system being born. Around the young star AB Aurigae lies a dense disk of dust and gas in which astronomers led by Ya-Wen Tang from ASIAA in 2017 have spotted two prominent spiral structures. The newly reported ‘twist’ further confirms the site where a planet may be forming. These features could be the first direct evidence of baby planets coming to existence.

“There are thousands of exoplanets identified so far, but little is known about how they form,” says Anthony Boccaletti who led the study from the Paris Observatory in Meudon, France. Astronomers know planets are born in dusty disks surrounding young stars, like AB Aurigae, as cold gas and dust clump together. The new observations with ESO’s VLT, published in Astronomy & Astrophysics, provide crucial clues to help scientists better understand this process.

“We need to observe very young systems to really capture the moment when planets form. But until now astronomers had rarely been able to take sharp and deep enough images of these young disks to directly discern regions of planetary formation,” says Ya-Wen Tang from ASIAA, Taiwan.

The new images feature a stunning spiral of dust and gas around AB Aurigae, located 520 light-years away from Earth in the constellation of Auriga (The Charioteer). Spirals of this type signal the presence of baby planets, which ‘kick’ the gas, creating disturbances in the disc in the form of a wave, somewhat like the wake of a boat on a lake . As the planet rotates around the central star, this wave gets shaped into a spiral arm. The very bright yellow ‘twist’ region close to the centre of the new AB Aurigae image (https://www.eso.org/public/images/eso2008d), which lies at about the same distance from the star as Neptune from the Sun, is one of these disturbance sites where the team believe a planet is being made.

This image shows the inner region of the disc around the young AB Aurigae star, where ESO’s Very Large Telescope has spotted signs of planet birth. The ‘twist’ (in dashed-white circle) marks the spot where a planet may be forming. This twist lies at about the same distance from the AB Aurigae star as Neptune from the Sun.

The image was obtained with the VLT’s SPHERE instrument in polarised light.

Credit: ESO/Boccaletti et al.

The images of the AB Aurigae system showing the disc around it. 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 disc, including the ‘twist’ (in very bright yellow) that scientists believe marks the spot where a planet is forming. This twist lies at about the same distance from the AB Aurigae star as Neptune from the Sun. The blue circle represents the size of the orbit of Neptune.

The images were obtained with the SPHERE instrument on ESO’s Very Large Telescope in polarised light.

Credit: ESO/Boccaletti et al.

Observations of the AB Aurigae system done a few years ago with the Atacama Large Millimeter/submillimeter Array (ALMA), in which ESO is a partner, provided the first hints for the ongoing planet formation around the star. In the ALMA images, scientists led by Ya-Wen Tang in 2017 spotted two spiral arms of gas close to the star, lying within the disk’s inner region. Then, in 2019 and early 2020, Boccaletti and a team of astronomers from France, Taiwan, the US and Belgium set out to capture a clearer picture by turning the SPHERE instrument on ESO’s VLT in Chile toward the star. The SPHERE images are the deepest taken of the AB Aurigae system to date.

By superimposing images of SPHERE and ALMA, the video visually marks out the locations of spiral arms and the newly spotted "twist".

Credit: ESO/Boccaletti et al., ALMA (ESO/NAOJ/NRAO)/Tang et al., ASIAA/Lauren Huang

With SPHERE’s powerful imaging system, astronomers could see the fainter light from small dust grains and emissions coming from the inner disc. They confirmed the presence of the spiral arms first detected by ALMA and also spotted another remarkable feature, a ‘twist,’ that points to the presence of ongoing planet formation in the disk. The twist is expected from some theoretical models of planetary formation. 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.

ESO is constructing the 39-metre Extremely Large Telescope, which will draw on the cutting-edge work of ALMA and SPHERE to study extrasolar worlds. As Boccaletti explains, this powerful telescope will allow astronomers to get even more detailed views of planets in the making. “We should be able to see directly and more precisely how the dynamics of the gas contributes to the formation of planets,” he concludes.

This research was presented in the paper “Are we witnessing ongoing planet formation in AB Aurigae? A showcase of the SPHERE/ALMA synergy” appears in Astronomy & Astrophysics.

The team is composed of A. Boccaletti (LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, CNRS, France), E. Di Folco (Laboratoire d’Astrophysique de Bordeaux, Université de Bordeaux, CNRS, France [Bordeaux]), E. Pantin (Laboratoire CEA, IRFU/DAp, AIM, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, CNRS, France), A. Dutrey (Bordeaux), S. Guilloteau (Bordeaux), Y. W. Tang (Academia Sinica, Institute of Astronomy and Astrophysics, Taipei, Taiwan), V. Piétu (IRAM, Domaine Universitaire, France), E. Habart (Institut d’astrophysique spatiale, CNRS UMR 8617, Université Paris-Sud 11, France), J. Milli (CNRS, IPAG, Univ. Grenoble Alpes, France), T. L. Beck (Space Telescope Science Institute, Baltimore, MD, USA), and A.-L. Maire (STAR Institute, Université de Liège, Belgium).

Links to Further Information:

2020.05.20 ESO Image Release of AB Aurigae: https://www.eso.org/public/news/eso2008/

2017.05.15 ASIAA Science Highlight: Spirals inside a dust gap of a young star forming disk

ALMA: https://www.eso.org/public/teles-instr/alma

VLT: https://www.eso.org/public/teles-instr/paranal-observatory/vlt

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