A twisted, swirling cloud of dust and gas more than 530 light-years away is not just a tumultuous wonder. It's a new puzzle piece about how planets evolve from tiny grains to giant globes.
Astronomers have obtained unusual new protoplanetary disk near-infrared images around the young star AB Aurigae. These images display dust-coming planets cause spiraling disturbances, which the researchers assume.
"In the early stage of planet formation, hydrodynamical simulations indicate that the accretion process generates an inner and outer spiral pattern at the planet location due to Lindblad resonances induced by disc-planet interactions," the researchers wrote.
"While this crucial step is well documented by theoretical works, observational [pieces of evidence] are rare and not fully conclusive."
The research has been published in Astronomy & Astrophysics.
Best photos just yet
The planetary formation is a particularly fascinating process. First, a star must form by spooling a giant disk of dust and gas that feeds into it. When this is done, astronomers think that the remaining disc begins to clump together to form other chunky bits found in planetary systems.
The electrostatic forces bring together tiny clumps of cold material. Then, as these clumps grow in size, they begin to generate sufficient gravity to attract more clusters, creating a dense, compact object.
The orbits of dust particles around the forming planet become disrupted during this process. The shape of their orbit grows elliptical, causing an oscillation between their nearest and farthest points.
If this oscillation is a multiple of the orbital period of the particle, it creates a resonance that should generate a spiral pattern-called a Lindblad resonance.
The spiral arms of massive galaxies are responsible for this resonance, astronomers believe. But what's written significant in the Universe is often written small too. In the rings of Saturn, the astronomers have seen the same physics at play. It should also be observed around the formation of planets.
Getting data isn't easy though
Except that protoplanetary discs aren't easy to watch. They are far away, and sometimes their star's light is so bright that it overshadows tiny features that might expose processes of planetary formation.
AB Aurigae gets into the picture here. It is one of its nearest stars - a young, under 10 million years old, and surrounded by a dense protoplanetary disk. Observations with the Atacama Large Millimeter / submillimeter Array (ALMA) showed rough spiral shapes in 2017, which might be sought after signatures of the formation of planets.
So, for a closer look, an international team of astronomers came in. Using the SPHERE facility attached to the Very Large Telescope in Chile, in December 2019 and January 2020, they made high-contrast observations of AB Aurigae in Near-Infrared.
These resulted in the darkest images of the star that we have seen, catching fainter light from tiny dust grains. These revealed an S-shaped disturbance in the protoplanetary disk that looks very much like the spiral density waves that astronomers would expect to see.
"The twist is expected from some theoretical models of planet formation," said astronomer Anne Dutrey of the Astrophysics Laboratory of Bordeaux in France.
"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 disc to accrete onto the forming planet and make it grow."
This putative protoplanet appears to be forming roughly equivalent to Neptune's distance from the Sun at a distance from her star. The exact size is difficult to gauge. However, based on a previous calculation of the accretion rates, the team estimates that it clocks around 4 and 13 times the Jupiter mass.
It is still not a fully confirmed outcome. But it does indicate that AB Aurigae is a promising candidate for follow-up observations with more powerful currently under construction telescopes.
These could confirm that what we are looking at is indeed a giant planet in the formation process, and more precisely, calculate its mass.
