ALMA image of the young star HL Tau and its protoplanetary disk. This best image ever of planet formation reveals multiple rings and gaps that herald the presence of emerging planets as they sweep their orbits clear of dust and gas. Credit: ALMA (NRAO/ESO/NAOJ); C. Brogan, B. Saxton (NRAO/AUI/NSF)
Stars form within clouds of gas and dust as they collapse under gravity. Over time, the surrounding dust particles acquire icy mantles which stick them together, forming icy snowballs which gradually grow to form larger-size rocks. Due to the rotation of the gas around the newly forming star, the gas and dust is flung out into a thin “protoplanetary” disk where asteroids, comets, and planets form.
Here gravity collects the protoplanets together into clumps which grow larger, sweeping up all the other dust close to them as they orbit the new star. Once these planets leave gaps in the disc, seen as dark rings, and collect the dust and gas into tighter and more confined zones.
Astronomers testing new high-resolution capabilities at the Atacama Large Millimeter/submillimeter Array (ALMA) have recently captured the best image ever of planet formation around an infant star. This revolutionary new image reveals in astonishing detail the planet-forming disk surrounding HL Tau, a Sun-like star located approximately 450 light-years from Earth in the constellation Taurus. Other astronomers had already discovered this system, but HL Tau is not observable in visible light because of the huge envelope of dust and gas which surrounds it.
But ALMA observes at much longer wavelengths than visible light, so it is able to see through the intervening dust to study the processes right at the core of this cloud. ALMA revealed never-before-seen features in this system, including multiple concentric rings separated by clearly defined gaps. These structures suggest that planet formation is already well underway around this remarkably young star.
“This is truly one of the most remarkable images ever seen at these wavelengths. The level of detail is so exquisite that it’s even more impressive than many optical images. The fact that we can see planets being born will help us understand not only how planets form around other stars but also the origin of our own Solar System,” said NRAO astronomer Crystal Brogan.
“These features are almost certainly the result of young planet-like bodies that are being formed in the disk,” said ALMA Deputy Director Stuartt Corder. “This is surprising since HL Tau is no more than a million years old and such young stars are not expected to have large planetary bodies capable of producing the structures we see in this image.”
“This new and unexpected result provides an incredible view of the process of planet formation. Such clarity is essential to understand how our own Solar System came to be and how planets form throughout the Universe,” said Tony Beasley, director of the National Radio Astronomy Observatory (NRAO) in Charlottesville, Virginia, which manages ALMA operations for astronomers in North America.
National Radio Astronomy Observatory