Now this is extraordinary. It's the sharpest picture ever made of a protoplanetary disc surrounding a young star. The image, which bears a striking resemblance to prior artistic impressions, is set to revolutionize our understanding of how planets form.

The new image was made possible by ALMA, the Atacama Large Millimeter/submillimeter Array. And as the name of the telescope implies, this picture was made at submillimeter wavelengths. It's also one of ALMA's first observations to be made at its near-final configuration, as this remarkable device is still being calibrated. The new image was taken as part of the testing and verification process for the telescope — so the best is still yet to come.

ALMA (NRAO/ESO/NAOJ); C. Brogan, B. Saxton (NRAO/AUI/NSF).

To make this observation, ALMA was pointed at HL Tauri, a young star located about 450 light-years away. The image reveals unexpectedly fine detail in the disc of material left over from star birth. You can actually see a series of concentric bright rings separated by gaps. There's almost an eerie resemblance to Saturn's rings.


ALMA/Hubble composite image of the region around the young star HL Tauri. ALMA (ESO/NAOJ/NRAO), ESA/Hubble and NASA/Judy Schmidt.

"When we first saw this image we were astounded at the spectacular level of detail," noted ALMA team member Catherine Vlahakis. "HL Tauri is no more than a million years old, yet already its disc appears to be full of forming planets. This one image alone will revolutionize theories of planet formation."

Indeed, given just how young this star is, this protoplanetary disc wasn't supposed to be so developed. This ALMA image suggests that the planet-formation process happens faster than we thought.


Artistic impression of a protoplanetary disc — before the ALMA image. Credit: University of Copenhagen/Lars Buchhave.

Stars like HL Tauri emerge from clouds of gas and fine dust, in regions which have collapsed under the effects of gravity. Eventually, dense hot cores form and ignite into stars. At first, these young stars are enveloped in the remaining gas and dust, which eventually settles into a disc, or protoplanetary disc. Over time, the dust particles will stick together, growing into clumps the size of sand grains and pebbles. They eventually grow into asteroids, comets and planets. Young planets will disrupt the disc and create rings — just like those gaps and holes that can be seen in the new ALMA image.


"Most of what we know about planet formation today is based on theory," said ESO Director General Tim de Zeeuw. "Images with this level of detail have up to now been relegated to computer simulations or artist's impressions. This high resolution image of HL Tauri demonstrates what ALMA can achieve when it operates in its largest configuration and starts a new era in our exploration of the formation of stars and planets."

Now all this said, those gaps may not be planets.

We have gaps in the dust of our own solar system from Jupiter resonance. Orbital resonance is just another form of natural resonance frequencies. You can see examples of it in everything from the gaps in the rings of Saturn to the orbital distance of moons around Jupiter to why large bodies in our solar system don't share any other large objects in the same orbits.

Astrophysicist Alex Parker has a nice set of images for our solar system of the gaps caused by Jupiter resonance:

He then took the HL Tauri system and drew in the locations of the most important orbital resonances that would result from there being a planet in the most prominent gap.

(The entire thread leading off from that Tweet is pretty awesome, with astrophysicists debating the rate of planetary formation, the lack of spiral gravity waves, and other such technical arcana.)

Overall, this is a totally incredible image, but be careful not to interpret it as each and every gap means a planet. Similarly, the apparent spokes could easily be imaging artifacts as opposed to planetary shadows.

Additional reporting by Mika McKinnon.

[ ESO | NRAO ]

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