An ALMA submillimeter-wavelength image unveils the dawn of planet formation around a surprisingly young star in unprecedented detail.

protoplanetary disk around HL Tauri
Planets are forming around HL Tauri, a young, variable star just 1 million years old. They’re leaving their imprints in the dusty disk leftover from the star’s formation, a protoplanetary system that spans 235 astronomical units (the distance between Earth and the Sun). The innermost disk gap appears at a radius of 20-30 a.u., roughly the size of Neptune’s orbit around the Sun. A second gap appears further out at 70 a.u., which would lie outside Pluto’s orbit, and still more gaps appear beyond that.
ALMA (NRAO/ESO/NAOJ) / C. Brogan / B. Saxton

Once the realm of theorist's and artist's imaginations, the process of planetary genesis has now been captured by ALMA, one of the world’s most powerful telescopes. And the astonishing image might revolutionize theories of how planets form.

The 66 dishes of the Atacama Large Millimeter/submillimeter Array (ALMA) can be placed as far as 16 kilometers (10 miles) apart, in effect combining their power into one 16-km-wide telescope. The result is crazy-fine detail at submillimeter wavelengths, a little-explored regime of the electromagnetic sky. The image above was using almost the full baseline; antennae were separated by 15 km.

ALMA has been fully operational since March 2013 but has continued to ramp up, adding antennae and possible configurations. Now the science team is testing the longest-baseline configurations, arrangements of the antennae that allow for the most detailed images ever recorded at submillimeter wavelengths.

In visible light, HL Tauri can’t even be seen — the newly formed star hides inside a cocoon of dust and gas. ALMA imaged the system at 1.3 mm (233 gigahertz) to cut through the dusty veil and reveal the planet-forming disk at its center. ALMA has imaged planet-forming gaps before, but never at this resolution: ALMA can make out details 35 milliarcseconds across in this image, the equivalent of 5 a.u. at HL Tauri's distance.

“These new observations really supersede any previous data on HL Tau,” says Laura Perez (NRAO). “In comparison with previous CARMA observations, this new ALMA image of HL Tau is 5 times better in spatial resolution and at least a factor of 20 more sensitive.”

Revolutionizing Planet Formation

The smallest gaps in the protoplanetary disk occur around 20 to 30 a.u. and near 70 a.u. Although no planets are detected at these wavelengths, the gaps are most likely the markers of a planet’s passage through the disk.

The discovery of disk gaps around a star less than 1 million years old is surprising — planets aren’t supposed to form so fast.

“It is early to do a comparison with theories; we are still doing the analysis,” says Leonardo Testi (University of Arcetri, Italy, and ESO), ALMA’s European Program Scientist. “But it’s clear that this is the youngest system where we see anything like this.”

In the core-accretion model, small dust grains collide and stick together, forming a small, rocky core that then begins collecting a gaseous envelope. But Testi says the core-accretion scenario, at least in its classical form, would have trouble building planets in less than 1 million years.

And there’s far more to this image than the gaps. “I am also super excited by the ripples,” Testi says. These undulations in density may hold clues on how dust grains stick together to form planets, a poorly understood process that’s key to the core-accretion scenario.

Technical analysis is just getting started, and once complete, the data will be made available to the worldwide scientific community. This is undoubtedly not the last you will hear about HL Tauri.

Image Fast Facts:

Date image taken: Between October 24th and 31st
Integration time: 4.5 hours
Wavelength / frequency: 1.3 mm / 233 GHz
Distance to HL Tauri: 450 light-years
Resolution: 35 milliarcseconds, which corresponds to 5 astronomical units at HL Tau’s distance.


Watch NRAO director Tony Beasley react to ALMA’s new image:

Read in-depth coverage on ALMA — including its capabilities and its promise to unveil our cosmic origins — in the November 2013 issue of Sky & Telescope.




Image of Peter Wilson

Peter Wilson

November 7, 2014 at 11:22 am

Can we get some clarification on the gapification? There are circular gaps between the rings (“grooves”?). There are gaps within the rings, rendering them incomplete. (“Voids”? There is a particularly prominent “void” in the 3rd ring, about 10 o’ clock.) Then there are arc-shaped gaps, or incomplete grooves, within the 2nd ring (“arcs”?).
Are astronomers looking for planets in the groovy, voidy, or arcky kind of gap?

You must be logged in to post a comment.

Image of Monica Young

Monica Young

November 7, 2014 at 5:32 pm

Peter, to my knowledge, astronomers think planets likely carved out the innermost two disk gaps shown in the image. As for everything else, I think all we can say for now is that it's fantastic detail - analysis is underway, and I'm sure we'll be hearing more soon about what created all these groovy, voidy, arc-y gaps.

You must be logged in to post a comment.

Image of Anthony Barreiro

Anthony Barreiro

November 7, 2014 at 2:40 pm

I would think the groovy gaps at 20-30 AU and 70 AU would be the likeliest places to look for planetesimals and protoplanets. I'm guessing that the more distant voids and arcs may be evidence of Oort-cloud-like comet formation.

By the way, this is a very exciting image! ALMA is an amazing instrument.

You must be logged in to post a comment.

Image of Antonio Mario

Antonio Mario

November 7, 2014 at 4:12 pm

@Peter: In the circular gaps, as the first paragraph of the 2nd section plus the figure caption suggest.

Hopefully it's clear to readers that, even though there is indeed essentially zero absorption by the cocoon dust in the sub-mm, what we're seeing at these wavelenghts is *emission* from the disk dust.

You must be logged in to post a comment.



November 7, 2014 at 4:52 pm

I thought that no planets were suppose to have formed past Saturn's orbit in our solar system with Uranus and Neptune later being thrown out, but this shows planets being formed at extremely large distances. How does this star compare to the sun in size to allow for this?

You must be logged in to post a comment.

Image of

November 8, 2014 at 12:36 pm

Your statement is probably true for OUR solar system with the inferred properties of the circumstellar disk out of which our planets formed 4.6 billion years ago. However, the properties of such disks have been observed to vary widely from star to star in terms of size and mass of available material (just as the properties of extrasolar planetary systems have been observed to vary widely). HL Tauri obviously has a disk which is much larger than the one the Sun sported long ago and will likely form large planets at distances much larger than those observed in our solar system.

You must be logged in to post a comment.

Image of Peter Rowen

Peter Rowen

November 7, 2014 at 4:55 pm

Saw on another website where someone had assumed a body orbiting in the the deepest "groove" and calculated the distances of various resonant orbits with that body. They matched up pretty well with the outer grooves, implying most or all may be Kirkwood gaps, but the inner groove matched up pretty poorly. Even that orbit is wider than the Sun-Jupiter distance, so clearly this system is still evolving.

You must be logged in to post a comment.

You must be logged in to post a comment.