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Exoplanets: ALMA reveals multi-planet birth ...

ALMA, the powerful new array of millimeter-wave dishes high in the Chilean Andes, has produced an extraordinarily detailed image of the protoplanetary disk around the newborn star HL Tauri, which reveals the disk's millimeter-wave glow. It is riddled with dense and sparse zones that betoken planets in the making. This astonishing image could revolutionize theories of how planets form. More such views are sure to come.

The 66 dishes of the Atacama Large Millimeter/submillimeter Array (ALMA) can be placed as far as 16 kilometers (10 miles) apart, producing nearly the resolving power of a 16-km-wide telescope. The result is crazyfine detail even at millimeter wavelengths, a little-explored part of the electromagnetic spectrum between the far infrared and microwave radio. The image here was made with the antennas separated by 15 km, almost the full baseline.

ALMA has been operational since March 2013 but has continued to ramp up, adding dishes and more 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 millimeter wavelengths.

In visible light, HL Tauri can't even be seen. The newly formed star still hides in a cocoon of dust and gas. ALMA imaged the system at 1.28 mm (233 gigahertz) to see through the dust to the planet-forming disk at its center. ALMA has imaged planet-induced gaps before, but never at this resolution: details 35 milliarcseconds across are resolved in the image, or 5 a.u. at HL Tauri's distance of 450 light-years.

"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 is 5 times better in spatial resolution and at least a factor of 20 more sensitive."

HL Tauri's solar system is much bigger than our own. All our planets except possibly Neptune would fit within the innermost visible gap. No planets are detected at ALMA's wavelengths, but each gap might mark a large body's orbit.

The discovery of disk gaps around a star less than 1 million years old is surprising --planets aren't supposed to be big enough to create such gaps this early.

And there's more to this image than the gaps. "I am also super excited by the ripples," says Leonardo Testi (University of Arcetri, Italy, and ESO). These undulations in density may hold clues on how dust grains stick together to start forming planets--a poorly understood process that's key to the standard, core-accretion scenario for how planets begin.

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Title Annotation:News Notes; Atacama Large Millimeter/submillimeter Array; HL Tauri
Author:Young, Monica
Publication:Sky & Telescope
Date:Feb 1, 2015
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