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