Astronomers have captured an image of a dense clump of dust orbiting around a young star — and they're saying it could be our first glimpse of a planet in the very earliest stages of formation. Back in 2014, astronomers released an unprecedented image of a planet-forming disk around a young star called HL Tau. The image, captured by the Atacama Large Millimetre/submillimeter Array (ALMA), showed gaps in the outer disk, likely caused by planet-like objects that are drawing in dust along their orbits. But the inner portion of the disk was relatively featureless; the thick dust within the interior was too opaque for the short one millimetre radio wavelengths received by ALMA.
The 2014 ALMA image of HL Tau. The inner region appeared to be relatively featureless. Image: ALMA (NRAO/ESO/NAOJ); C. Brogan, B. Saxton (NRAO/AUI/NSF).
Now, after tuning into the star at a 7mm wavelength with the Karl G. Jansky Very Large Array (VLA), an international team of astronomers have imaged this inner region, revealing a distinct clump of dust. At this longer wavelength, dust emissions appears thinner, allowing for a more detailed look at the thick interior. The details of this work now appears at Astrophysical Journal Letters (pre-print here).
"We believe this clump of dust represents the earliest stage in the formation of protoplanets, and this is the first time we've seen that stage," noted Thomas Henning, of the Max Planck Institute for Astronomy (MPIA), in a news release.
The combined 2016 ALMA/VLA image of HL Tau. Image: Carrasco-Gonzalez, et al.; Bill Saxton, NRAO/AUI/NSF
The clump is about three to eight times the mass of the Earth, and it contains grains as large as 1cm in diameter. The inner protoplanetary disk is where Earth-like planets likely form. Over time, gravity brings these clumps of dust closer and closer together. Eventually, they collect enough mass to form solid bodies that continue to grow into planets.
"This is an important discovery, because we have not yet been able to observe most stages in the process of planet formation," added Carlos Carrasco-Gonzalez from the Institute of Radio Astronomy and Astrophysics (IRyA) of the National Autonomous University of Mexico (UNAM). "This is quite different from the case of star formation, where, in different objects, we have seen stars in different stages of their life cycle. With planets, we haven't been so fortunate, so getting a look at this very early stage in planet formation is extremely valuable."
HL Tau is located about 450 light-years away, and it's only about a million years old. According to pre-existing theory, protoplanetary disks aren't supposed to be so well developed at such an early stage. But as the VLA data shows, the planet-formation process occurs faster than we thought.