Planetary systems form from the same cosmic clouds that give birth to stars, but the exact mode of formation is still a matter of debate. In 2014, the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile snapped a spectacular image of a planetary system forming around the star HL Tauri, which showed rings of material orbiting a star. Now, it's been seen in even better detail.
To observe what is going in the inner portion of the disk, astronomers used the Very Large Array (VLA) in New Mexico. The observations showed a large clump of dust containing roughly three to eight times the mass of Earth.
"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," said Thomas Henning from the Max Planck Institute for Astronomy, and one of the project leaders, in a statement.
The inner region is rich in dust grains up to 1 centimeter (0.4 inches) in diameter, and astronomers think regions like these are responsible for the formation of Earth-like planets. Over time, the particles interact and merge to form larger "pebbles," eventually becoming a full planet. The findings are available online, and a paper will be published in the Astrophysical Journal Letters.
ALMA image of HL Tauri (left). VLA image showing clump of dust (right). Carrasco-Gonzalez, et al.; Bill Saxton, NRAO/AUI/NSF.
HL Tauri is a very young star, about 1 million years, so the presence of protoplanets is somewhat surprising. For this region, the researchers suggested that the observed clumps could be produced by gravitational instabilities in the disk, and this might be the earliest stage of planet formation.
"This is an important discovery because we have not yet been able to observe most stages in the process of planet formation," said Carlos Carrasco-Gonzalez from the Institute for Radio Astronomy and Astrophysics (IRyA), the other project leader. "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."