An international team of researchers has uncovered what they believe is the missing plot point in the story of how planets are born.
For years, scientists were aware that planets are formed out of the swirling disk of dust and gas found around newly-made stars via accretion. Still, a major mystery has long remained as astronomers trying to model the process have come face-to-face again and again with the fact that once the dust grows to a certain size, it self-destructs, either by colliding with other grains or by being drawn into their central star.
By imaging the outer regions of a young solar system known as Oph IRS 48, however, astronomers have proposed a new theory.
In order to save dust grains from oblivion, the study, published in the journal Science, describes a vortex capable of providing an area of higher pressure that then acts like a cocoon, protecting planets and other space objects while they grow into objects capable of surviving on their own.
Creating the dust trap, as the scientists are calling it, requires a helping hand by a very large object, such as a gas-giant or companion star, however. As this large object then moves through the disk, it clears a path around the star and produces the essential eddies and vortices in its wake.
Sure enough, while previous studies of Oph IRS 48 may not have hinted at a dust trap, they did reveal a large gap between the inner and outer portions of this disk, a telltale sign that something huge is clearing the path.
Moreover, by using the new Atacama Large Millimeter/Submillimeter Array (ALMA) telescope, the astronomers were able to do something they never could before: simultaneously observe both the gas and the much larger dust grains. In so doing, the scientists discovered a lopsided bulge in the outer portion of the disk.
"At first the shape of the dust in the images was a complete surprise for us," Nienke van der Marel, a doctoral student at Leiden Observatory in the Netherlands and lead author on the paper, said in a news release. "Instead of the ring we had expected to see, we found a very clear cashew-nut shape. We had to convince ourselves that this feature was real, but the strong signal and sharpness of the ALMA observations left no doubt about the structure.”
Though the ALMA observations only showed the outer structure of the disk, the principle would still be the same closer to the star where rocky planets would form, the scientists explained.
“This structure we see with ALMA could be scaled down to represent what may be happening in the inner solar system where more Earth-like rocky planets would form,” said Til Birnstiel, a researcher at the Harvard-Smithosonian Center for Astrophyics and co-author of the paper.
At 50 times the distance of the Earth to the Sun, Birstiel explained that what the astronomers are currently viewing is likely the equivalent to the Kuiper Belt or Oort Cloud, where comets are believed to originate.