In a vast disc of gas and dust particles circling a young star, scientists have found evidence of a hypothesized but never-seen dust trap that may solve the mystery of how planets form.
We know planets that orbit stars are abundant throughout our galaxy, and likely throughout the universe as well, but until recently, scientists weren't exactly sure how those planets came to be.
The working theory is that they grew over time as tiny bits of dust collided and stuck together -- eventually forming comets, rocky planets and the cores of gaseous planets over millions of years.
But there is a problem with that theory: Once these tiny bits of dust grow to the size of pebbles or boulders, they are likely to either smash into one another and break apart, or spiral toward their central star where their growth is inhibited.
Theoretical astronomers had hypothesized that the flat discs of dust and gas that often surround young stars might occasionally contain dust traps -- an area in the disc where the gas is more dense and can create a barrier that keeps more substantial bits of dust from falling toward the star.
And then, quite by accident, a team of scientists found evidence of one in a disc around the massive young star Oph IRS 48, about 400 light-years from Earth.
The team, led by Nienke van der Marel, a doctoral student at Leiden Observatory in the Netherlands, was using ALMA, an array of radio telescopes in Chile, to observe just the gas in the disc. But ALMA also gave them data about the dust in the ring, "for free," Van Der Marel said.
When the researchers looked at the dust data, they were confused: They had expected to see dust particles distributed evenly throughout the disc, but the images they saw showed the dust clumped on about one-third of the disc in the shape of a cashew.
"The first time we saw the image of the dust, we thought there must be something wrong with the data," Van Der Marel told the Los Angeles Times. "But we had a really high, clear signal so it was clear it wasn't a mistake. Then we started looking into possibilities that could explain the separation between the gas and the dust."
It turns out that they had just made the first observation of a dust trap.
This dust trap was caused by a large gaseous planet or perhaps a small star that is also circling the central star, Van Der Marel said. She and her colleagues had observed that there was a hole in the gas disc, likely filled by one of these two types of bodies (but they are not sure which one).
The gas around this hole is more dense than the rest of the disc, and can keep the dust particles that get stuck behind it from falling toward the central star.
Van Der Marel said this particular dust trap is not likely to create planets because of its location in the disc, but it could create comets as large as .6 miles across. Van Der Marel describes it as "comet factory."
Next, Van Der Marel said she planned to use ALMA to look for dust traps that are closer to their stars, where planet formation is more likely. A study describing the dust trap was published in Science this week.
See below for video of how scientists envision the dust trap works.