Ring of cold dust could indicate the existence of giant planets
If water trickles through a gutter, there’s not much of it anywhere at any time. But if you place a barrier in the gutter, the water starts to pile up: new drops of water continue to arrive, but they can’t stream away anymore.
Something similar is happening in the disk of gas and dust that surrounds the young star Sz91. The pile-up of dust particles, observed with ALMA, may indicate the presence of giant planets orbiting the star.
The disk of Sz91 contains lots of gas and fine dust that cannot be seen by ALMA. However, the 66-dish observatory can see the millimeter/submillimeter radiation from larger particles, about the size of grains of sand. These particles plough their way through the gas in the disk. As a result, they are expected to slow down and spiral inward, until they end up in the star. Dust grains that migrate from the outer parts of the disk to the inner parts are comparable to water drops trickling through a gutter.
However, if planets have formed in the inner part of the disk, their gravity clears up the gas, creating a hole in the disk. If there is no gas to slow them down, dust grains no longer spiral inward. Instead, they remain stuck at the outer edge of the hole. In a sense, the hole is a ‘barrier’ that prevents dust grains to further migrate toward the star.
Since new dust grains continue to arrive from the outer regions of the disk, they pile up at this ‘barrier’, just like drops of water pile up at the barrier in the gutter. This creates a relatively dense ring of large dust grains at the outer edge of the central hole.
This is exactly what ALMA has observed in the disk that surrounds the star Sz91. Apparently, the disk contains a central hole where giant planets already have formed. The ALMA observations of Sz91 shed light on the migration of dust particles, and on the interactions between planets and disks.
Interestingly, the pile-up of dust grains may lead to the formation of additional planets. In the ring, the grains are much closer to each other than elsewhere in the disk, so they can more easily start to accumulate into ever larger objects.
Sz91 is a young star: it formed only some 5 million years ago. It is part of a star-forming region at a distance of 650 light-years in the southern constellation Lupus the Wolf. The star is only half as massive as our own Sun. Like many new-born stars, it is surrounded by a flattened disk of gas and dust – the building material of new planets. The hole in the disk of Sz91 that ALMA has observed is over three times as wide as the orbit of the planet Neptune in our own Solar System.
The ALMA observations of the dust ring of Sz91 were carried out by a Chilean team of astronomers led by Héctor Cánovas of the Universidad de Valparaíso. They have described their results in an article in the scientific journal Monthly Notices of the Royal Astronomical Society.