Spiral-in-a-gap hints at one or two planets around young star
Astronomers have detected a beautiful spiral pattern in the disk of material around a young star. It looks like the gas in the disk is somehow stirred up, probably by one or two planets.
Young stars are surrounded by flat disks of gas and dust. The star AB Aurigae is no exception. From Earth, we see the disk almost exactly face-on. The dust particles in the disk emit radiation at millimeter wavelengths, which can be observed by ALMA. The radiation from the dust is color-coded red in the ALMA image.
If you look closely at the image, you will see that there’s dust close to the star, and also at a much larger distance. In other words: the dust disk contains a very wide gap – a region where dust is almost absent.
However, the gap is not really empty. It contains a lot of gas, including carbon monoxide. ALMA can also observe the millimeter-wave radiation from this gas. The gas is color-coded blue in the image. For some reason, the gas is not distributed evenly. Instead, it shows a beautiful spiral pattern.
To explain the wide gap in the dust disk and the spiral pattern in the gas, the astronomers suggest that AB Aurigae must have at least one massive planet. The planet would orbit the star at a distance of some 9 to 12 billion kilometers – more than twice the distance between the Sun and the planet Neptune in our own solar system.
There may even be a second planet in the system, at a smaller distance of 4.5 billion kilometers from the star. The second planet could explain why the gap contains so little dust.
Unfortunately, the planets cannot be observed by ALMA or by any other telescope. They’re just too small, and the star is too far away. So we won’t know for sure if they’re really there. The outer planet may even be a small companion star, like a brown dwarf.
AB Aurigae is a young star in the constellation Auriga, the Charioteer. The star is 2.4 times more massive and about 50 times more luminous than our own Sun. Still, it can only be seen with the aid of a telescope, because of its large distance of some 450 light-years. Like most young stars, AB Aurigae is surrounded by a flat, rotating disk of gas and dust. Such a disk is called a protoplanetary disk because it contains the material from which planets may clump together. In the case of AB Aurigae, some planets may already have been formed.
The spiral pattern in the gas distribution around AB Aurigae was discovered by a large international team of astronomers, led by Ya-Wen Tang of the Institute of Astrophysics and Astronomy, Academia Sinica in Taiwan. The results have been published in The Astrophysical Journal, a professional astronomy magazine.