A new observatory in Chile has given scientists a greater understanding of a star's planetary system.
ALMA, an anacronym for Atacama Large Millimeter/submillimeter Array, though only part built, has captured images of a star known as Fomalhaut. The star, which is located 25 light years from Earth, has a disc of dusty material encircling it. While the existence of the ring had been previously known, ALMA has imaged it in such striking detail that it has helped to solve a planet-sized mystery.
ALMA has revealed that the inner and outer edges of the dusty disc have very well defined edges. Previous Hubble Space Telescope visible light images had shown blurry edges of the disc, as the telescope was only capable of detecting small dust grains which were blown outwards by the star’s radiation. However, ALMA observes at longer wavelengths, which allowed it to detect larger dust grains. These grains are around one millimetre in diameter and are not influenced by the star’s radiation, thus bringing the edge of the ring into focus.
Combining the information about the disc edges with computer simulations enabled astronomers to conclude that the dust ring is kept in place by two planets. Such an effect is also seen in our own Solar System on a smaller scale, where Saturn’s moons Prometheus and Pandora shepherd the gas giant’s F-ring.
Planets shepherding material into a narrow ring around Fomalhaut. Credit: ALMA (ESO/NAOJ/NRAO)/B. Saxton
As a planet that lies within the dust ring surrounding Fomalhaut orbits the star faster than the dust particles, it will transfer some energy to the particles and push them outwards. Similarly, the planet exterior to the dust ring is moving slower than the dust ring, which will decrease the energy of the dust particles and make them fall inward. Thus the overall motion of the two planets keeps the dust ring in check.
The size of the planets were also calculated to be no bigger than a few Earth radii, because if they were too big they would destroy the dust ring. This is much smaller than the original Saturn-size prediction of the inner planet based on a Hubble Space Telescope image. The smaller sized planet offers an explanation to why follow-up observations with infrared telescopes failed to detect the planet, as it is small enough to remain hidden at these particular wavelengths.
ALMA will consist of 66 radio antenna, and is currently under construction in Chile. The observations of Fomalhaut were made with just a quarter of these antenna, and when the full array comes online much better pictures can be taken.
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“ALMA may be still under construction, but it is already the most powerful telescope of its kind. This is just the beginning of an exciting new era in the study of discs and planet formation around other stars", said ESO’s Bill Dent.