ALMA reveals workings of nearby planetary system
Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have discovered that planets orbiting the star Fomalhaut must be much smaller than originally thought. The discovery was made possible by exceptionally sharp ALMA images of a disc, or ring, of dust orbiting Fomalhaut, which lies about 25 light-years from Earth.
The ALMA images show that both the inner and outer edges of the thin, dusty disc have very sharp edges. That fact, combined with computer simulations, led the scientists to conclude that the dust particles in the disc are kept within the disc by the gravitational effect of two planets — one closer to the star than the disc and one more distant.
The ALMA observations traced dust grains about 1 millimetre in diameter that are not moved by the star’s radiation. They clearly reveal the disc’s sharp edges and ringlike structure, which indicate the gravitational effect of the two planets. The planets are not larger than Saturn; they are larger than Mars but no larger than a few times the size of the Earth. The masses of these planets must be small; otherwise the planets would destroy the ring.
The ALMA research shows that the ring’s width is about 16 times the distance from the Sun to the Earth, and is only one-seventh as thick as it is wide. The whole ring is about 140 times the Sun-Earth distance from the star. In our own Solar System, Pluto is about 40 times more distant from the Sun than the Earth.
Above: 1. A view of the dust ring around Fomalhaut from the Atacama Large Millimeter/submillimeter Array (ALMA). The underlying blue picture shows an earlier picture obtained by the Hubble. 2. Particles in the dust ring are kept inside the ring by gravitational interactions with planets. The faster-moving inner planet transfers energy to dust particles, moving them outward, deeper into the ring. The slower-moving outer planet removes energy from the particles, causing them to drop inward.