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Istituto italiano di astrofisica - national institute for astrophisics

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ALMA Reveals Workings of Nearby Planetary System

A new observatory still under construction has given astronomers a major breakthrough in understanding a nearby planetary system and provided valuable clues about how such systems form and evolve. 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. It helps resolve a controversy among earlier observers of the system. 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.

Their calculations also indicated the probable size of the planets — larger than Mars but no larger than a few times the size of the Earth. This is much smaller than astronomers had previously thought. In 2008, a NASA/ESA Hubble Space Telescope image had revealed the inner planet, then thought to be larger than Saturn, the second largest planet in our Solar System. However, later observations with infrared telescopes failed to detect the planet.

That failure led some astronomers to doubt the existence of the planet in the Hubble image. Also, the Hubble visible-light image detected very small dust grains that are pushed outward by the star's radiation, thus blurring the structure of the dusty disc. The ALMA observations, at wavelengths longer than those of visible light, traced larger 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 two planets.

The 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. "Because of the small size of the planets near this ring and their large distance from their host star, they are among the coldest planets yet found orbiting a normal star," added Aaron Boley.


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