The ring and planets of Formalhaut
New observations from the new ground-based submillimeter ALMA telescope in Chile have given astronomers a clearer picture of the ring/disk of debris orbiting the nearby star Formalhaut. Moreover, the data suggests that two planets that are shepherding that ring are smaller than previously thought, 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.
If Formalhaut’s debris ring encircled our Sun, it would be located more than three times farther away than Pluto. This means that the two shepherding Earth-sized planets are equivalent to the objects in the Kuiper Belt in our solar system, though much larger. As they orbit far from their star, they get very little heat and light, and are thus among the coldest exoplanets yet found.
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