Optical images of the nearby star Fomalhaut show a ring of dust orbiting the central star. This dust is in many respects expected
to be similar to the zodiacal dust in the solar system. The ring displays a clear brightness asymmetry, attributed to asymmetric
scattering of the central starlight by the circumstellar dust grains. Recent measurements show that the bright side of the
Fomalhaut ring is oriented away from us. This implies that the grains in this system scatter most of the light in the backward
direction, in sharp contrast to the forward-scattering nature of the grains in the solar system. In this letter, we show that
grains considerably larger than those dominating the solar system zodiacal dust cloud provide a natural explanation for the
apparent backward scattering behavior. In fact, we see the phases of the dust grains in the same way as we can observe the
phases of the Moon and other large solar system bodies. We outline how the theory of the scattering behavior of planetesimals
can be used to explain the Fomalhaut dust properties. This indicates that the Fomalhaut dust ring is dominated by very large
grains. The material orbiting Fomalhaut, which is at the transition between dust and planetesimals, can, with respect to their
optical behavior, best be described as micro-asteroids.
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