  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.
