FOMALHAUT (Alpha Piscis Austrini). Star with
planet.
This wonderful first magnitude star of northern-
hemisphere autumn, usually pronounced "fo-ma-low," slides slowly in
lonely grandeur above the southern horizon (as seen from
the north) during the months of
October and November. Well to the south of the
Great Square of
Pegasus, Fomalhaut marks for us the otherwise dim constellation
Piscis Austrinus,
the Southern Fish, not surprisingly also south of
the more well-known zodiacal constellation Pisces, the Fishes. The
Alpha star of the constellation, the name means the "fish's mouth,"
and comes from a longer Arabic phrase meaning "the mouth of the
southern fish." It at first seems like yet another ordinary white
"class A" star similar to, though a bit cooler than, Vega in Lyra (which passes nearly overhead in
temperate latitudes) with a surface temperature of about 8500
Kelvin. It is quite close, only 25 light years away, from which we
calculate a luminosity 16 times greater than the
Sun. Almost the
same distance as Vega, it is over a full magnitude fainter to the
eye as a result of somewhat lower mass, which results in a lower
surface temperature and smaller size. In 1983 an orbiting
satellite called IRAS discovered far more infrared radiation coming
from the star than expected. Infrared -- radiation which has waves
longer than red light -- is a signature of a cool source. The
radiation is coming from a huge disk of matter four times the
dimension of our planetary system that surrounds the star much like
those that encompass Vega and Denebola.
The disk is thought to be made of icy dust particles that have been
warmed by the star. The planets of our Solar System almost
certainly formed from the accumulation of dust in just such a disk.
Recent observations of Fomalhaut's disk shows a hole in the middle.
Could the hole be the result of planets that have removed the dust?
So far none have been detected. But keep your eye on the lonely
one to the south.
The Planet.
Recent observations of Fomalhaut's disk shows a hole in the middle.
Could the hole be the result of planets that have removed the dust?
So far none have been detected within the hole. But in a stunning
discovery announced in November of 2008, one was found by direct
imaging to be orbiting
within the disk. With a mass of under 3 Jupiters, it lies at a
huge distance of 115 Astronomical Units from its parent star and
has an orbital period estimated at around 875 years. A modest
eccentricity causes the distance to change between 102 and 127 AU.
Along with the planets of HR 8799 Pegasi, Fomalhaut's has the honor
of being the first to be directly seen.
Written by Jim Kaler10/9/98; modified 11/14/08).
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