Several multiple planet systems have been identified. The first,
Upsilon Andromedae, is a triple planet system that was discovered by the
Lick and AFOE teams in 1999. This announcement has been followed by
the detection of several double planet systems.
The long time baseline of observations at Lick Observatory, coupled with high
precision, frequent observations make this sample ideal for an investigation
of the statistical rate of occurance of multiple planet systems.
Fischer et al. established a set of stars on the Lick planet search
project that met the following criteria:
one known planet
at least 2 years of observations
Twelve stars met these selection criteria. The characteristics of these stars are listed
in the table below. After accounting for the velocity variation attributable to
the one known planet, we evaluated the residual velocities for evidence of a
second trend that could be consistent with Keplerian velocity variations.
velocity trends observed in at least half of these stars,
suggest that known planet-bearing stars appear to harbor
a distant detectable companion more often than other stars in our