Rory Barnes
Lunar and Planetary Lab, University of Arizona

The First Successful Prediction of an Extrasolar Planet

The first-discovered multiple planet extrasolar systems (as well as
the Solar System) appear to have formed in dynamically "packed"
configurations; additional planets could not occupy orbits in between
those that were known and still be stable. Although the majority of
systems exhibit this appearance, a minority did not. This incosistency
led to the supposition that all planetary systems are packed, and that
stable gaps between planets still harbor planets, they just haven't
been detected yet. I will describe how numerical integrations of
planetary systems led to the "Packed Planetary Systems" hypothesis,
and how subsequent investigations were able to provide quantitative
estimates of where additional companions would be discovered. This
model recently received an important boost with the discovery of HD
74156 d, detected in between the previously known planets of that
system. Moreover, this planet was discovered within 2 sigma of the
most stable orbit identified in the pre-discovery predictions. This
discovery marks the first successful prediction of an extrasolar
planet, as well as only the second successful prediction of a planet
ever (the first being Neptune in the 1840's). I will also describe
more recent work to search for a simple description of the boundary of
dynamical instability in planetary system, work which reinforces the
notion that all planetary systems lie near the edge of dynamical
instability.