Formation and Abundance of Planets: What Might Be Out There?
Alan P. Boss Department of Terrestrial Magnetism Carnegie Institution of Washington 5241 Broad Branch Road, NW Washington, DC 20015 Email: boss@axp1.ciw.edu A new era in the search for life outside the Solar System was entered in 1995, when the first definitive evidence for the existence of a companion with a mass comparable to that of Jupiter orbiting a solar-type star was presented by Mayor \& Queloz (1995). Since that epochal moment, the pace of discovery has been rapid, so that even newly-published review articles (e.g., Marcy \& Butler 1998) are soon out of date. As of May 1999, there was good evidence for at least 20 planetary-mass companions to solar-type stars, as well as a growing number of objects that appear to be better classified as brown dwarf stars. With the discovery of the first confirmed extrasolar planetary {\it system} around a solar-type star, Upsilon Andromedae with a hot Jupiter and two multiple-Jupiter-mass outer planets (Butler et al. 1999), a second milestone
