While the planets in our solar system are astonishingly diverse, their orbital motion is extremely orderly. All of them move around the Sun in approximately the same orbital plane, in the same direction, and primarily in circular orbits. Since 1995, Paul Butler and his team have discovered over half of the planets found orbiting nearby stars. Unlike the situation in our own solar system, most of these extrasolar planets have elongated, eccentric orbits. However, some are very close to their parent stars in circular orbits with periods of as little as three days. Because the detection technique is limited to large planets, most of these newly found objects have masses on a par with those of Jupiter or Saturn.

Butler and colleagues have developed the most precise method to date for finding these remote bodies: the precision Doppler velocity technique. The system works by detecting, via the Doppler effect, the wobble of a star used by the gravitational attraction of a massive orbiting object. The information also allows the team to infer the planet’s mass, its orbital period, and the size of the orbit.

After further refinements to their method, in 2002 Butler and team announced the smallest planetary find yet—one with a mass just 40 times that of Earth. They also announced the discovery of the first true analogue to our own solar system—three planets in mostly circular orbits around the star 55 Cancri. The outermost planet in the system, at between 3.5 and 5 times Jupiter's mass and at a distance of 5.9 astronomical units (AU) from its star, is analogous to Jupiter, which is 5.2 AU from our Sun (1 astronomical unit is the distance from the Earth to the Sun).

Butler’s work is part of a multiyear project to carry out the first reconnaissance of all 2,000 nearby Sun-like stars within 150 light-years of the solar system (1 lightyear is about 9.4 trillion kilometers). His team is currently monitoring about 1,700 stars, including 1,000 Northern Hemisphere stars with the Keck telescope in Hawaii and the UCO Lick Observatory telescope in California, and 300 Southern Hemisphere stars with the Anglo-Australian telescope in New South Wales, Australia. The remaining Southern Hemisphere stars are being surveyed with Carnegie’s new Magellan telescopes in Chile. By 2010 the researchers hope to have completed their planetary census. They will then be able to tell what percentage of stars have planets, how many systems are similar to our own, and the different characteristics these systems exhibit. The ultimate goal is to find planets that resemble the Earth.