Properties of extrasolar planets

To date, 97 extrasolar planets are known, most having M sin i < 10 M-JUP (for updates, see http://exoplanets.org). The distribution of masses rises rapidly toward the lower masses, dN x M-0.7, with the lowest M sin i being 0.12 M-JUP (for HD 49674). The distribution of orbit size reveals a minimum near a = 0.3 AU and an increasing number of planets in larger orbits (dN/dlog a > 0). Indeed more extrasolar planets are known orbiting beyond 1 AU than within, with signs of a large population beyond 3 AU. We report the first planet orbiting beyond 5 AU, the outermost of three planets around 55 Cancri. There is an obvious lack of massive planets (M > 4 M-JUP) orbiting within 0.3 AU, indicating that the migration mechanism is either inefficient or too efficient for them. The orbital eccentricities of all extrasolar planets are spread nearly uniformly from e = 0 to 0.7, indicating that some robust mechanism commonly occurs that pumps the eccentricities. Clues to this mechanism come from the multiple-planet systems of which 10 are known. Four of these systems reveal secular and mean motion resonances while the other planetary systems are "hierarchical", consisting of widely separated orbits. These two system architectures may result from multiple planets migrating in a viscous disk sometimes leading to their mutual capture in resonances. Subsequent eccentricity pumping and other perturbations may result in close encounters leading to scattering and ejection of planets. This scenario would explain the resonances commonly seen and the eccentric orbits common among single planets. Finally, there are indications of a less perturbed population of giant planets at 3-10 AU that would be the analogs of our solar system.