The Oort cloud

Dynamical studies during the past two decades have greatly increased our understanding of the Oort cloud of comets which surrounds the solar system. Cometary orbits in the cloud evolve under the complex interaction of stellar, galactic, and giant molecular cloud perturbations, as well as planetary and nongravitational perturbations when the comets re-enter the planetary region. Evidence has continued to build for a dense, inner Oort cloud of comets which acts as a reservoir to replenish the outer cloud as comets there are stripped away. The total number of comets in the dynamically active outer Oort cloud is on the order of 10(12), and about five times that number in the unseen inner Oort cloud, with a total estimated mass of similar to 38 Earth masses. Temporal variations in the flux of comets from the Oort cloud into the planetary region by a factor of four are possible due to the varying galactic tide, and by a factor of similar to 300 due to perturbations by stars penetrating the Oort cloud or close GMC encounters. The most intense cometary ''showers'' may result in biological extinction events on the Earth. Comets in the Oort cloud are processed by galactic cosmic rays, heated by nearby supernovae, eroded by interstellar dust impacts, and disrupted by mutual collisions (in the inner cloud). A detailed estimate of the Oort cloud's dynamical history is not possible because of the inability to reconstruct the Sun's varying galactic motion over the history of the solar system, and because of uncertainty over where comets actually formed. However, it is likely that a substantial fraction of the original Oort cloud population has been lost to interstellar space. We are approaching the time when Oort clouds around other stars may be detectable, though searches to date have so far been negative.