Solar System

The central, and most important object in our solar system is the Sun. It is the largest and most massive object in the solar system—its diameter is 109 times that of Earth, and it is 333,000 times more massive. The extent of the solar system is determined by the gravitational attraction of the Sun. Indeed, the boundary of the solar system is defined as the surface within which the gravitational pull of the Sun dominates over that of the galaxy. Under this definition, the solar system extends outwards from the Sun to a distance of about 100,000 AU. The solar system is much larger, therefore, than the distance to the remotest known planet, Pluto, which orbits the Sun at a mean distance of 39.44 AU.

The Sun and the solar system are situated some 26,000 light years from the center of our galaxy. Traveling at a velocity of 137 MPH (220 km/h), the Sun takes about 240 million years to complete one orbit about the galactic center, and since its formation the Sun has completed about 19 such trips. As it orbits the center of the galaxy the Sun also moves in an oscillatory fashion above and below the galactic plane (the Sun's motion is similar to that of a carousel fair-ground ride) with a period of about 30 million years. During their periodic sojourns above and below the plane of the galaxy, the Sun and solar system suffer gravitational encounters with other stars and giant molecular clouds. These close encounters result in the loss of objects (essentially dormant cometary nuclei located in the outer Oort cloud) that are on, or near, the boundary of the solar system. These encounters also nudge some cometary nuclei toward the inner solar system, where they may be observed as long-period comets.