Definition of planet object can be irregular in shape, but beyond that point gravity begins to pull an object towards its own centre of mass until the object collapses into an ellipsoid. (None of the large objects of the Solar System are truly spherical. Many are spheroids, and several, such as the larger moons of Jupiter and Saturn and the dwarf planet , have been further distorted into ellipsoids by rapid rotation or tidal forces, but still in hydrostatic equilibrium.) However, there is no precise point at which an object can be said to have reached hydrostatic equilibrium. As Soter noted in his
Solar System shaped by two separate forces: the solar wind and the Sun's gravity. The limit of the solar wind's influence is roughly four times Pluto's distance from the Sun; this "heliopause", the outer boundary of the heliosphere, is considered the beginning of the interstellar medium. The Sun's Hill sphere, the effective range of its gravitational dominance, is thought to extend up to a thousand times farther and encompasses the theorized Oort cloud. The heliosphere is a stellar-wind bubble, a region of space dominated by the Sun, which radiates at roughly 400 km/s its solar wind, a stream of charged particles, until