The principal component of the Solar System is the Sun or Sol (astronomical symbol ); a main sequence G2 star that contains 99.86% of the system's known mass and dominates it gravitationally.[4] Jupiter and Saturn are the Sun's two largest orbiting bodies and account for more than 90% of the system's remaining mass. (The currently hypothetical Oort cloud would also hold a substantial percentage were its existence confirmed).[5] All of the planets (and most other objects) also orbit with the Sun's rotation; in a counter-clockwise direction as viewed from a point above the Sun's north pole. There is a direct relationship between how far away a planet is from the Sun and how quickly it orbits. Mercury, the closest to the Sun, travels the fastest, while Neptune, being much farther from the Sun, travels more slowly. Objects orbit in an ellipse around the Sun, so an orbiting object's distance from the Sun varies in the course of its year. Its closest approach to the Sun is known as its perihelion while its farthest point from the Sun is called its aphelion. Although the orbits of the planets are nearly circular (with perihelions roughly equal to their aphelions), many comets, asteroids and objects of the Kuiper belt follow highly elliptical orbits with large differences between perihelion and aphelion. The paths of objects around the Sun travel according to a law of planetary motion discovered by German astronomer Johannes Kepler in the early 1600's. The sun is slightly off to the side of the center of each ellipse at a point called a focus. The focus is actually a point just outside the centre of the Sun called the barycenter of the solar system. Astronomers most often measure distances within the solar system in astronomical units or AU. One AU is the average distance between the Earth and the Sun or roughly 149 598 000 km (93,000,000 mi). Pluto is roughly 39 AU from the Sun while Jupiter lies at roughly 5.2 AU. Informally, the Solar System is sometimes divided into separate zones. The first zone, known as the inner Solar System, includes the four terrestrial planets and the main asteroid belt. The outer Solar System is sometimes defined as "everything beyond the asteroids". Alternatively, the term may be used to describe the region beyond Neptune, with the four gas giants considered a separate "middle zone".[6] The orbits of the bodies in the solar system to scale (clockwise from top left)One common misconception is that the orbits of the major objects within the Solar System (planets, Pluto and asteroids) are equidistant. To cope with the vast distances involved, many representations of the Solar System simplify these orbits by showing them the same distance apart. However, in reality, with a few exceptions, the Solar System is arranged so that the farther a planet or belt is from the Sun, the larger the distance between it and the previous orbit. For example, Venus is approximately 0.33 AU farther out than Mercury while Jupiter is 1.9 AU from the farthest extent of the asteroid belt and Neptune's orbit is roughly 20 AU farther out than that of Uranus. Attempts have been made to determine a correlation between these distances (see Bode's Law) but to date there is no accepted theory that explains the orbital distances.