Solar System
The Solar System's location in the Milky Way is a factor in the evolutionary history of life on Earth. Its orbit is close to circular, and orbits near the Sun are at roughly the same speed as that of the spiral arms. Therefore, the Sun passes through arms only rarely. Because spiral arms are home to a far larger concentration of supernovae, gravitational instabilities, and radiation that could disrupt the Solar System, this has given Earth long periods of stability for life to evolve. The Solar System also lies well outside the star-crowded environs of the galactic centre. Near the centre, gravitational tugs from nearby stars could perturb bodies in the Oort cloud and send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. The intense radiation of the galactic centre could also interfere with the development of complex life. Even at the Solar System's current location, some scientists have speculated that recent supernovae may have adversely affected life in the last 35,000 years, by flinging pieces of expelled stellar core towards the Sun, as radioactive dust grains and larger, comet-like bodies.
List of gravitationally rounded objects of the Solar System
A planet is defined according to the 2006 International Astronomical Union (IAU) definition as a body in orbit around the Sun that was large enough to have achieved hydrostatic equilibrium and to have cleared the neighbourhood around its orbit. The practical meaning of "cleared the neighborhood" is that a planet is comparatively massive enough for its gravitation to control the orbits of all objects in its vicinity. By the IAU's definition, there are eight planets in the Solar System; four terrestrial planets (Mercury, Venus, Earth and Mars) and four giant planets, which can be divided further into two gas giants (Jupiter and Saturn) and two ice giants (Uranus and Neptune). When excluding the Sun, the four giant planets account for more than 99 percent of the mass of the Solar System.