Solar System
The overall structure of the charted regions of the Solar System consists of the Sun, four relatively small inner planets surrounded by a belt of mostly rocky asteroids, and four giant planets surrounded by the Kuiper belt of mostly icy objects. Astronomers sometimes informally divide this structure into separate regions. The inner Solar System includes the four terrestrial planets and the asteroid belt. The outer Solar System is beyond the asteroids, including the four giant planets. Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune.
Discovery and exploration of the Solar System
Although it is debatable when the Solar System was truly "discovered", three 19th century observations determined its nature and place in the Universe beyond reasonable doubt. First, in 1838, Friedrich Bessel successfully measured a stellar parallax, an apparent shift in the position of a star created by Earth's motion around the Sun. This was not only the first direct, experimental proof of heliocentrism, but also revealed, for the first time, the vast distance between the Solar System and the stars. Then, in 1859, Robert Bunsen and Gustav Kirchhoff, using the newly invented spectroscope, examined the spectral signature of the Sun and discovered that it was composed of the same elements as existed on Earth, establishing for the first time a physical link between Earth and the heavens. Then, Father Angelo Secchi compared the spectral signature of the Sun with those of other stars, and found them virtually identical. The realisation that the Sun was a star led to the hypothesis that other stars could have systems of their own, though this was not to be proven for nearly 140 years.