For many thousands of years, people, with a few notable exceptions, did not believe the solar system existed. The Earth was believed not only to be stationary at the centre of the universe, but to be categorically different from the divine or ethereal objects that moved through the sky. The conceptual advances of the 17th century, led by Nicolaus Copernicus, Galileo Galilei, Johannes Kepler, and Isaac Newton, led gradually to the acceptance of the idea not only that Earth moved round the Sun, but that the planets were governed by the same physical laws that governed the Earth, and therefore could be similar to it. Timeline of solar system astronomy Galileo's telescopeThe first exploration of the solar system was conducted by telescope, with astronomers learning that the Moon and other planets possessed such Earthlike features as craters, ice caps, and seasons. Galileo Galilei was the first to discover physical details about the individual bodies of the Solar System. He discovered that the Moon was cratered, that the Sun was marked with sunspots, and that Jupiter had four satellites in orbit around it.[56] Christiaan Huygens followed on from Galileo's discoveries by discovering Saturn's moon Titan and the shape of the rings of Saturn. [57] Giovanni Domenico Cassini later discovered four more moons of Saturn, the Cassini division in Saturn's rings, and the Great Red Spot of Jupiter.[58] In 1682, Edmund Halley realised that repeated sightings of a comet were in fact recording the same object, returning regularly once every 75-6 years. This proved once and for all that comets were not atmospheric phenomena, as had been previously thought, and was the first evidence that anything other than the planets orbited the Sun.[59] In 1781, William Herschel was looking for binary stars in the constellation of Taurus when he observed what he thought was a new comet. In fact, its orbit revealed that it was a new planet, Uranus, the first ever discovered.[60] In 1801, Giuseppe Piazzi discovered Ceres, a small world between Mars and Jupiter that was initially considered a new planet. However, subsequent discoveries of thousands of other small worlds in the same region led to their eventual separate reclassification: asteroids.[61] In 1846, discrepancies in the orbit of Uranus led many to suspect a large planet must be tugging at it from farther out. Urbain Le Verrier's calculations eventually led to the discovery of Neptune.[62] Further discrepancies in the orbits of the planets led Percival Lowell to conclude yet another planet, "Planet X" must still be out there. After his death, his Lowell Observatory conducted a search, which ultimately led to Clyde Tombaugh's discovery of Pluto in 1930. Pluto was, however, found to be too small to have disrupted the orbits of the outer planets, and its discovery was therefore coincidental. Like Ceres, it was initially considered to be a planet, but after the discovery of many other similarly sized objects in its vicinity it was eventually reclassified as a Kuiper belt object.[62] In 1992, astronomers David Jewitt of the University of Hawaii and Jane Luu of the Massachusetts Institute of Technology discovered 1992 QB1, the first object found beyond Neptune in 62 years. This object proved to be the first of a new population, which came to be known as the Kuiper Belt; an icy analogue to the asteroid belt of which such objects as Pluto and Charon were deemed a part.[63] Many of the largest of these objects, such as Chaos, Quaoar, Varuna and Ixion, where discovered by astronomer Mike Brown.[64] In 2005, Mike Brown announced the discovery of Eris, a Scattered disc object larger than Pluto and the largest object discovered in orbit round the Sun since Neptune.[65] Space exploration The Pale Blue Dot photo, a photo of Earth as a tiny dot (taken 4 billion miles from Earth by Voyager 1 at the edge of the solar system)Since the start of the space age, a great deal of exploration has been performed by unmanned space missions that have been organized and executed by various space agencies. The first probe to land on another solar system body was the Soviet Union's Luna 2 probe, which impacted on the Moon in 1959. Since then, increasingly distant planets have been reached, with probes landing on Venus in 1965, Mars in 1976, the asteroid 433 Eros in 2001, and Saturn's moon Titan in 2005. Spacecraft have also made close approaches to other planets: Mariner 10 passed Mercury in 1973. The first probe to explore the outer planets was Pioneer 10, which flew by Jupiter in 1973. Pioneer 11 was the first to visit Saturn, in 1979. The Voyager probes performed a grand tour of the outer planets following their launch in 1977, with both probes passing Jupiter in 1979 and Saturn in 1980 – 1981. Voyager 2 then went on to make close approaches to Uranus in 1986 and Neptune in 1989. The Voyager probes are now far beyond Neptune's orbit, and astronomers anticipate that they will encounter the heliopause which defines the outer edge of the solar system in the next few years.[66][67] All planets in the solar system have now been visited to varying degrees by spacecraft launched from Earth, the last being Neptune in 1989. Through these unmanned missions, humans have been able to get close-up photographs of all of the planets and, in the case of landers, perform tests of the soils and atmospheres of some. No Kuiper belt object has been visited by a man-made spacecraft. Launched in 19 January 2006, the New Horizons is currently enroute to becoming the first man-made spacecraft to explore this area. This unmanned mission is scheduled to fly by Pluto in July 2015. Should it prove feasible, the mission will then be extended to observe a number of other Kuiper belt objects.[68]