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How were the planets of the solar system discovered?
All inner planets, as well as Mars, Jupiter and Saturn are well known since prehistorical times by direct visual sight.
These planets were subject of observation, and the Sumerians were the first to study these planets as a science (Astronomy), and as a base to predict the future (Astrology).
Jupiter and Saturn's movements were systematically observed by the Babylonian astronomers, from the 7th or 8th century BC.
There is a dispute about who first acknowledged Uranus as a planet, not as a star as previously thought.
Sir William Herschel presented his conclusions to the Royal Society in 1783, but Russian astronomer and mathematician Anders Johan Lexel (born in Sweden) was the first to defend Uranus was a planet, and he also calculated its orbit around the Sun, when Herschel still considered Uranus a comet.
However, Uranus was formally discovered by Herschel, using for the first time an optical telescope to find a new planet.
Neptune was the first planet discovered by mathematical prediction rather than by telescope obervation. Its discovery is also disputed between Le Verrier and Johann Galle...
The dwarf planet Pluto, considered the nineth planet of the Solar System on his own merit until 2006, was discovered only in 1930 by the young astronomer Clyde Tombaugh, died in 1997 at age 90.
It was the first time a planet was discovered using photography. Tombaugh used a new technology to compare photographs from the sky taken at different times, called a «Blink Comparator».
Part of Tombaugh ashes will orbit Pluto, sent inside a small probe orbiter carried aboard the New Horizons spacecraft mission, which will arrive Pluto in 2015).
Although the International Astronomical Union keeps insisting Pluto is not a planet, the scientifical community is not unanimous on this formal decision taken after the discovery of Eris, a Kuiper Belt object larger and heavier than Pluto.
Asteroids and other trans-Neptunian objects are not discussed here.