Well, it’s hard to give an exact figure. Between 1930 when Pluto was discovered by the American Clyde Tombaugh, and 2006, there were 9 planets. Going outwards from the sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.
But in 2006, another American, Mike Brown, nicknamed PlutoKiller, showed that Pluto is not the same as other planets. Why? Because it’s too small and not special enough; it’s just like many other objects in the region of Neptune, such as Eris, which he discovered in 2006. So Pluto became a dwarf planet.
And the number of “real” planets was down to 8.
Until 20 January 2016 that is… when Mike Brown, him again, and Konstantin Batygin, working together, said they had discovered a 9th planet by deduction. According to the 2 astronomers, only the presence of a planet 10 times more massive than the Earth can explain why certain dwarf planets behave strangely. In fact, like a shepherd, Planet 9 drives its flock of dwarf planets in the same direction. It’s so far away, it takes 10 to 20,000 years to orbit the sun! No giant telescope has ever observed it to prove it actually exists. Thanks to the Internet, 60,000 astronomers have joined the search for Planet Nine. So, today there are 8 observable planets and a ninth “calculated” one… Until the next discovery anyway!