Question 3: the correct answer is 8. Did you get it right? Award yourself 1 point.
Question 4 of 10
What is the planet closest to the Sun?
Why, in question 3: How many planets does the Solar System have?
The answer is 8: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Pluto was considered the ninth planet until 2006, when the International Astronomical Union created a new definition and classified it as a dwarf planet.
Why Pluto stopped being a planet
Under the 2006 definition, a planet must orbit the Sun, have enough mass to be nearly round, and have cleared the neighbourhood around its orbit. Pluto meets the first two, but it shares its region with many other objects.
The change came after the discovery of Eris in 2005, an object about the size of Pluto and farther from the Sun. Either both would be planets, along with others that might still be found, or neither would.
Today, besides Pluto and Eris, Ceres, in the asteroid belt, and Haumea and Makemake, in the region beyond Neptune, are recognised as dwarf planets.
In 2015, the New Horizons probe flew past Pluto and revealed icy mountains and a large, light-coloured plain shaped like a heart.
