Since the discovery of Pluto in 1930, many children grew up learning that the solar system had nine planets. This changed in the late 1990s, when astronomers began arguing about whether Pluto was really a planet. Finally, in 2006 the International Astronomical Union decided to leave Pluto out and classified it as a dwarf planet, reducing the list of planets in the solar system to eight.
However, astronomers are looking for another possible planet in our solar system after various scientific evidences of its existence were discovered in 2016. This supposed Planet Nine, also called Planet X, could have one ten times the mass of Earth and about 5,000 times the mass of Pluto. Another theory holds that it is not actually a planet, but a black hole. However, the puzzle has not yet been solved.
The order of the planets in the solar system (starting from closest to the Sun to further away) is as follows: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and later the possible Planet Nine. If you decide to include Pluto, it would go right after Neptune in the list of planets.
Curiosities of the planets
The four planets closest to the Sun (Mercury, Venus, Earth, and Mars) are called terrestrial planets because their surfaces are rocky. Pluto also has a rocky surface although it is frozen, so it has never been included in this grouping.
The four outer planets (Jupiter, Saturn, Uranus, and Neptune) are sometimes called Jovian planets due to their enormous size compared to the terrestrial planets. They are composed mainly of gases such as hydrogen, helium, and ammonia.
Jupiter and Saturn are called gas giants, while Uranus and Neptune are known as ice giants. This is because Uranus and Neptune have more atmospheric water and other ice-forming molecules (such as methane, phosphene, or hydrogen sulfide) that crystallize in the clouds of these planets.
