And then there are Eight.
How many planets are there in our solar system? Nine? I do not know about what your textbook says (they’re ancient news anyway), but let’s count again (starting from the one nearest to the sun):
- Mercury
- Venus
- Earth
- Mars
- Jupiter
- Saturn
- Uranus
- Neptune and…
Wait! Pluto IS no more considered a planet (as of Mid 2008). (Do you know?) Why is this so?
The definition of “planet” set in 2006 by the International Astronomical Union (IAU) states that in the Solar System a planet is a celestial body that:
- is in orbit around the Sun,
- has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
- has “cleared the neighbourhood” around its orbit. (From wikipedia. For more information click here)
Too complicated? Yeah. So let’s just trim the reasons down into very simple terms. In short, Pluto is TOO SMALL and does not have a ROUND ORBIT that makes it the same as other planets. (And there are too much ‘rubbish’ on its path) So, it’s a planet no more. Not exactly no more, only with a “dwarf” as a surname. A Dwarf Planet. A new category for similar spherical giants (compared to other titans) 
How big is Pluto anyway?
Diameter comparison of Pluto and Earth and their moons (the one on the upper right is our moon, not Pluto)
PlFrom left to right: Mercury, Venus, Earth, and Mars (sizes to scale)
Okay, Pluto’s small. Even smaller than our moon. So that kind of places it in the dwarf planet category. Pluto was discovered in 1930, and one of the main difference Pluto has compared to other planets is that it has an oblong orbit (somewhat oval) compared to the others. Maybe this diagram will show it more clearly
oblong orbit
sources: here
Another one: (from here)
Now in 3D...
So, that’s it…keep in that in your mind: there ARE eight planets in our Solar System. How about the one in your textbooks? How am I supposed to know? Why don’t you ask your teacher?
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