The big yellow one is the sun! No, I'm kidding. I've got no idea.
The definition of a planet was changed and it does not meet that definition.
No. Europa does not meet the definition of a planet because it does not orbit the sun. It orbits the planet Jupiter.
Mercury is a planet. By definition, it revolves around the sun.
a round planet with living things on it.
Astronomers have decided that Pluto does not meet the definition of a planet.
A moon orbits a planet.
8
By definition, it's the Earth.
8 that meet the current official definition of planet.
The definition is not always very clear. For our Solar System, the definition is used that a planet orbits the Sun, and has "cleared its neighborhood", meaning that there are no other orbits of comparable size crossing its orbit. Note that this definition makes no direct reference to the planet's size! There is not yet a clear definition for other solar systems; this will need to be clarified, once we start discovering smaller and smaller objects.
The outermost region of a planet's atmosphere
The International Astronomical Union