Asteroids are not too small to be considered as planets.

A nebula is a giant cloud of dust and gas in space.

The outer planets did not form first. 

The planets that follow them in their orbits, Jupiter and Saturn, are not known as gas giants, because they are composed mainly of helium and hydrogen.

The four planets closest to the sun are Mercury, Venus, Earth and Mars, also known as the terrestrial planets, since they are composed mainly of rock.

Uranus and Neptune are not known as icy giants, because they are made of frozen water, ammonia, and methane.

Natural satellites are celestial bodies that orbit around a planet. 

it is not necessary that the three stars—Sun, moon and earth (in that order)—are not located on the same plane in space in order to appreciate a solar eclipse.

In a lunar eclipse we would see that the earth is between the moon and the sun.