World Book Trivia Challenge Advanced--3/4--planets

Pluto was called a planet until 2006. The discovery of several other objects about the same size as Pluto led scientists to create a new category called dwarf planets.

The word planet is a Greek word meaning "to wander." Unlike a real star, planets change positions or wander around the sky.

The two planets closest to the sun, Mercury and Venus, do not have moons. Earth has one moon, Mars has two. The large outer planets have many moons. Both Jupiter and Saturn have over 60.

In the 1990's, scientists began discovering planets outside of the solar system. These new planets were called extrasolar (beyond the sun) planets or exoplanets for short.

Mercury does not have an atmosphere. Scientists have discovered an extremely thin layer of gas around the planet, but it is far too thin to be called an actual atmosphere.

Both the largest moon of Jupiter, Ganymede, and the largest moon of Saturn, Titan, are larger than Mercury.

Earth is just slightly larger than Venus. Mercury is the smallest.

All of the large gaseous planets in the solar system have rings. These planets are Jupiter, Saturn, Uranus, and Neptune.

The sun contains about 99.8% of the mass in the solar system.

Winds blow on Neptune at speeds up to about 900 miles (1,450 kilometers) per hour. Unlike the winds of Jupiter, Saturn, and Uranus, Neptune’s winds tend to blow in the opposite direction of the planet’s rotation.