After Friday's spectacle, a "planet parade" of this size won't appear in the night sky for several years, experts say.

Inner planets are the four planets closest to the Sun: Mercury, Venus, Earth and Mars. They are collectively characterized by a solid surface and small size relative to the outer gas giants.

Clear skies will also be necessary to see all the planets. The planets in the solar system orbit the sun, just as Earth does. Every planet orbits at a different speed and distance. During these ...

This phenomenon known as a "planet parade," will feature Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune all present at the same time along a line in the night sky on Friday, NASA says.