The Sun is the center of the solar system and contains 99 percent of the solar system’s mass. The seven outer planets are named after their average densities, from least to most dense. Mercury, Venus, Earth, Mars, and Pluto are the closest planets to the Sun, while Saturn and Uranus have moons and ring systems. Pluto was officially listed as a planet in 1930, and now orbits beyond Neptune. Recently, an icy object was discovered farther from the Sun than Pluto.
The outer planets are compared to the Earth by shrinking their sizes by a factor of a billion. If Earth were only one tenth of its current size, the Moon would be 30 centimeters across. The Sun would be 150 meters from Earth. The other planets would be five centimeters across and be roughly the size of a large grapefruit. Pluto would be the size of a grapefruit and Saturn would be about the size of an orange. The inner planets, including Neptune and Uranus, would be about twenty blocks away.
The planets orbit the Sun in a plane called the ecliptic. Mercury, Neptune, and Earth all have circular orbits; Pluto has the highest elliptical and highly inclined orbit, which makes it closer to the Sun. Pluto’s axis of rotation is almost perpendicular to the ecliptic, except for Uranus. And while we can’t really see their surfaces, we can still see how far they are from the Sun, and how close they are to the Earth.
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