Planets

Until the end of the 18th century, humans knew of five planets—Mercury, Venus, Mars, Jupiter, and Saturn—in addition to Earth. When viewed without a telescope, planets appear to be dots of light in the sky. They shine steadily, while stars seem to twinkle. Twinkling results from turbulence in Earth's atmosphere. Stars are so far away that they appear as tiny points of light. A moment of turbulence can change that light for a fraction of a second. Even though they look the same size as stars to unaided human eyes, planets are close enough that they take up more space in the sky than stars do. The disks of planets are big enough to average out variations in light caused by turbulence and therefore do not twinkle.

Between 1781 and 1930, astronomers found three more planets—Uranus, Neptune, and Pluto. This brought the total number of planets in our solar system to nine. However, in 2006 the International Astronomical Union (IAU)—the official body that names objects in the solar system—reclassified Pluto as a dwarf planet. The IAU rulings reduced the number of official planets in the solar system to eight. In order of increasing distance from the Sun, the planets in our solar system are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Astronomers call the inner planets—Mercury, Venus, Earth, and Mars—the terrestrial planets. Terrestrial (from the Latin word terra, meaning “Earth”) planets are Earthlike in that they have solid, rocky surfaces. The next group of planets—Jupiter, Saturn, Uranus, and Neptune—is called the Jovian planets, or the giant planets. The word Jovian has the same Latin root as the word Jupiter. Astronomers call these planets the Jovian planets because they resemble Jupiter in that they are giant, massive planets made almost entirely of gas. The mass of Jupiter, for example, is 318 times the mass of Earth. The Jovian planets have no solid surfaces, although they probably have rocky cores several times more massive than Earth. Rings of chunks of ice and rock surround each of the Jovian planets. The rings around Saturn are the most familiar. See also Planetary Science.

The planet Uranus (the bright blue object) is surrounded by its five largest satellites clockwise from top left, Ariel, Umbriel, Oberon, Titania, and Miranda, in this collage created from photographs taken by the United States Voyager 2 spacecraft in 1986.

Pluto is tiny, with a mass about one five-hundredth the mass of Earth. Pluto seems out of place, with its tiny, solid body out beyond the giant planets. Many astronomers believe that Pluto is just one of a group of icy objects in the outer solar system. These objects orbit in a part of the solar system called the Kuiper Belt. In 2006 the International Astronomical Union (IAU) reclassified Pluto as a dwarf planet because it had a rounded shape from effects of its own gravity but it was not massive enough to have cleared the region of its orbit of other bodies. Other dwarf planets in the solar system include Eris, an icy body slightly larger than Pluto that also orbits in part of the Kuiper Belt, and Ceres, a rocky body that orbits in the asteroid belt.

Most of the planets have moons, or satellites. Earth’s Moon has a diameter about one-fourth the diameter of Earth. Mars has two tiny chunks of rock, Phobos and Deimos, each only about 10 km (about 6 mi) across. Jupiter has more than 60 satellites. The largest four, known as the Galilean satellites, are Io, Europa, Ganymede, and Callisto. Ganymede is even larger than the planet Mercury. Saturn has more than 50 satellites. Saturn’s largest moon, Titan, is also larger than the planet Mercury and is enshrouded by a thick, opaque, smoggy atmosphere. Uranus has nearly 30 known moons, and Neptune has at least 13 moons. Some of the dwarf planets also have satellites. Pluto has three moons; the largest is called Charon. Charon is more than half as big as Pluto. Eris has a small moon named Dysnomia.