I.

Introduction

Planet, a round body in space that orbits a star. To be a planet, a body must be big enough to settle into a rounded shape from the inward pull of its own gravitation. A planet shines by reflecting light and not by releasing nuclear energy the way a star does. Our solar system has eight major planets—Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune—and a number of small, dwarf planets, including Pluto, Eris, and Ceres. A planet-like body that revolves around a larger planet is called a satellite or moon rather than a planet. Planets are distinct from asteroids and comets, smaller bodies that also orbit stars.

The study of planets is called planetary science. Areas of research include the composition, structure, and evolution of planets and planetary systems. New discoveries have made scientists rethink the best way to define what objects should be called planets. Important properties that help determine what should be called a planet include the object’s size, its shape, its location, what it is made of, and the way it formed.

Ideas about planets have changed many times in the history of science. For thousands of years, most people thought that planets circled around Earth instead of the Sun. Many people also believed that planets gave off their own light and were made of a substance called ether that was different from ordinary matter found on Earth. The modern understanding of planets began with the pioneering scientists Nicolaus Copernicus and Galileo in the 16^th and early 17^th centuries. They showed that planets orbit the Sun and are made of ordinary matter like the Earth.

II.

Solar System Planets

Astronomers have changed the count of official planets in our solar system several times. Copernicus counted six planets: Mercury, Venus, Earth, Mars, Jupiter, and Saturn. Using telescopes, astronomers discovered Uranus in 1781 and Neptune in 1846. Pluto was thought to be a planet when it was discovered in 1930.

For most of the 20th century scientists counted nine planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Beginning in the 1990s, astronomers began finding additional small Pluto-like bodies in the outer solar system in a region called the Kuiper Belt. One of these so-called Kuiper Belt Objects (KBOs) turned out to be larger than Pluto and so seemed to qualify as the tenth planet in the solar system. Discovered in 2005 and now called Eris, the distant body forced scientists to find a clearer definition of a planet. The responsibility for defining a planet fell to the International Astronomical Union (IAU), a body of astronomers and other scientists that decides on official names for objects in the solar system and beyond.

According to resolutions passed by the IAU in 2006, a “classical planet” orbiting the Sun must have a rounded shape from effects of its own gravity and must be the dominant object in its region of space. To be the dominant object, the planet must have been massive enough for its own gravitation to clear away other objects in the neighborhood of its orbit. Such objects were either drawn into the planet as the planet formed or were sent off into completely different orbits. The IAU now recognizes eight classical planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

The IAU put Pluto in a new category called “dwarf planets.” Like “classical planets,” dwarf planets orbit the Sun and have rounded shapes. However, dwarf planets are not massive enough for their gravity to clear other bodies from the neighborhoods of their orbits. Dwarf planets are found in regions of the solar system that contain swarms of smaller objects. The asteroid belt between Jupiter and Mars is filled with small rocky bodies, while the Kuiper Belt beyond Neptune and much more distant Oort Cloud contain icy comet-like bodies. Official dwarf planets now include Ceres in the asteroid belt and Pluto and Eris in the Kuiper Belt. More dwarf planets will be formally recognized in the future.

The IAU’s official definitions of “classical planet” and “dwarf planet” are not meant to apply to bodies outside our solar system. The new definitions are also controversial and have met opposition from a large number of astronomers and planetary scientists. Other researchers, however, have supported the downgrading of Pluto to a dwarf planet as recognition of its origin among the comet-like bodies orbiting the Sun beyond Neptune.

Another resolution passed by the IAU in 2006 designated bodies that are smaller than planets and dwarf planets as “small solar system bodies.” Such objects include asteroids and comets. The IAU keeps an official numbered catalog of “minor planets,” which include dwarf planets, Kuiper Belt Objects (KBOs), and asteroids—but not comets—in the solar system.

In addition to the official definitions decided on by the IAU, scientists use a number of other terms for solar system bodies that share some properties with planets. The term planetoid has been used for objects too small to be planets, most commonly asteroids but also some KBOs. Such smaller objects are also sometimes called planetesimals. The term planetesimal is a broad term that can include comets and is widely used to refer to the objects from which planets are thought to form by accretion—the process of collecting together into larger and larger bodies. A forming planet is called a protoplanet. The general term planetary body can refer to both major and minor planets (asteroids, KBOs), as well as to natural satellites (moons) that are similar to planets in composition but orbit around larger planets.

The planets in our solar system are commonly grouped by their composition and their sizes. The major inner planets Mercury, Venus, Earth, and Mars are called terrestrial (meaning “Earth-like”) or rocky planets, referring to a rocky outer crust around a mantle and a core. The major outer planets Jupiter, Saturn, Uranus, and Neptune are called giant planets, referring to their large size—the term Jovian (meaning “Jupiter-like”) planets is also used for such planets. Jupiter and Saturn are also classified as gas giant planets for their massive atmospheres of hydrogen and helium around a rocky core. Uranus and Neptune are classified as ice giant planets, composed mostly of water in the form of a hot, high-pressure slushy solid or “ice” under an atmosphere of hydrogen and helium and surrounding a rocky core. Pluto and other rounded icy bodies in the Kuiper Belt (KBOs) have been called ice dwarfs, referring to their small size and outer layers of solid ice around a rocky core.

III.

Planets Beyond Our Solar System

Scientists have long thought that other stars may have planets the way the Sun does. Beginning in the mid-1990s astronomers began finding planets that orbit other stars, thanks to powerful telescopes and computers to analyze data. Known as extrasolar planets or exoplanets, over 200 of these distant planets have been detected. For more information, see Extrasolar Planets.

There is currently no official scientific definition of a planet that includes the known range of extrasolar planets. An upper size limit on exoplanets can be set, however. Bodies that have about 13 times the mass of Jupiter but less than 8 percent the mass of the Sun are called brown dwarfs. Brown dwarfs are considered a kind of failed star. Anything less massive than a brown dwarf might be considered a planet if it orbits a star.

Objects called planemos (short for “planetary mass objects”) are too small to be brown dwarfs and are similar to giant extrasolar planets—except that they float free in space rather than orbiting a star.

Even stranger kinds of planets have been detected orbiting pulsars, a kind of dense neutron star left by supernova explosions. The planets may have formed from debris created in the catastrophic explosion of a massive star. What such planets are made of is not known.

IV.

Formation of Planets

According to current theories, stars and planets form in different ways. Stars result from the gravitational collapse of a cloud of gas and dust that is drawn in toward a central body that becomes more and more massive until it begins to fuse hydrogen in its core. When a star forms it may be surrounded by a rotating disk of leftover debris made of gas, dust, and ice.

Planets are thought to take shape by accretion (clumping together) of the tiny dust and ice grains in the disk. The objects collect into planetesimals. The planetsimals gradually accumulate into larger and larger bodies that can build into protoplanets that have a spherical shape from the effects of their own gravity. When a planet is large enough to have settled into a rounded shape, it is said to be in hydrostatic equilibrium. See Planetary Science: Formation of Planets.

V.

Planets and People

A.

Ancient Ideas

The word planet comes from the Greek word planetes, meaning “wanderer.” Seen from Earth, the naked-eye planets Mercury, Venus, Mars, Jupiter, and Saturn change their positions against the background of stars over periods of weeks and months. People once used the term “wandering stars” for planets. The two brightest heavenly bodies, the Sun and the Moon, have more regular motions but were also called planets in ancient times, for a total of seven planets. Explaining the strange motions of the planets posed a major challenge to the ancient philosophers and astronomers. See History of Astronomy.

In the 4th century BC, the Greek philosopher Aristotle asserted that Earth was the stationary center of the universe and the heavens revolved around the Earth. Heavenly bodies such as stars and planets were made of pure ether, a substance totally different from the imperfect form of matter found on Earth. The stars and planets both shined from their own fiery heat and had perfect circular motions. Aristotle’s views were widely accepted for centuries and prevailed in European thought up until the Renaissance.

Ancient astrologers believed that the motions of the planets affected events on Earth and guided the lives of individuals. Because the planets all orbit in the plane of the solar system, the Sun, Moon, and planets appear to move through a single band of constellations of stars called the zodiac. Special significance was attached to the passage of a planet through a particular constellation of the zodiac.

Many ancient cultures associated the planets with gods and goddesses, or other supernatural beings. The English names for the planets Mercury, Venus, Mars, Jupiter, and Saturn date from their ancient identification with Roman gods. Educated Romans saw the planets not as gods themselves but as heavenly objects that represented qualities of the particular gods. Thus the bright, majestically moving planet Jupiter was “Jupiter’s star,” and fiery red Mars was associated with god of war and called “Mars’s star.”

B.

Understanding Planets

The traditional notion of seven planets lasted until the 16th century when the Polish astronomer Nicolaus Copernicus proposed that the Sun was at the center of the universe, not Earth. In the Copernican system Earth became a planet that orbited Sun and the Moon became a satellite that orbited Earth. Copernicus’s system redefined the planets as worlds similar to Earth and not perfect objects made of pure ether.

The true scientific study of planets began with the Italian astronomer and physicist Galileo in the early 17th century. Galileo used a telescope to study the surface of the Moon, the phases of Venus, and the moons of Jupiter. His observations fully confirmed that the Moon and the planets were worlds like Earth. Galileo clashed with the Roman Catholic Church over his support for the Sun-centered Copernican system, however. Authorities in Catholic Church put Galileo on trial and forced him to say that his scientific ideas were wrong.

As astronomers after Galileo made more discoveries about the solar system, they had to decide what should be called a planet. The German-born British astronomer William Herschel discovered the planet Uranus in 1781 but first thought it was a comet. Uranus was not officially recognized as a planet until more observations showed it had a circular orbit beyond Saturn.

In 1801 Italian astronomer Giuseppe Piazzi found a body orbiting between Mars and Jupiter. Named Ceres, the body was thought to be the “missing” planet that should exist between Mars and Jupiter. Other astronomers found additional bodies with similar orbits. For a few decades these objects were called planets. After scientists detected many more such small bodies, they decided to call them asteroids rather than true planets. (Ceres is now considered a dwarf planet.)

The planet Neptune was found in 1846 based on slight changes in the orbit of Uranus, likely caused by gravity from another planet further out in the solar system. The calculations were so accurate that the first astronomer who made the search with a telescope located the new planet in a few hours.

In 1859, an astronomer claimed to find a small body orbiting nearer the Sun than Mercury. The discovery was hailed as a new planet named Vulcan. Later, more careful searches failed to find Vulcan again and astronomers decided that the supposed innermost planet never existed.

The existence of a large planet beyond Neptune was proposed by the American astronomer Percival Lowell to explain slight irregularities apparently detected in Neptune’s orbit. (Lowell’s calculations contained errors and no irregularities actually existed.). After Lowell’s death, the Lowell Observatory in Flagstaff, Arizona, hired the young amateur astronomer Clyde Tombaugh to search for Lowell’s so-called Planet X. In 1930, Tombaugh found a small point of light that moved like a planet only a few degrees away from one of the positions predicted by Lowell. The object was named Pluto and hailed as the ninth planet in the solar system.

Pluto turned out to be far smaller than the planet Lowell had imagined, however. In fact, the discovery of Pluto was a lucky accident that rewarded Clyde Tombaugh’s diligent work and not a confirmation of Lowell’s ideas. Whether Pluto deserved to be called a planet at all later became a topic of scientific debate. The International Astronomical Union made Pluto a dwarf planet in 2006, leaving eight major (or “classical” planets) in the solar system.

The most important information about planets has come from ambitious space exploration missions. Spacecraft have been able to fly by or orbit all of the eight major planets in the solar system. Probes have even landed on the surfaces of Mars and Venus, providing photographs and scientific data. The public worldwide has shared in these discoveries through television broadcasts, Web sites, movies, and books. Planets have become something very different from the distant orbs of burning ether most people once thought they were.