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.