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Planet facts for kids

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Mercury Venus
Earth Mars
Jupiter Saturn
Uranus Neptune
The eight known planets of the Solar System
Mercury, Venus, Earth, and Mars
Jupiter and Saturn (gas giants)
Uranus and Neptune (ice giants)

Shown in order from the Sun and in true color. Sizes are not to scale.

A planet is a large object such as Jupiter or Earth that orbits a star. It is smaller than a star, and it does not make light.

Planets are shaped like a slightly squashed ball (a spheroid). Objects that orbit planets are called satellites. A star and everything which orbits it are called a star system.

There are eight planets in our Solar System. Pluto used to be called a planet, but in August 2006, the International Astronomical Union decided it was a dwarf planet instead. There are four more known dwarf planets in the Solar System, Ceres, Makemake, Eris and Haumea.

The name "planet" is from the Greek word πλανήτης (planetes), meaning "wanderers", or "things that move". Until the 1990s, people only knew of those in the Solar System. As of June 2011, we know of 563 other planets. All of these newly found planets are orbiting other stars: they are extrasolar planets. Sometimes people call them "exoplanets".

In the Solar System

Planets2008
Planets of the Solar System

The planets in the Solar System have names of Greek or Roman gods, apart from Earth, because people did not think Earth was a planet in old times. However, Earth is occasionally referred by the name of a Roman god: Terra. Other languages, for example Chinese, use different names. Moons also have names of gods and people from classical mythology, or from the plays written by Shakespeare.

Planets

Here is a list of planets in the Solar System. They are ordered by how close they are to the Sun, nearest first.

Planet Symbol
Mercury Astronomical symbol for Mercury
Venus Astronomical symbol for Venus
Earth Astronomical symbol for Earth
Mars Astronomical symbol for Mars
Jupiter Astronomical symbol for Jupiter
Saturn Astronomical symbol for Saturn
Uranus Astronomical symbol for Uranus
Neptune Astronomical symbol for Neptune

Mythology and naming

Naming of planets differs between planets of the Solar System and Exoplanets (planets of other planetary systems). Latter are commonly named after their parent star and their order of discovery within its planetary system, such as Proxima Centauri b.

The names for the planets of the Solar System (other than Earth) in the English language are derived from naming practices developed consecutively by the Babylonians, Greeks and Romans of antiquity. The practice of grafting the names of gods onto the planets was almost certainly borrowed from the Babylonians by the ancient Greeks, and thereafter from the Greeks by the Romans.

In ancient Greece, the two great luminaries, the Sun and the Moon, were called Helios and Selene, two ancient Titanic deities; the slowest planet, Saturn, was called Phainon, the shiner; followed by Phaethon, Jupiter, "bright"; the red planet, Mars was known as Pyroeis, the "fiery"; the brightest, Venus, was known as Phosphoros, the light bringer; and the fleeting final planet, Mercury, was called Stilbon, the gleamer. The Greeks assigned each planet to one among their pantheon of gods, the Olympians and the earlier Titans:

  • Helios and Selene were the names of both planets and gods, both of them Titans (later supplanted by Olympians Apollo and Artemis);
  • Phainon was sacred to Cronus, the Titan who fathered the Olympians;
  • Phaethon was sacred to Zeus, Cronus's son who deposed him as king;
  • Pyroeis was given to Ares, son of Zeus and god of war;
  • Phosphoros was ruled by Aphrodite, the goddess of love; and
  • Stilbon with its speedy motion, was ruled over by Hermes, messenger of the gods and god of learning and wit.
Olympians
The Greek gods of Olympus, after whom the Solar System's Roman names of the planets are derived

Although modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the Roman Empire and, later, the Catholic Church, use the Roman (Latin) names rather than the Greek ones.

When the Romans studied Greek astronomy, they gave the planets their own gods' names: Mercurius (for Hermes), Venus (Aphrodite), Mars (Ares), Iuppiter (Zeus) and Saturnus (Cronus). Some Romans, following a belief possibly originating in Mesopotamia but developed in Hellenistic Egypt, believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts went Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon (from the farthest to the closest planet). Therefore, the first day was started by Saturn (1st hour), second day by Sun (25th hour), followed by Moon (49th hour), Mars, Mercury, Jupiter and Venus. Because each day was named by the god that started it, this became the order of the days of the week in the Roman calendar.

In English, Saturday, Sunday, and Monday are straightforward translations of these Roman names. The other days were renamed after Tīw (Tuesday), Wōden (Wednesday), Þunor (Thursday), and Frīġ (Friday), the Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus, respectively.

Earth's name in English is not derived from Greco-Roman mythology. Because it was only generally accepted as a planet in the 17th century, there is no tradition of naming it after a god. The name originates from the Old English word eorþe, which was the word for "ground" and "dirt" as well as the world itself. As with its equivalents in the other Germanic languages, it derives ultimately from the Proto-Germanic word erþō, as can be seen in the English earth, the German Erde, the Dutch aarde, and the Scandinavian jord. Many of the Romance languages retain the old Roman word terra (or some variation of it) that was used with the meaning of "dry land" as opposed to "sea". The non-Romance languages use their own native words. The Greeks retain their original name, Γή (Ge).

Non-European cultures use other planetary-naming systems.

When subsequent planets were discovered in the 18th and 19th centuries, Uranus was named for a Greek deity and Neptune for a Roman one (the counterpart of Poseidon). The asteroids were initially named from mythology as well. Pluto was given a classical name, as it was considered a major planet when it was discovered.

Types of planets

Image of the Solar System planets. Top image shows the Terrestrial planets and the bottom image shows the Gas giants.

Astronomers speak about major (or true) planets, and minor planets, which are smaller objects that go around the Sun. Some examples of "minor planets" are asteroids, comets, and trans-Neptunian objects.

Planets in the Solar System are of three sorts:

  • Terrestrial or rocky: Planets that are similar to Earth — in them is mostly rock: Mercury, Venus, Earth, Mars.
  • Jovian or gas giant: These planets are mostly made of gas: Jupiter, Saturn, Uranus, Neptune. Uranian planets are a special sort of gas giants, they have more hydrogen and helium.
  • Icy: Sometimes people also have a third sort, for bodies such as Pluto (though Pluto is no longer called a planet by everyone). These planets are mostly made of ice.

Many objects in the Solar System that are not planets are also "icy". Examples are the icy moons of the outer planets of the Solar System (like Triton).

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See also

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