Planet

From Wikipedia

A planet is a large object such as Earth, Jupiter, etc. that orbits a star. It is smaller than a star, and it does not make light. Planets are ball-shaped (spheres). Objects that orbit planets are called moons. There are eight planets in our solar system. Pluto used to be called a planet, but in August 2006, a group of astronomers decided it should be called a dwarf planet instead. There are two more dwarf planets, Ceres and Eris.

The name "planet" is from the Greek word πλανήτης (planetes), meaning "wanderers", or "things that move". Until the 1990s, people only knew of the eight that were in our solar system then. As of January 2007, we know of 209 other planets. All of the newly found planets are in other solar systems: they are extrasolar planets. Sometimes people call them "exoplanets".

[edit] In our solar system

The planets in our 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 of Shakespeare.

[edit] Planets

Here is a list of planets in our solar system. The first planets are closer to the Sun.

• Mercury
• Venus
• Earth
• Mars
• Jupiter
• Saturn
• Uranus
• Neptune

[edit] Types of Planets

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 Earth's 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 less 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 our solar system (like Triton).

Some people think the Earth and moon are a double planet, because:

• The Moon is 1.5 times larger than Pluto.
• The gravitational force of the Sun on the moon is more than double the gravitational force of the Earth on the moon.

[edit] Extrasolar planets

If a planet goes around another star besides our Sun, it is called an extrasolar planet. The extrasolar planets that we have found until today have masses about the same or larger than the gas giants in our solar system.

But there are three planets, orbiting a burned-out star, that are about the same size as the terrestrial planets. Also, a planet orbiting the star mu Arae has a mass of about 14 times that of the Earth.

It isn't clear whether the newly found large planets are similar to gas giants in our solar system. Perhaps they are entirely different. Some of the newly found planets, "hot Jupiters", orbit very close to their parent star, in nearly circular orbits. Because of this, they get much more radiation from the star than the gas giants in our solar system. So it is not certain that they are the same sort of planet.

Because other stars are so far away, it is very difficult to find planets orbiting other stars. The National Aeronautics and Space Administration of the United States has a program to make an artificial satellite called the "Terrestrial Planet Finder". This satellite could find "smaller" planets with masses similar to terrestrial planets.

Today, with our technology, people can only find extrasolar planets that are so big and so close to a star that their gravity makes the star move quite a lot. When we can build better telescopes, smaller planets might be found farther away.

If a planet does not go around any star, it is called an interstellar planet. Today, we do not know of any interstellar planet. But people think they are possible, because of computer simulations.

[edit] See also

• List of planets
• Dwarf planets

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San · Mekyuri · Vines · Giraun · Mas · Ceres · Jyupita · Saten · Yurenes · Nepsyun · Pluto · Eris
Planets · Dwarf planets · Moons: Terrestrial · Martian · Jovian · Saturnian · Uranian · Neptunian · Plutonian · Eridian
Small bodies:   Meteoroids · Asteroids/Asteroid moons (Asteroid belt) · Centaurs · TNOs (Kuiper belt/Scattered disc) · Comets (Oort cloud)
See also astronomical objects, the solar system's list of objects, sorted by radius or mass, and the Solar System Portal