There are eight planets in the 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 2017, we know of 3,610 other planets. These newly found planets are orbiting other stars: they are extrasolar planets. Sometimes people call them "exoplanets".
Origin of the planetsEdit
The planets are composed of material quite different from the Sun. The Sun is almost entirely composed of hydrogen, and some helium. Its energy comes from the conversion of hydrogen to helium. In contrast, the "terrestrial" planets are composed almost entirely of larger atoms and molecules which cannot have come from the Sun. It follows, therefore, that the material which forms those planets must have come from another source or sources. Those sources were the atoms formed in supernovae explosions by much larger and much shorter-lived stars in the Sun's neighbourhood as it moved in the galaxy. This material was captured by the Sun's gravity and pulled along to become the basic material out of which the planets condensed. The same considerations apply to other planetary systems in the galaxy.
The gas giants are composed of hydrogen gas from the same source as the Sun, plus (at their centres) higher "metallic" elements like the terrestrial planets. This is known from the drive-by data gathered by satellites, and by drive-into probes like the Galileo (spacecraft). There is a huge amount of data collected about each planet. The data is stored on computer files, and there are summary volumes in print.
In the Solar SystemEdit
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. The names of the moons of Uranus are from the plays written by Shakespeare.
Here is a list of planets in the Solar System. They are ordered by how close they are to the Sun, nearest first.
Types of planetsEdit
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).
- Jean Schneider. "Interactive Extra-solar Planets Catalog". The Extrasolar Planets Encyclopedia. Retrieved 2011-06-23. CS1 maint: discouraged parameter (link)
- Charles H. Lineweaver 2001. An estimate of the age distribution of terrestrial planets in the Universe: quantifying metallicity as a selection effect. Icarus. 151 (2): 307–313. 
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- Safronov, Viktor Sergeevich 1972. Evolution of the protoplanetary cloud and formation of the Earth and the planets. Israel Program for Scientific Translations. ISBN 0-7065-1225-1