Planets are shaped like a slightly squashed ball (called aspheroid). Objects that orbit planets are calledsatellites. A star and everything which orbits it are called astar system.
The name "planet" is from theGreek wordπλανήτης (planetes), meaning "wanderers", or "things that move".
Until the 1990s, people only knew the planets in the Solar System. Since then 4,905 extrasolar planets (exoplanets) have been discovered in 3,629 planetary systems (January 2022 data). The count includes 808 multi-planetary systems. Known exoplanets range in size from gas giants about twice as large as Jupiter down to just over the size of the Moon. About 100 of these planets are roughly the size as Earth. Nine of these orbit in thehabitable zone of their star.[1][2] Also extrasolar moons (moons orbiting extrasolar planets) have been observed.[3]
The planets are made of elements that are very different from the Sun (which is mostlyhydrogen).[4][5] The Sun is mostly made up ofhydrogen, with somehelium. Its energy comes from converting hydrogen to helium. In contrast, the planets are mostly made up of larger atoms and molecules whichcould not have come from the Sun.[6] The materials of planets must have come from another source or sources. Those sources were atoms made in earliersupernovae explosions near the Sun's path as it moved through its part of theMilky Way. The material captured by the Sun'sgravity formed the planets. The same thing happened in other planetary systems in the galaxy.[7]
Thegas giants are made up of hydrogen gas like the Sun, plus (at their centres) metallic elements like the terrestrial planets.
The planets in the Solar System have names ofGreek orRoman gods, except for 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 exampleChinese, use different names. Moons also have names of gods and people from classicalmythology. The names of the moons ofUranus are from theplays written byShakespeare.
There are three types of planets in the Solar System. They are:
Terrestrial orrocky: These are planets that are like Earth. They are mostly made up ofrocks. They include: Mercury, Venus, Earth, Mars.
Jovian: These planets are much bigger than our home planet. They are mostly made ofgas orices. They include: Jupiter, Saturn, Uranus, Neptune.
Gas giant: These giant planets are mostly made ofgas. They include: Jupiter, Saturn.
Ice giant: These giant planets are mostly made ofices. They include: Uranus, Neptune.
Icy: These planets are mostly made ofices. They include: Pluto, Orcus, Haumea, Makemake, Quaoar, Sedna, Gonggong, Eris. 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 (likeTriton).
↑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.
↑Williams J. 2010. The astrophysical environment of the solar birthplace..Contemporary Physics.51 (5): 381–396.
↑Though the gas giants do have hydrogen: because their gravity is strong enough to hang on to the small molecules.
↑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
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