Planet

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A planet, as defined by the International Astronomical Union (IAU) for the Solar System, is a body that orbits the Sun, is massive enough to be rounded by its own gravity, and has cleared its neighbouring region of planetesimals. No formal definition has been made for extrasolar planets.

The term planet is an ancient one having ties to history, science, myth, and religion. The planets were originally seen as a divine presence; as emissaries of the gods. Even today, many people continue to believe the movement of the planets affects their lives, although such a causation is rejected by the scientific community. As scientific knowledge advanced, the human perception of the planets changed over time, incorporating a number of disparate objects. Even now there is no uncontested definition of what a planet is. In 2006, the IAU officially adopted a resolution defining planets within the Solar System. This definition has been both praised and criticized, and remains disputed by some scientists.

The planets were initially thought to orbit the Earth in circular motions; after the development of the telescope, the planets were determined to orbit the Sun, and their orbits were found to be elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes and shared such features as ice-caps and seasons. Since the dawn of the Space Age, close observation by probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics and even hydrology. Since 1992, through the discovery of hundreds of extrasolar planets (planets around other stars), scientists are beginning to observe similar features throughout the Milky Way Galaxy.

Under IAU definitions, there are eight planets in the Solar System (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and 277 known extrasolar ones.[3] The Solar System also contains at least three dwarf planets (Ceres, Pluto, and Eris). Many of these planets are orbited by one or more moons, which can be larger than small planets. Planets are generally divided into two main types: large, low-density gas giants and smaller, rocky terrestrials. less

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Uranus		Topic	William Herschel	Mar 13, 1781	Uranus ( or ) is the seventh planet from the Sun and the third-largest and fourth-most massive planet in the solar system. It is named after the ancient Greek deity of the sky (Uranus, ), the father of Kronos (Saturn) and grandfather of Zeus (Jupiter). Uranus was the first planet discovered in modern times. Though it is visible to the naked eye like the five classical planet, it was never recognized as a planet by ancient observers due to its dimness. Sir William Herschel announced its...
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Mars		Topic			Mars (pronounced ) is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance as seen from Earth. Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact crater of the Moon and the volcano, valley, desert and polar ice caps of Earth. It is the site of Olympus Mons, the highest known mountain in the Solar System, and...
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Venus		Topic			Venus (pronounced ) is the second-closest planet to the Sun, orbiting it every 224.7 Earth days. It is the brightest natural object in the night sky, except for the Moon, reaching an apparent magnitude of −4.6. Because Venus is an inferior planet, from Earth it never appears to venture far from the Sun: its elongation reaches a maximum of 47.8°. Venus reaches its maximum brightness shortly before sunrise or shortly after sunset, for which reason it is often called the Morning Star or the...
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Mercury		Topic			Mercury (pronounced ) is the innermost and smallest planet in the solar system (since Pluto was re-labelled as a dwarf planet), orbit the Sun once every 88 days. Mercury is bright when viewed from Earth, ranging from −2.0 to 5.5 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun (greatest elongation) is only 28.3°: It can only be seen in morning and evening twilight. Comparatively little is known about it; the first of two spacecraft to approach...
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Jupiter		Topic			Jupiter (pronounced ) is the fifth planet from the Sun and the largest planet within the Solar System. It is two and a half times as massive as all of the other planets in our Solar System combined. Jupiter, along with Saturn, Uranus and Neptune, is classified as a gas giant. Together, these four planets are sometimes referred to as the Jovian planet, where Jovian is the adjectival form of Jupiter. The planet was known by astronomer of ancient times and was associated with the mythology and...
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Saturn		Topic			Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Along with the planets Jupiter, Uranus and Neptune it is classified as a gas giant (also known as a Jovian planet, after the planet Jupiter). It was named after the Roman god Saturnus, equated to the Greek Kronos (the Titan father of Zeus) and the Babylonian Ninurta. Saturn's symbol represents the god's sickle (Unicode: ). The planet Saturn is composed of hydrogen, with small proportions...
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Neptune		Topic	Urbain Le Verrier	Sep 23, 1846	Neptune (, AmE: ) is the eighth and farthest planet from the Sun in the Solar System. It is the fourth largest planet by diameter, and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 Earth masses and less dense. The planet is named after the Roman god of the sea. Its astronomical symbol is , a stylized version of the god Neptune's trident. Discovered on September 23, 1846, Neptune was the first planet found by...
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Earth		Topic			Earth (pronounced ) is the third planet from the Sun and is the largest of the terrestrial planet in the Solar System in diameter, mass and density. It is also referred to as the Earth, Planet Earth, the World, and Terra. Home to millions of species, including human, Earth is the only place in the universe where life is known to exist.Scientific evidence indicates that the planet formed 4.54 billion years ago, and life appeared on its surface within a billion years. Since then, Earth's...
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