Planet

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Mercury Venus
Earth Mars
Jupiter Saturn
Uranus Neptune
Pluto 15760 Albion
The nine planets of the Solar System (not including (181708) 1993 FW), plus Pluto:
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 celestial object that does not undergo nuclear reactions or orbits a star or stars. Planets are round in shape and can destroy anything that comes into their orbits. They can be small or large, or be made out of gas, liquid, or solid material. The planets of the Solar System orbit the Solar System Barycenter, and indirectly, the Sun. The Earth is a planet.

The International Astronomical Union defines three criteria needed for a celestial body to be considered a planet:

  1. It must have sufficient mass for its own gravity to pull it into a round, near spherical shape.
  2. It must orbit (explicitly) the Sun.
  3. It must be able to "clear its neighborhood" and orbit of debris such as asteroids and trojans.

Planets are generally less massive than sub-stellar brown dwarfs (less than 0.01 solar masses or ~20 times the mass of Jupiter). Planets that orbit stars other than the Sun are called exoplanets. The nearest orbits the nearest star to the Sun, Proxima Centauri.

Count History[edit]

Before the life of Pythagoras, the Enlightenment, and the Renaissance, there were 8 planets:

  1. Sun
  2. Moon
  3. Mercury
  4. the Morning Star
  5. Venus (the Evening Star)
  6. Mars
  7. Jupiter
  8. Saturn

During the Enlightenment and Renaissance, the Earth was discovered to be a planet and orbit the Sun along with the others. Before then, Pythagoras found that the Morning Star and Venus were the same. The planet count was reduced to six:

  1. Mercury
  2. Venus
  3. Earth
  4. Mars
  5. Jupiter
  6. Saturn

Discoveries by various astronomers such as William Herschel and Urbain LeVerrier increased the count to 16:

  1. Mercury
  2. Venus
  3. Earth
  4. Mars
  5. Ceres
  6. 2 Pallas
  7. 3 Juno
  8. 4 Vesta
  9. 5 Astraea
  10. 6 Hebe
  11. 7 Iris
  12. Jupiter
  13. Saturn
  14. Uranus
  15. Neptune
  16. Pluto

In the 1900s, it was found that the "planets" from Ceres to 7 Iris all orbited in the same path, and so were thought to be the remains of a planet dubbed "Phaeton". However, this theory was quickly refuted. Those "planets", however, were nonetheless reclassified as star-like "asteroids" instead, orbiting in the asteroid belt. In 1993, two new planets were discovered,[1] leaving the Solar System with 11 planets:

  1. Mercury
  2. Venus
  3. Earth
  4. Mars
  5. Jupiter
  6. Saturn
  7. Uranus
  8. Neptune
  9. Pluto
  10. 15760 Albion[1]
  11. (181708) 1993 FW[1]

In 2006, the IAU came up with their current conditions for a body to be considered a planet. Pluto was not able to clear its neighborhood of debris due to its location in the Kuiper Belt, and was reclassified as a dwarf planet, leaving the Solar System with 10:

  1. Mercury
  2. Venus
  3. Earth
  4. Mars
  5. Jupiter
  6. Saturn
  7. Uranus
  8. Neptune
  9. 15760 Albion[1]
  10. (181708) 1993 FW[1]

The subcategory for dwarf planets was also introduced, with the conditions being:

  1. It must have sufficient mass for its own gravity to pull it into a round, near spherical shape.
  2. It must orbit (explicitly) the Sun.
  3. It must not be a moon or natural satellite.

Many people are against these conditions, especially since they were taught in their studies that Pluto was a planet. One scientist came up with a revised definition, that would allow for a total of 397 total Solar System planets, including the Earth's Moon.[2]

References[edit]


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