有冇人可以知道'行星'的分類及其解釋

According to the IAU's current definitions there are eight planets in the Solar System. In increasing distance from the Sun, they are:

1. (☿) Mercury

2. (♀) Venus

3. (⊕) Earth

4. (♂) Mars

5. (♃) Jupiter

6. (♄) Saturn

7. (♅) Uranus

8. (♆) Neptune

The larger bodies of the Solar System can be divided into categories based on their composition:

＊Terrestrials:

Planets (and possibly dwarf planets) that are similar to Earth — with bodies largely composed of rock: Mercury, Venus, Earth and Mars. If including dwarf planets, Ceres would also be counted, with as many as three other asteroids that might be added.

Terrestrial planets all have roughly the same structure: a central metallic core, mostly iron, with a surrounding silicate mantle. The Moon is similar, but lacks an iron core. Terrestrial planets have canyons, craters, mountains, and volcanoes. Terrestrial planets possess secondary atmospheres — atmospheres generated through internal vulcanism or comet impacts, as opposed to the gas giants, which possess primary atmospheres — atmospheres captured directly from the original solar nebula.

＊Gas giants:

Planets with a composition largely made up of gaseous material and are significantly more massive than terrestrials: Jupiter, Saturn, Uranus, Neptune. Ice giants are a sub-class of gas giants, distinguished from gas giants by their depletion in hydrogen and helium, and a significant composition of rock and ice: Uranus and Neptune.

Gas giants may have a rocky or metallic core—in fact, such a core is thought to be required for a gas giant to form—but the majority of its mass is in the form of the gaseous hydrogen and helium, with traces of water, methane, ammonia, and other hydrogen compounds. (Although familiar to us as gases on Earth, these constituents are expected to be compressed into liquids or solids deep in a gas giant's atmosphere.)

Unlike rocky planets, which have a clearly defined difference between atmosphere and surface, gas giants do not have a well-defined surface; their atmospheres simply become gradually denser toward the core, perhaps with liquid or liquid-like states in between. One cannot "land on" such planets in the traditional sense. Thus, terms such as diameter, surface area, volume, surface temperature and surface density may refer only to the outermost layer visible from space.

All four planets rotate relatively rapidly, which causes wind patterns to break up into east-west bands or stripes. These bands are prominent on Jupiter, muted on Saturn and Neptune, and barely detectable on Uranus. Uranus has an extreme tilt unlike the other gas giants that causes extreme seasons.

Finally, all four are accompanied by elaborate systems of rings and moons. Saturn's rings are the most spectacular, and were the only ones known before the 1970s. As of 2006, Jupiter is known to have the most moons, with sixty-three.

＊Ice dwarfs:

Objects that are composed mainly of ice, and do not have planetary mass. The dwarf planets Pluto and Eris are ice dwarfs, and several dwarf planetary candidates also qualify.

An ice dwarf is a planetary body that is larger than the nucleus of a normal comet and much icier than any asteroid. They are believed to be up to a few hundred kilometers across, and they are found mostly in the Oort Cloud and the Kuiper Belt in very large numbers.

Reference:

http://en.wikipedia.org/wiki/Planet

http://en.wikipedia.org/wiki/Terrestrial_planet

http://en.wikipedia.org/wiki/Gas_giant

http://en.wikipedia.org/wiki/Ice_dwarf

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A planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion in its core, and has cleared its neighbouring region of planetesimals.[1][2]

After stars and stellar remnants, planets are some of the most massive objects known to humanity. They play an important part in the structure of planetary systems, and are also considered, along with large moons, the most feasible environment for life.[3] Thus planetary science is essential not only to comprehend the structure of the universe, but also to better understand the development of life, and to aid the search for extraterrestrial intelligence. Additionally, the planets visible from Earth have played a vital role in the shaping of human culture, religion and philosophy in numerous civilisations. 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.

Under IAU definitions, there are eight planets in the Solar System (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and also 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. There have also been more than two hundred planets discovered orbiting other stars.[4] Planets are generally divided into two main types: large, low-density gas giants and smaller, rocky terrestrials. Dwarf planets, a separate category, can either be terrestrials or frozen ice dwarfs.