Each civilization and their respective language has had their own names for each of the planets, since their existence has been known. Often they arise from mythology, or in relation to things noted previously in nature. The following is a list of each planets name in Japanese (romaji, kanji, hiragana),and a brief description of the kanji in the name.

太陽(たいよう) - Taiyou - The Sun

太(たい), "tai" means, well, big(can't miss the sun now can we?).
陽(よう), "you" is all things positive, the yang, sunshine(opposite to darkness, yin, and the negative)

水星(すいせい) - Suisei - Mercury

水(すい) - "sui" (also read "mizu") is the kanji for water. This kanji was chosen before the true nature of Mercury was known.
星(せい) - "sei" is a kanji that is appended to all planets except earth, generally meaning star.

金星(きんせい) - Kinsei - Venus

金(きん) - "kin" is the kanji for gold, most likely for the color venus takes when visible.

liveforever says: The "kin" kanji in "kinsei", Venus, can also mean "metal" or "money/coinage". "Metal-star" is a reasonable translation.

地球(ちきゅう) - Chikyuu - Earth

地(ち) - "chi" is the kanji that represents the earth (soil), the ground.
球(きゅう)- "kyuu" menas ball or sphere.
Which explains all the Gundam references to various things such as the Earth Sphere Alliance

月(つき) - Tsuki - The Moon
While not a planet, it deserves a mention...
The moon has only one kanji, which serves two purposes. One is simply to refer to the moon (as a heavenly object), the other refers to the months in a year, such as now, ichigatsu, January, the first month (this pattern is not iron clad. One month is "ichigetsu", two months is "futatsuki," but February is "nigatsu.") This originated from the fact that the lunar cycle takes approximately one month.

火星(かせい) - Kasei - Mars

火(か) - "ka" means fire, evidenced by the red color of Mars.

木星(もくせい) - Mokusei - Jupiter

 木(もく) - "moku" means tree or wood, most likely from the way Jupiter's clouds resemble woods grain.
 Incidentally, all of Jupiter and Saturn's moons use the Roman names, and are written in katakana.

土星(どせい) - Dosei - Saturn

土(ど) - "do" too means earth, or soil. Incidentally, it also stands for the country of Turkey.

天王星(てんのうせい) - Tennousei - Uranus

天(てん) - "ten" means the skies or the heavens

王(のう) - "nou" is an unusual reading, commonly the kanji is read "ou." It signifies a king or magistrate. One could infer that Uranus was the "Ruler of the Skies"


海王星(かいおうせい) - Kaiousei - Neptune

海(かい) - "kai" (also read "umi") means the sea, or the ocean.
The Japanese name for Neptune is likely a cross cultural translation done from the Roman god of the sea. Had the planet been discovered later (after the Meiji Era had begun, around 1868), it would have been written in katakana from its Roman name (as Neptune was discovered 22 years earlier, in 1846). This may be the case for both Uranus and Pluto as well.

冥王星(めいおうせい) - Meiousei - Pluto

冥(めい) - "mei" represents dark. Pluto, being so small, is hard to find/see without constant observation.
Being an "ou" like Uranus and Neptune, Pluto therefore would be the ruler of the dark, or the night.

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