Names of the planets in Japanese

Look at the Japanese names for the planets and a quiet surprise shows up: Mars is a fire star, Venus a metal star, Mercury a water star. The list does not follow Roman gods the way English does. It follows the five classical elements of Chinese thought, then pins the character 星 (sei, “star”) after each one.

That is why the planets in Japanese often feel readable even before you open a dictionary. The kanji tell a story—sometimes philosophical rather than geological—and the same elements reappear in the days of the week. Below you will find the names with readings, literal meanings, a map of that system, space vocabulary, and example sentences.

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Names of the planets in Japanese

Most classical planet names pair an element or a title with 星. Earth is the exception: 地球 (chikyū) means “earth sphere,” not “earth star.” The general word for planet, 惑星 (wakusei), literally suggests a “bewildering” or “wandering” star—an old way of talking about lights that move against the fixed background of the sky.

English	Japanese	Reading	Literal sense
Planet	惑星	wakusei	wandering / deluding star
Solar system	太陽系	taiyōkei	sun + system
Sun	太陽	taiyō	great yang (also 日)
Moon	月	tsuki	moon / month
Mercury	水星	suisei	water star
Venus	金星	kinsei	metal / gold star
Earth	地球	chikyū	earth sphere
Mars	火星	kasei	fire star
Jupiter	木星	mokusei	wood star
Saturn	土星	dosei	earth / soil star
Uranus	天王星	tennōsei	king of heaven star
Neptune	海王星	kaiōsei	king of the seas star
Pluto	冥王星	meiōsei	dark / underworld king star

You may also see ムーン (mūn) for “moon” in loanword style, and 月光 (gekkō) for moonlight. Note that 彗星 (suisei, comet) is a different word from 水星 (suisei, Mercury): same reading, different kanji and meaning.

Five elements, not Roman gods

English learners often expect Japanese planet names to mirror Mars, Venus, and Mercury as deities. For the five classical planets visible to the naked eye, the logic is different. Japanese (via Chinese) maps them to the five phases or elements (五行, gogyō): water, metal, fire, wood, and earth.

• Mercury 水星 — water
• Venus 金星 — metal (often read as “gold”)
• Mars 火星 — fire
• Jupiter 木星 — wood
• Saturn 土星 — earth as soil, not “Earth the planet”

So Saturn is not named because it is a rocky world; it is the “earth-element star” in that traditional system. Mars is “fire star” for the same symbolic reason, even though the planet is cold. The names encode a worldview, not a chemistry lab report.

Those same elements show up in Japanese weekdays: 火曜日 (kayōbi, Tuesday) shares 火 with Mars, 水曜日 (suiyōbi) shares 水 with Mercury, and so on through wood, metal, and earth. Learning the planet list also strengthens the days of the week. For calendars and date words, see days, months, and years in Japanese; for sun and moon vocabulary, see Japanese terms for sun and moon.

Outer planets and Pluto

Uranus, Neptune, and Pluto were named later, after Western astronomy spread. Their Japanese forms keep the “king” idea from Greco-Roman myth, filtered through Chinese characters:

• 天王星 — heavenly king (Uranus)
• 海王星 — sea king (Neptune)
• 冥王星 — underworld / dark king (Pluto)

Since 2006, Pluto is classed as a dwarf planet: 準惑星 (junwakusei). In everyday talk people still say 冥王星; textbooks and news may add the dwarf-planet label when precision matters. The solar system is usually described with eight planets, not nine.

Talking about space in Japanese

Once the planet list is clear, these words open conversations about the universe, science news, and anime that leaves the ground:

• 銀河系 (gingakei) — galaxy (literally a silver-river system; 銀河 alone is also common)
• 小惑星 (shōwakusei) — asteroid (small planet)
• 宇宙 (uchū) — universe / space
• 大気 (taiki) — atmosphere (great air / qi of the sky)
• 食 (shoku) — eclipse root; 月食 lunar eclipse, 日食 solar eclipse
• 流星 (ryūsei) — meteor / shooting star
• 宇宙人 (uchūjin) — extraterrestrial (person of space)
• 重力 (jūryoku) — gravity (power of weight)
• 宇宙飛行士 (uchū hikōshi) — astronaut (space aviator)
• 星座 (seiza) — constellation
• 衛星 (eisei) — satellite (natural or artificial, by context)
• 天の川 (amanogawa) — the Milky Way (river of heaven)
• 軌道 (kidō) — orbit
• ブラックホール (burakku hōru) — black hole
• 超新星 (chōshinsei) — supernova
• 渦巻銀河 (uzumaki ginga) — spiral galaxy
• 白色矮星 (hakushoku waisei) — white dwarf
• パルサー (parusā) — pulsar
• クエーサー (kuēsā) — quasar
• 南十字星 (minami jūjisei) — Southern Cross
• 彗星 (suisei) — comet
• 黄道帯 (kōdōtai) — zodiac belt
• 天文学者 (tenmongakusha) — astronomer
• 準惑星 (junwakusei) — dwarf planet

Kanji reward slow looking: 天の川 paints a river across the sky, 宇宙飛行士 breaks into universe + flight + specialist. That is part of the fun of Japanese space vocabulary—you can often reconstruct a word from pieces you already know. When you want to check readings and example sentences, a dictionary like Jisho helps. Japan’s own space agency, JAXA, is another natural place to meet these terms in real headlines.

Sentences about space in Japanese

Short models you can reuse and rewrite:

My father has been to space.

父は宇宙へ行ったことがある。
Chichi wa uchū e itta koto ga aru.

The moon orbits the Earth.

月は地球の周りを回っている。
Tsuki wa chikyū no mawari o mawatte iru.

The Sun has eight planets.

太陽には８つの惑星がある。
Taiyō ni wa yattsu no wakusei ga aru.

What language would extraterrestrials speak?

宇宙人はどんな言語を話すのだろうか。
Uchūjin wa donna gengo o hanasu no darō ka.

We all live on Earth.

私たちはみんな地球に住んでいる。
Watashitachi wa minna chikyū ni sunde iru.

Jupiter is the largest planet in the solar system.

木星は太陽系で最大の惑星である。
Mokusei wa taiyōkei de saidai no wakusei de aru.

Mercury is the planet closest to the Sun.

水星は太陽に一番近い惑星です。
Suisei wa taiyō ni ichiban chikai wakusei desu.

Neptune is the eighth planet of the solar system.

海王星は太陽系の8番目の惑星だ。
Kaiōsei wa taiyōkei no hachi-banme no wakusei da.

Earth is not a star; it is a planet.

地球は恒星ではなく惑星です。
Chikyū wa kōsei de wa naku wakusei desu.

Long ago, when Pluto was still counted as a planet, there was a boy named Tom.

昔々、冥王星がまだ惑星だったころ、トムという男の子がいました。
Mukashi mukashi, Meiōsei ga mada wakusei datta koro, Tomu to iu otoko no ko ga imashita.

Learn the elements on the list first, then the outer names, then a handful of space words. After that, every new anime spaceship, news clip about JAXA, or weekday on a calendar starts to look less random—and more like a system you can actually read.