惑星（わくせい、wakusei）
- planet

The Japanese word 惑星 (wakusei) means "planet". It refers to a celestial body that orbits a star, such as the Sun, and is massive enough for its gravity to pull it into a nearly round shape. It does not produce light through nuclear fusion. 惑星 are typically larger than asteroids and comets.

⚫︎ 地球は太陽系の惑星の一つです。 Translation: Earth is one of the planets in the solar system. ⚫︎ 火星は赤い惑星として知られています。 Translation: Mars is known as the red planet. ⚫︎ 宇宙飛行士は様々な惑星を探査します。 Translation: Astronauts explore various planets.

You use 惑星 when talking about planets in general, specific planets, or planetary science. It is used in both formal and informal contexts. When discussing a specific planet, you would typically use 火星 (Kasei - Mars), 金星 (Kinsei - Venus), etc.

惑星 is typically written using kanji, as in 惑星. The reading is always "wakusei" in hiragana. There are no other standard ways to write this word, nor are there variations in the okurigana.

There are no direct synonyms in common usage. The concept might be described using related terms, but 惑星 itself is the most appropriate word to use.

There aren't specific antonyms, as 惑星 is a descriptive term. However, you could contrast it with other celestial objects such as stars 恒星 (kousei), moons 月 (tsuki), or asteroids 小惑星 (shouwakusei).

太陽系には八つの惑星があります。 ・Translation: There are eight planets in the solar system. ・Explanation: This sentence simply states a fact about the solar system and its planetary composition.

科学者は新しい惑星を発見することに興奮しています。 ・Translation: Scientists are excited about discovering new planets. ・Explanation: This sentence describes the excitement of scientists in planetary exploration and discovery.

The word 惑星 is a Sino-Japanese word, a compound of two kanji. 惑 (waku) means "to be perplexed or confused" and 星 (sei) means "star". This refers to the ancient observation that planets "wander" among the fixed stars, appearing to move in complex paths against the backdrop of the constellations, unlike the seemingly fixed position of the stars.