Description: 天王星是距離太陽第七遠的行星。它是一顆冰巨行星，半徑約 2.5 萬公里，大約是地球半徑的四倍。天王星的質量是地球質量的 14.5 倍，內核是堅硬的岩石，周圍有一層高壓水、甲烷和氨。在早期的外太陽系中，這些化學物質被凍結。年輕的天王星吸積了這些 "冰"，因此被稱為 "冰巨行星"。天王星的外部大氣層是一層厚厚的、浮腫的氫和氦。它的自轉軸相對於軌道平面傾斜超過 90 度。天王星有一個行星環。

它與太陽的典型距離約為 29 億千米，約為 19 個天文單位（地球-太陽距離）。天王星完成一個軌道大約需要 84 年。天王星有超過 25 顆已知的衛星或天然衛星。

在良好的條件下，肉眼幾乎看不到天王星，因此在現代之前，天王星並沒有被認定為行星。1781 年，威廉-赫歇爾將天王星確定為行星。因此，它是自古以來發現的第一顆行星。天王星以希臘天神的名字命名。

Related Terms:

• 天文單位
• 巨行星
• 冰雪巨人
• 衛星
• 海神
• 外行星
• 行星環
• 太陽系
• 自轉

Term and definition status: The original definition of this term in English have been approved by a research astronomer and a teacher
The translation of this term and its definition is still awaiting approval

This is an automated transliteration of the simplified Chinese translation of this term

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