eight principal planets的用法和樣例：
例句
用作名詞 (n.)
- The earth is one of the eight Planets.
地球是八大行星之一。 - Leading astronomers declared Thursday that Pluto is no longer a planet under historic new guidelines that downsize the solar system from nine planets to eight.
要天文學(xué)家周四宣布，在發(fā)布歷史性的新方針后，太陽(yáng)系九大行星縮減為八大行星，冥王星已不再是行星。
