地球

It combines 地 ('ground / earth') and 球 ('ball / sphere'), so it literally reads 'earth-ball,' a transparent compound that maps neatly onto the idea of a planet. It's used in geography, science, and environmental contexts: 保护地球 ('protect the Earth'), 地球上 ('on Earth'). English speakers should note the split that trips learners up: 地球 is the planet, while 地 alone means 'ground / land / floor' and 土 means 'soil / dirt.'

You don't call the ground 地球. It's neutral in register and equally at home in a child's textbook or a scientific report. Related planet-and-space vocabulary follows the same 球 pattern: 月球 (yuèqiú, the Moon) and 星球 (xīngqiú, a celestial body/planet).