A planet is a celestial body that orbits a star and meets three specific criteria established by the International Astronomical Union in 2006: it must be massive enough for its gravity to pull it into a roughly spherical shape, it must orbit a star (not another planet), and it must have cleared its orbital path of other debris through gravitational dominance. This definition emerged from centuries of astronomical observations and was refined when discoveries like Pluto's small size and crowded orbital neighborhood challenged traditional classifications.</p><p>Our solar system contains eight planets, divided into terrestrial planets (Mercury, Venus, Earth, Mars) with rocky surfaces, and gas giants (Jupiter, Saturn, Uranus, Neptune) composed primarily of hydrogen and helium. Beyond our solar system, astronomers have discovered thousands of exoplanets orbiting distant stars, revolutionizing our understanding of planetary formation and the potential for life elsewhere. These worlds display incredible diversity, from scorching hot Jupiters to potentially habitable Earth-like worlds, expanding our definition of what planetary systems can look like.
Examples
**Example:** Earth is the densest planet in our Solar System, while Saturn is so low in density it would float in water. Jupiter, the largest planet, could contain all other planets combined with room to spare.
Practical Applications
The planet classification system helps astronomers organize and study celestial bodies systematically, enabling better understanding of solar system formation and evolution. This framework is crucial for exoplanet research, where scientists identify and categorize thousands of distant worlds using similar criteria. Understanding planetary characteristics aids in astrobiology research, climate studies, and space mission planning for exploration targets.