bolygózó

A bolygózó, also known as a planet, is a celestial body that orbits a star and is massive enough for its own gravity to pull it into a nearly spherical shape. Planets are primarily composed of rock and/or gas, and they do not produce their own light; instead, they reflect the light of their parent star. The term "planet" is derived from the Greek word "planetes," which means "wanderer," reflecting the ancient observation that planets appear to move against the background of fixed stars.

The International Astronomical Union (IAU) defines a planet as a celestial body that:

1. Is in orbit around the Sun.

2. Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.

3. Has cleared the neighborhood around its orbit.

This definition was adopted in 2006 and has since been used to classify planets within our solar system and beyond. According to this definition, there are eight planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto, once considered the ninth planet, was reclassified as a "dwarf planet" due to its failure to clear its orbital neighborhood.

Planets can be further categorized based on their composition and location within a star system. Terrestrial planets, like Earth, are primarily composed of rock and metal, while gas giants, such as Jupiter, are composed mostly of hydrogen and helium. Ice giants, like Uranus and Neptune, have a significant amount of icy materials in their composition. The study of planets, including their formation, evolution, and potential for harboring life, is a major focus of planetary science and astrobiology.