Planet

Astronomical body orbiting a star

A collage of planets in the Solar System, to scale.

A planet is a celestial body that orbits a star, is massive enough to be rounded by its own gravity, and has cleared its neighboring region of planetesimals. The term "planet" is ancient, with ties to history, science, mythology, and religion. The planets were originally seen by many early cultures as divine or as emissaries of deities. As scientific knowledge advanced, the human perception of planets changed, incorporating a more scientific understanding of their nature.

Characteristics

Planets are generally divided into two main types: large, low-density gas giants and smaller, rocky terrestrial planets. There are also dwarf planets, which are similar to planets but have not cleared their orbital path.

Orbital Dynamics

Planets orbit stars in elliptical paths, with the star at one of the foci of the ellipse. The orbital period of a planet is the time it takes to complete one orbit around its star. This period is influenced by the planet's distance from the star and the star's mass.

The Solar System, showing the orbits of the planets.

Physical Properties

Planets vary widely in size, composition, and atmosphere. Terrestrial planets like Earth are composed primarily of rock and metal, while gas giants like Jupiter are composed mostly of hydrogen and helium. The axial tilt of a planet affects its seasons and climate.

Diagram showing the axial tilt of planets.

Atmospheres

The atmosphere of a planet is a layer of gases surrounding it. The composition and thickness of a planet's atmosphere can vary greatly, influencing surface conditions and the potential for life. For example, Venus has a thick, toxic atmosphere, while Mars has a thin atmosphere.

Formation

Planets form from the protoplanetary disk of dust and gas that surrounds a young star. Over time, particles in the disk collide and stick together, forming larger bodies called planetesimals. These planetesimals can further coalesce to form planets.

A protoplanetary disk around a young star.

Planetary Migration

During their formation, planets can migrate from their original orbits. This migration can be caused by interactions with the protoplanetary disk or with other planets. Such migrations can lead to the current arrangement of planets in a star system.

An artist's impression of a collision between asteroids around a young star.

Exoplanets

Exoplanets are planets that orbit stars outside our Solar System. The study of exoplanets has expanded our understanding of planetary systems and the potential for life elsewhere in the universe.

Graph showing the number of exoplanet detections per year.

Related pages

• Star
• Moon
• Asteroid
• Comet
• Solar System
• Exoplanet