Planet

A nearly round celestial body that orbits a star or stellar remnant and has cleared its orbital neighborhood of smaller bodies.

Hydrostatic equilibrium: A state in which gravity and pressure balance within a body, shaping it into a nearly spherical form. This balance explains why sufficiently massive planets become nearly round.
Protoplanetary disk: A rotating disk of gas and dust surrounding a young star, where planets can form. Planet formation begins in the material orbiting a young star.
Dwarf planet: A nearly round body orbiting the Sun that has not cleared its orbital neighborhood and is not a moon. Dwarf planets satisfy the shape and orbit criteria but fail the neighborhood-clearing criterion.
Mercury: The smallest planet in the Solar System and the closest to the Sun. Its small size and proximity to the Sun mark one extreme among Solar System planets.
Orbit: The path of an object moving under the gravitational influence of another body. A planet’s orbit defines its path around a star or stellar remnant.
Planetesimal: A small solid body formed by the growth of dust in a young planetary system. Planetesimals are the building blocks that collide and grow into planets.
Small Solar System body: A Solar System object that is neither a planet, dwarf planet, nor natural satellite. These bodies orbit the Sun without becoming round through self-gravity.
Venus: The second planet from the Sun, with a dense carbon-dioxide atmosphere and intense surface heat. Venus shows how a rocky planet can develop an extreme greenhouse atmosphere.
Gravity: The fundamental interaction by which masses attract one another. Gravity gathers planetary material and governs planetary motion.
Planetary migration: The change in a planet’s orbit caused by gravitational interactions with a disk or other bodies. Migration can rearrange a system and help planets clear or share orbital regions.

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