Planets

Planets are celestial bodies that orbit stars, have sufficient mass for their gravity to shape them into roughly spherical forms, and have cleared their orbital neighborhoods of other sizable bodies. In the solar system, eight bodies meet these criteria. Pluto was previously considered a planet, but is now classified as a dwarf planet because it has not cleared its orbit.

Planets are commonly categorized by composition. Terrestrial planets are rocky and metal-rich, and include Mercury, Venus, Earth, and Mars. Giant planets are much larger and fall into two groups: gas giants, such as Jupiter and Saturn, dominated by hydrogen and helium, and ice giants, such as Uranus and Neptune, with larger histories of water, methane, and ammonia ices.

Not all worlds orbiting stars are planets. Objects that orbit stars but have not cleared their orbits are termed dwarf planets, while many small bodies orbiting stars are not planets. Beyond the solar system, thousands of exoplanets have been discovered around other stars, detected mainly through the transit method and the radial-velocity method.

Planets are believed to form in protoplanetary disks around young stars, through accretion of solid material and gas. They can host moons and exhibit a wide range of orbits and compositions. Some exoplanets, such as hot Jupiters that orbit very close to their stars, challenge simple formation ideas, while others resemble more familiar planets in the solar system.

Habitability discussions focus on the habitable zone—the region around a star where liquid water could exist—though actual habitability depends on many factors, including atmosphere, geology, and stellar activity.