The word planet comes from the Greek word planḗtai, meaning wanderers. In ancient usage, the term applied to the Sun, the Moon, and the five points of light visible to the naked eye that moved independently against the fixed background of stars: Mercury, Venus, Mars, Jupiter, and Saturn.
A planet is a large, rounded astronomical body that orbits a star, a stellar remnant, or a brown dwarf, and is not itself one of those things. By the most restrictive current definition, the Solar System contains eight planets: the four terrestrial planets Mercury, Venus, Earth, and Mars, and the four giant planets Jupiter, Saturn, Uranus, and Neptune. The leading explanation for how planets form is the nebular hypothesis, which proposes that an interstellar cloud collapses out of a nebula to produce a young protostar surrounded by a protoplanetary disk. Within that disk, material accumulates through gravity in a process called accretion, gradually building planets over vast stretches of time.
Planets have carried religious significance throughout human history. Across many cultures, celestial bodies were identified with gods, and those mythological associations have carried forward into the naming conventions applied to newly discovered bodies in the Solar System.
Earth itself came to be understood as a planet only when heliocentrism displaced geocentrism during the sixteenth and seventeenth centuries.
Earth remains the only planet confirmed to support life. The conditions required for habitability are shaped by many factors, including atmospheric composition. Mars, for instance, reaches temperatures during equatorial daytime that would allow liquid water to exist at the surface, but its atmospheric pressure is far too low to make that possible.
Planet formation begins with a vast, rotating cloud of gas and dust left behind after a new star ignites. Over millions of years, dust grains collide and stick together, building up into pebbles, then boulders, then mile-wide bodies called planetesimals. Continued collisions and gravitational merging gradually produce full-sized planets. If a rocky core accumulates roughly ten times the mass of Earth, it can draw in enormous quantities of surrounding gas and grow into a giant planet like Jupiter. Material left over after this process either collides with the new planets or is scattered away, leaving a finished planetary system behind.
Our understanding of what qualifies as a planet has shifted over time. Pluto was considered the ninth planet from its discovery in 1930 until 2006, when it was reclassified as a dwarf planet on the grounds that it has not cleared other debris from its orbital zone. Ceres followed a similar path in reverse, briefly recognized as a planet before being reclassified first as an asteroid and later also as a dwarf planet. Earlier discoveries including Vesta and Juno were also temporarily called planets before being moved into the asteroid category. Eris, found in 2005, was initially described as a tenth planet before also being designated a dwarf planet, and its discovery was a direct catalyst for Pluto’s demotion. The Solar System currently recognizes eight major planets and five dwarf planets, the essential distinction being that major planets have cleared their orbital neighborhoods while dwarf planets share their orbital zones with similar objects.
The names given to the planets of the Solar System in English derive from naming traditions developed in sequence by the Babylonians, Greeks, and Romans. The practice of assigning divine names to planets was almost certainly adopted by the ancient Greeks from the Babylonians and then passed from the Greeks to the Romans. The Babylonians named Venus after Ishtar, the Akkadian name for the Sumerian goddess of love; Mars after Nergal, their god of war; Mercury after Nabu, their god of wisdom; Jupiter after Marduk, their chief deity; and Saturn after Ninurta, their god of farming. The parallels between Greek and Babylonian naming are too consistent to have developed independently. The correspondence was not perfect, given differences in mythology. The Babylonian Nergal, for instance, was a god of war and therefore matched to the Greek Ares, but Nergal also governed pestilence and ruled the underworld, roles Ares did not share.
In ancient Greece the Sun and Moon were named Helios and Selene after two Titanic deities. Saturn, the slowest-moving planet, was called Phainon, the shiner. Jupiter was Phaethon, meaning bright. Mars was Pyroeis, the fiery. Venus, the brightest object in the sky after the Sun and Moon, was Phosphoros, the light bringer. Mercury was Stilbon, the gleamer. Each planet was then associated with a specific deity from the Greek pantheon.
China and the East Asian nations historically shaped by Chinese culture, including Japan, Korea, and Vietnam, developed a naming system grounded in the five Chinese classical elements. Mercury became the water star, Venus the gold star, Mars the fire star, Jupiter the wood star, and Saturn the soil star.
A day on Venus is longer than its year. The planet takes 243 Earth days to complete one full rotation but only 225 days to complete one orbit around the Sun.
Jupiter has 95 known moons, and that count continues to grow as surveys identify smaller previously undetected ones.
Saturn is the least dense planet in the Solar System. It is so light relative to its volume that it would float if placed in a body of water large enough to hold it.
As Mercury’s iron core cooled and contracted over billions of years, its surface crumpled inward, producing massive cliff formations that give it a wrinkled appearance sometimes compared to a dried prune.
Neptune’s winds are the fastest in the Solar System, reaching speeds exceeding 1,200 miles per hour.
Uranus rotates at an axial tilt of nearly 98 degrees, essentially rolling on its side as it orbits the Sun. The most widely accepted explanation is a massive collision in the distant past.
Earth is not a perfect sphere. It bulges slightly at the equator and is flattened at the poles, a shape described as an oblate spheroid.
The tallest known mountain in the Solar System sits on Mars. Olympus Mons rises to approximately 13.6 miles, or 22 kilometers, above the surrounding terrain, making it roughly two and a half times the height of Everest.
Estimates suggest there are more planets than stars in the Milky Way, with scientists calculating at least one planet per star on average, implying billions of planets in our galaxy alone.
On Neptune and Uranus, conditions deep in the atmosphere may produce diamond hail. Intense pressure breaks methane down into carbon, which compresses into diamond crystals that then sink toward the planetary core.