Planet

A planet is a large, rounded astronomical body that orbits a star, stellar remnant, or brown dwarf, but is not a star itself. Our Solar System has eight planets: the terrestrial planets Mercury, Venus, Earth, and Mars, and the giant planets Jupiter, Saturn, Uranus, and Neptune. These planets formed from a cloud of gas and dust that collapsed around the young Sun, creating a disk of material from which they grew by pulling in more matter over time.

The word planet comes from ancient Greek, meaning "wanderers," because these objects move across the sky compared to the fixed stars. Early astronomers thought the Sun, Moon, and five bright objects in the sky were planets too. As telescopes improved, more objects were discovered, leading to debates about what should count as a planet. Today, astronomers have a clear definition for planets in our Solar System, but they still use the word for many other objects, too.

Scientists have found thousands of planets around stars beyond our Solar System, called exoplanets. These planets come in many surprising forms, like very hot giant planets that orbit their stars very closely. Some may even be in positions where liquid water could exist, making them possible places for life. However, Earth is still the only planet we know that supports life.

Formation

Main article: Nebular hypothesis

Planets are thought to form from clouds of gas and dust called nebulae. As these clouds collapse, a young star forms in the center, surrounded by a spinning disk of material called a protoplanetary disk. Tiny particles in this disk stick together through a process called accretion, growing bigger over time. These growing pieces, called planetesimals, attract more material and eventually form larger bodies known as protoplanets.

Once a protoplanet becomes large enough, it can start gathering a thick atmosphere, which helps it pull in even more material. This process can lead to the formation of different kinds of planets, such as rocky terrestrial planets or giant planets with lots of gas and ice. As the young star shines brightly, it clears away the remaining disk, leaving behind planets orbiting the star.

Planets in the Solar System

Main article: Solar System

There are eight planets in our Solar System, starting from the closest to the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Jupiter is the biggest planet, while Mercury is the smallest.

Planets are grouped by what they are made of. The rocky planets—Mercury, Venus, Earth, and Mars—are called terrestrials. The giant planets—Jupiter, Saturn, Uranus, and Neptune—are much bigger and made of different materials. Jupiter and Saturn, the gas giants, are mostly hydrogen and helium. Uranus and Neptune, the ice giants, have more water, methane, and ammonia in them.

Exoplanets

Main article: Exoplanet

An exoplanet is a planet that orbits a star outside our Solar System. As of February 26, 2026, scientists have confirmed 6,256 exoplanets in 4,664 different star systems, with many systems having more than one planet. These planets come in many sizes, from giants about twice the size of Jupiter down to bodies just a bit larger than our Moon.

The first exoplanets were discovered in 1992 around a fast-spinning star called a pulsar. The first exoplanet found around a normal star like our Sun was discovered in 1995. Since then, space telescopes like Kepler have helped find many more, including planets about the size of Earth. Some of these Earth-sized worlds lie in the "habitable zone," where conditions might be right for liquid water—a key ingredient for life as we know it. Scientists continue to discover exoplanets in many different places, some very close to their stars and others extremely far away.

Attributes

All planets share some basic traits, even though each one looks and behaves a little differently. One big thing they have in common is how they move around stars. In our Solar System, the planets all go around the Sun in the same direction — counter-clockwise if you look down from above the Sun’s north pole. Some planets found around other stars, called exoplanets, might go the opposite way.

Planets also have something called an orbit, which is the path they take around their star. Their year — the time it takes to finish one orbit — depends on how far they are from their star. The farther they are, the longer their year is. Even though their orbits aren’t perfect circles, every planet gets closest to its star at one point (called perihelion) and farthest away at another (called aphelion).

Dynamic characteristics

Planets spin around an imaginary line called an axis. How long it takes for a planet to spin once is called its day. Most planets spin the same way they orbit their star, but some, like Venus and Uranus, spin the opposite way. Some planets, like Jupiter, have very short days, while others, like Venus, take a very long time to spin once.

Physical characteristics

Gravity pulls planets into shapes that are almost round. Bigger planets can keep more gases close to them, which gives them thick atmospheres. Smaller planets might lose these gases into space. The way a planet’s inside is put together — with heavier materials sunk in the middle and lighter ones on top — is called differentiation. This process helps create things like magnetic fields, which protect planets from harmful space weather.

Secondary characteristics

Many planets have natural satellites, or moons, that orbit around them. Earth has one moon, while the giant planets have many. Some of these moons have landscapes and features similar to planets. The giant planets also have rings made of tiny particles and dust. Scientists think these rings might come from moons that got too close to their planet and broke apart.

History and etymology

The idea of planets has changed a lot over time. Long ago, people thought planets were divine lights in the sky. Today, we know about planets not just in our Solar System, but also around other stars. What counts as a planet has changed many times. At one point, even moons and objects like Pluto were called planets, but ideas have shifted over time.

Ancient people like the Babylonians in Mesopotamia were among the first to study the planets. They watched the sky and wrote down how planets moved. Later, ancient Greeks also studied the planets and came up with their own ideas about how they moved around Earth. Through history, many cultures have had their own ways of understanding these wandering lights in the sky.

Mythology and naming

The names of the planets in our Solar System come from ancient cultures. The Babylonians, Greeks, and Romans gave the planets the names of their gods. For example, the Babylonians named Venus after Ishtar, their goddess of love, and Mars after Nergal, their god of war. The Greeks and Romans followed similar naming traditions, using their own gods’ names for the planets.

Different cultures have their own ways of naming planets. In India, planets are named based on the Navagraha system. In China and nearby countries, planets are named after the five elements—water, metal, fire, wood, and earth. Each culture’s names reflect its own myths and beliefs about the stars and planets.

Main article: Names of the days of the week and classical planet