Planet

A planet is a large, rounded astronomical body that moves around a star, stellar remnant, or brown dwarf, but it is not a star itself. Our Solar System has eight planets. The terrestrial planets are Mercury, Venus, Earth, and Mars. The giant planets are Jupiter, Saturn, Uranus, and Neptune. These planets formed from a cloud of gas and dust that came together around the young Sun. They grew by pulling in more material over time.

The word planet comes from ancient Greek and means "wanderers." This is because these objects move in the sky compared to the stars, which seem to stay in the same place. Early astronomers thought the Sun, Moon, and five bright objects in the sky were planets too. As telescopes got better, more objects were found, which caused debates about what should be called a planet. Today, scientists have a clear way to decide what is a planet in our Solar System, but they also use the word for many other objects.

Scientists have found thousands of planets around stars beyond our Solar System. These are called exoplanets. These planets come in many different shapes and sizes, like very hot giant planets that orbit their stars very closely. Some might be in places where liquid water could exist, which could make them possible homes for life. But Earth is the only planet we know that supports life.

Formation

Main article: Nebular hypothesis

Planets start from clouds of gas and dust called nebulae. These clouds fall inward, forming a young star in the middle. Around the star is a spinning disk of material called a protoplanetary disk. Tiny pieces in the disk stick together through a process called accretion, getting bigger over time. These pieces are called planetesimals, and they pull in more material to become larger bodies known as protoplanets.

When a protoplanet gets big enough, it can pull in a thick atmosphere, helping it gather even more material. This leads to different kinds of planets, like rocky ones or giant ones full of gas and ice. As the young star shines, it clears away the disk, leaving planets that orbit the star.

Planets in the Solar System

Main article: Solar System

There are eight planets in our Solar System. They are, from closest to the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Jupiter is the largest planet and 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, contain more water, methane, and ammonia.

Exoplanets

Main article: Exoplanet

An exoplanet is a planet that goes around a star that is not our Sun. These planets are found far from our Solar System. They come in many different sizes, from very big ones to ones just a little bigger than our Moon.

The first exoplanets were found in 1992 around a special kind of fast-spinning star called a pulsar. The first one found around a star like our Sun was discovered in 1995. Since then, special space telescopes have helped find many more of these planets, including some about the size of Earth. Some of these planets are in areas where it might be possible for liquid water, which is important for life. Scientists keep finding new exoplanets in many places, both close to and far from their stars.

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 special 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 old stories and traditions. The Babylonians, Greeks, and Romans named the planets after their gods. For example, the Babylonians called Venus after Ishtar, their goddess of love, and Mars after Nergal, their god of war. The Greeks and Romans did the same, using names of their own gods.

Different places have different ways of naming planets. In India, planets are named using the Navagraha system. In China and nearby lands, planets are named after five elements: water, metal, fire, wood, and earth. Each culture’s names show what they believed about the stars and planets.

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