A planet is a large object that orbits a star. To be a planet, an object must be massive enough for gravity to have squeezed it into a spherical, or round, shape,. It must also be large enough for gravity to have swept up any rocky or icy objects from its path, or orbit, around the star.
Scientists believe planets begin to form when a dense cloud of dust and gas, called a nebula, spins around a newly formed star. Gradually, gravity causes the bits of matter in the nebula to clump together. Slowly, these clumps accumulate and grow. Eventually, these clumps become planets.
Earth is one of eight planets that circle the star we call the sun. Together, the sun, the planets, and smaller objects such as moons make up our solar system.
The four planets closest to the sun—Mercury, Venus, Earth, and Mars—are called terrestrial planets. These planets are solid and rocky like Earth (terra means “earth” in Latin). Earth is the largest of the four terrestrial planets, and Mercury is the smallest. All are surrounded by a layer of gas, or atmosphere. Their atmospheres vary in density from Mercury’s extremely thin atmosphere to Venus’, which is thick with clouds of sulfuric acid.
The four planets that are more distant from the sun—Jupiter, Saturn, Uranus, and Neptune—are called gas giants. Gas giants are huge compared with Earth, and they do not have solid surfaces. They are big balls of gas. Jupiter and Saturn are composed mostly of hydrogen and helium. Uranus and Neptune have greater proportions of water vapor, ammonia, and methane. Each of the four gas giants also has a ring system. A planet’s rings are made of ice, dust, and small rocks. Saturn’s ring system is the largest.
Every planet except Mercury and Venus has at least one natural satellite, or moon. A planet’s moon orbits it as it revolves around the sun. Jupiter, Saturn, and Uranus each have dozens of moons.
In addition to orbiting a star, planets also rotate, or spin, around an axis. An axis is an invisible line that runs through the center of a planet. One complete rotation is called a day. A day on Earth is about 24 hours. A day on Jupiter takes only 9.8 hours. Venus has the longest day of any planet in our solar system. It takes 243 Earth days for Venus to make a complete turn on its axis.
Unlike stars, planets do not experience nuclear fusion, the process of combining tiny particles called atoms to release energy. Nuclear fusion creates radiation (heat and light) and makes stars glow. Because planets do not have nuclear fusion, they do not produce their own light. Instead, they shine with light reflected from a star. When we see planets in the night sky, such as Venus, the so-called "Evening Star," we're seeing reflected sunlight.
Because there are trillions of stars in the universe, there are very likely billions of planets. But until the early 1990s, the only known planets were in our solar system. Since then, however, scientists have discovered more than 400 planets orbiting other stars. These are called extrasolar planets, or exoplanets.
Exoplanets appear to be fairly small from our viewpoint on Earth. Telescopes usually cannot observe exoplanets directly, so astronomers have had to come up with methods to detect them. One method astronomers use is to look for a slight wobble in a star’s movement. This wobble is the result of the gravitational pull of a nearby planet. Most of the exoplanets discovered so far are gas giants.