Our solar system is home. It contains one star, eight planets, hundreds of moons, millions of asteroids, countless comets and a vast region of icy objects stretching far beyond the most distant planet. Each world is unique: a scorching planet where lead would melt, a red desert with the tallest volcano known, giant planets with storms larger than Earth, and icy moons that may hide oceans beneath their surfaces. This explainer takes you on a tour of the solar system, from the Sun to its far frontier, and explains how it all formed.
Measuring distances
Distances in the solar system are so large that astronomers use a special unit: the astronomical unit (AU), defined as the average distance between Earth and the Sun, about 93 million miles (150 million kilometers). Light, traveling at about 186,000 miles (300,000 kilometers) per second, takes about 8 minutes and 20 seconds to travel from the Sun to Earth.
The Sun
At the center lies the Sun, an ordinary star that contains about 99.86 percent of all the mass in the solar system. It is a giant ball of hot gas, mostly hydrogen and helium, about 865,000 miles (1.39 million kilometers) across, roughly 109 times the diameter of Earth. In its core, at about 27 million degrees Fahrenheit (15 million degrees Celsius), hydrogen fuses into helium, releasing the energy that lights and warms the planets.
The Sun's gravity holds the solar system together. Its light and heat make life on Earth possible. It also emits a constant stream of charged particles, the solar wind, which flows outward through the solar system.
The inner, rocky planets
The four planets closest to the Sun are small, rocky worlds with solid surfaces.
Mercury
Mercury is the smallest planet and the closest to the Sun, about 0.39 AU away. It has almost no atmosphere to trap heat, so temperatures swing from about 800 °F (430 °C) during the day to about −290 °F (−180 °C) at night. Its surface is heavily cratered, like the Moon. A year on Mercury lasts only 88 Earth days.
Venus
Venus is similar in size to Earth but very different in character. It is covered by thick clouds of sulfuric acid and has a crushing atmosphere of carbon dioxide, about 90 times the pressure at Earth's surface. A runaway greenhouse effect makes Venus the hottest planet, with surface temperatures around 870 °F (465 °C), hot enough to melt lead. Venus rotates very slowly and backward compared with most planets, so the Sun rises in the west there.
Earth
Earth is the only known world with life and the only planet with liquid water oceans on its surface. Its atmosphere of nitrogen and oxygen, its magnetic field and its distance from the Sun create conditions that allow life to thrive. Earth has one large moon, which stabilizes its tilt and creates ocean tides.
Mars
Mars, the "Red Planet," gets its color from iron oxide, essentially rust, in its soil. It has a thin atmosphere of carbon dioxide, polar ice caps and the largest volcano in the solar system, Olympus Mons, about 13.6 miles (22 kilometers) high. Mars also has a canyon system, Valles Marineris, long enough to stretch across the continental United States. Evidence from rovers and orbiters shows that liquid water once flowed on its surface. Mars has two small moons, Phobos and Deimos.
The asteroid belt
Between Mars and Jupiter lies the asteroid belt, a region containing millions of rocky objects. Most are small, but the largest, Ceres, is about 590 miles (950 kilometers) across and is classified as a dwarf planet. Despite how it looks in movies, the asteroid belt is mostly empty space; spacecraft have flown through it many times without danger. The asteroids are leftover building blocks from the formation of the planets, kept from forming a planet by Jupiter's strong gravity.
The giant planets
Beyond the asteroid belt lie four giant planets. They are much larger than the rocky planets and have no solid surface like Earth's.
Jupiter
Jupiter is the largest planet, more than twice as massive as all the other planets combined. It is a gas giant made mostly of hydrogen and helium. Its striped appearance comes from bands of clouds, and its Great Red Spot is a giant storm larger than Earth that has been observed for centuries. Jupiter has dozens of moons, including the four large Galilean moons discovered by Galileo in 1610:
- Io, the most volcanically active body in the solar system.
- Europa, which has an icy crust that likely covers a global ocean of liquid water.
- Ganymede, the largest moon in the solar system, even larger than Mercury.
- Callisto, one of the most heavily cratered objects known.
Saturn
Saturn is famous for its spectacular rings, made of countless pieces of ice and rock, ranging from tiny grains to boulders as large as houses. The rings are hundreds of thousands of kilometers wide but in most places only about 30 feet (10 meters) to a few hundred meters thick. Saturn is a gas giant like Jupiter and is so low in density that it would float in water if there were a bathtub big enough. Its largest moon, Titan, has a thick atmosphere and lakes of liquid methane and ethane. Another moon, Enceladus, shoots jets of water vapor from an ocean beneath its icy surface.
Uranus
Uranus is an ice giant, made of water, ammonia and methane ices around a rocky core, with an atmosphere of hydrogen, helium and methane. Methane absorbs red light, giving Uranus its pale blue-green color. Uranus rotates on its side, with its axis tilted by about 98 degrees, possibly because of a massive collision long ago. This gives it extreme seasons, with each pole facing the Sun for about 42 years at a time.
Neptune
Neptune, the most distant planet, is also an ice giant and has a deep blue color. It has the fastest winds in the solar system, reaching more than 1,200 miles per hour (2,000 kilometers per hour). Neptune was the first planet found through mathematical prediction: astronomers noticed that Uranus's orbit was being disturbed and calculated where an unseen planet must be. Neptune was discovered in 1846 very close to the predicted position. Its largest moon, Triton, orbits backward and may be a captured object from the Kuiper Belt.
The planets at a glance
| Planet | Average distance from Sun (AU) | Diameter compared with Earth | Length of year | Known type |
|---|---|---|---|---|
| Mercury | 0.39 | 0.38 | 88 days | rocky |
| Venus | 0.72 | 0.95 | 225 days | rocky |
| Earth | 1.00 | 1.00 | 365 days | rocky |
| Mars | 1.52 | 0.53 | 687 days | rocky |
| Jupiter | 5.2 | 11.2 | 11.9 years | gas giant |
| Saturn | 9.5 | 9.4 | 29.5 years | gas giant |
| Uranus | 19.2 | 4.0 | 84 years | ice giant |
| Neptune | 30.1 | 3.9 | 165 years | ice giant |
Why Pluto is a dwarf planet
Pluto was considered the ninth planet from its discovery in 1930 until 2006. That year, the International Astronomical Union defined a planet as an object that orbits the Sun, is massive enough to be rounded by its own gravity and has "cleared the neighborhood" around its orbit of other objects. Pluto meets the first two criteria but not the third, because it shares its region with many other icy bodies. It was reclassified as a dwarf planet, along with Ceres, Eris, Haumea and Makemake.
In 2015, NASA's New Horizons spacecraft flew past Pluto and revealed a surprisingly active world with mountains of water ice, glaciers of nitrogen ice and a heart-shaped plain.
The Kuiper Belt and the Oort Cloud
Beyond Neptune lies the Kuiper Belt, a doughnut-shaped region of icy bodies, including Pluto and other dwarf planets. It extends from about 30 to 50 AU from the Sun. Many short-period comets come from here.
Much farther out, perhaps from about 2,000 to 100,000 AU, astronomers believe there is a vast, spherical shell of icy objects called the Oort Cloud. It has never been observed directly but is thought to be the source of long-period comets. Its outer edge may extend a quarter of the way or more to the nearest star.
Comets
Comets are icy bodies sometimes called "dirty snowballs." When a comet approaches the Sun, its ices turn into gas, releasing dust and forming a glowing cloud called a coma and long tails that always point away from the Sun, pushed by sunlight and the solar wind. Famous comets include Halley's Comet, which returns about every 76 years and will next appear in 2061.
How the solar system formed
About 4.6 billion years ago, a cloud of gas and dust began to collapse, perhaps triggered by a nearby exploding star. As it collapsed, it spun faster and flattened into a disk. Most of the material gathered in the center, forming the Sun. In the surrounding disk, dust grains stuck together, forming larger and larger bodies, eventually planets.
Close to the hot young Sun, only rock and metal could condense, forming the small rocky planets. Farther out, beyond what astronomers call the frost line, ices could form as well, allowing planets to grow larger and capture thick atmospheres of hydrogen and helium, becoming the giant planets. Leftover material became asteroids, comets and Kuiper Belt objects. This is why all the planets orbit the Sun in the same direction and in nearly the same plane.
Questions readers ask
How many planets are in the solar system? Eight: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Pluto is classified as a dwarf planet.
Which planet is the hottest? Venus, because its thick carbon dioxide atmosphere traps heat, even though Mercury is closer to the Sun.
Do other planets have moons? Yes. All planets except Mercury and Venus have moons. Saturn and Jupiter each have dozens.
Could there be life elsewhere in the solar system? Scientists consider places like Mars, Europa and Enceladus the most promising, because of evidence of past or present liquid water. No life has been found so far.
Where does the solar system end? There is no single edge. The heliopause, where the solar wind meets interstellar space, lies about 120 AU away; the Voyager spacecraft crossed it in 2012 and 2018. The Sun's gravity extends much farther, out to the Oort Cloud.
In short
The solar system consists of the Sun, which holds almost all its mass, four rocky inner planets, the asteroid belt, two gas giants and two ice giants, plus dwarf planets like Pluto, the icy Kuiper Belt and the distant Oort Cloud. Each world has its own personality, from scorching Venus to ringed Saturn and sideways Uranus. All of it formed together from a spinning disk of gas and dust about 4.6 billion years ago. Exploring our neighborhood helps us understand how planets form and whether life might exist beyond Earth.