Solar System

The Solar System is the gravitationally bound system of the Sun and the objects that orbit it. The eight planets are the main objects, and Earth is one of them. The Solar System formed about 4.6 billion years ago when a big cloud of gas collapsed, creating the Sun and a flat disk where the planets formed.

The Sun has almost all of the Solar System's mass. Inside the Sun's core, hydrogen is fused into helium. This makes energy that we see as sunlight. This energy creates the heliosphere.

The eight planets are the biggest objects that orbit the Sun. The four closest planets are Mercury, Venus, Earth, and Mars. These are called the terrestrial planets or the planets of the inner Solar System. Earth and Mars are in the habitable zone, where it is warm enough for liquid water.

Beyond the frost line, about five astronomical units from the Sun, are the outer Solar System. Here are the gas giants: Jupiter and Saturn, and the ice giants: Uranus and Neptune. Jupiter and Saturn have most of the non-stellar mass of the Solar System.

Definition

The Solar System includes the Sun and all the objects that are pulled toward it by gravity and move around it. According to the International Astronomical Union, the Solar System has the Sun, its eight planets, and other space objects that move around the Sun. NASA also calls the Solar System a planetary system, which means the Sun and everything that orbits it.

The way we write "Solar System" can change. When we use it as a name, we write it with a capital "S" and "S". But when we talk about any system that looks like ours, we might write it with small letters. The International Astronomical Union has rules about naming space objects, and they use both ways in their guidelines.

Formation and evolution

Main article: Formation and evolution of the Solar System

The Solar System formed about 4.6 billion years ago when a big cloud of gas and dust collapsed. This collapse made the Sun and a flat, spinning disk of material. From this disk, the planets, moons, and smaller objects formed by coming together.

The Sun now makes up most of the Solar System's mass. It will keep shining for about another 5 billion years. After that, it will expand and shrink down to a small, dim remnant called a white dwarf. During this time, the inner planets like Mercury and Venus may vanish, and Earth might become a place where nothing can live.

General characteristics

Astronomers divide the Solar System into parts. The inner Solar System has Mercury, Venus, Earth, Mars, and the asteroid belt. The outer Solar System has Jupiter, Saturn, Uranus, Neptune, and the Kuiper belt. The area beyond Neptune is a special part of the Solar System.

The Sun is the main part of the Solar System. It is a G-type main-sequence star and has most of the system's mass. It also controls the system with its gravity. The four largest planets, called giant planets, have most of the remaining mass. Jupiter and Saturn have more than half of this mass together. The Sun is mostly made of hydrogen and helium, like Jupiter and Saturn. The planets and other big objects move near a flat plane called the ecliptic. Most planets have moons that orbit them. The giant planets also have rings made of tiny particles.

Sun

Main article: Sun

The Sun is the star at the center of our Solar System. It is much bigger and more massive than any other object in the system. The Sun creates high temperatures and pressures in its core. This lets it turn hydrogen into helium. This process is called nuclear fusion. It releases a lot of energy that we feel as sunlight.

The Sun shines steadily because it is a main-sequence star. Its temperature is just right—not too hot and not too cold. The Sun also sends out a constant stream of charged particles called the solar wind. This wind fills the space around the Sun and creates a region called the heliosphere. Sometimes, the Sun has bursts of activity. These can create beautiful lights in the sky near the poles called aurorae.

Inner Solar System

Main article: Terrestrial planet

The inner Solar System has rocky planets and asteroids close to the Sun. These objects are made of rocks and metals. They are all inside an area called the frost line, about 5 astronomical units from the Sun.

The four inner planets—Mercury, Venus, Earth, and Mars—are dense and rocky. They have few or no moons and no rings. They are made of minerals and metals like iron and nickel. Three of these planets have atmospheres that create weather. All of them show signs of past volcanic activity and other changes on their surfaces.

• Mercury is the smallest planet. It has a gray surface and big temperature changes. It has no moons.
• Venus has a thick, hot atmosphere and a very hot surface. It also has no moons.
• Earth is the only place we know of with life and liquid water. It has a protective magnetic field and one moon, called the Moon. The Moon has a cratered surface.
• Mars is red because of iron oxide on its surface. It has ice caps and a thin atmosphere. Mars has two small moons, Phobos and Deimos.

Asteroids

Main article: Asteroid

Asteroids are small, rocky bodies that orbit the Sun. Most are found in the asteroid belt, a region between Mars and Jupiter. These objects range from just a few meters to hundreds of kilometers in size. The largest asteroid, Ceres, is also a dwarf planet. Other notable asteroids include Vesta and Pallas. Some asteroids share similar orbits and are grouped into families. Others have stable positions called Lagrange points, where they share an orbit with a larger planet like Jupiter.

Outer Solar System

The outer region of the Solar System has the giant planets and their large moons. The centaurs and many short-period comets orbit here too. Because these objects are farther from the Sun, they have more water, ammonia, and methane than the inner planets.

Outer planets

Main article: Giant planet

The four outer planets, called giant planets, make up most of the mass orbiting the Sun. They all have many moons and ring systems. Jupiter and Saturn are mostly gas. Uranus and Neptune are made of icy materials. Jupiter has many moons, including the large Galilean moons. Saturn has bright rings and many moons, with Titan having its own atmosphere. Uranus orbits on its side. Neptune is the farthest planet, and its moon Triton has active geysers.

Centaurs

Main article: Centaur (small Solar System body)

Centaurs are icy bodies between Jupiter and Neptune. They were pushed inward by the outer planets. Some, like 2060 Chiron, show comet-like activity when close to the Sun. The largest known centaur, 10199 Chariklo, has its own ring system.

Trans-Neptunian region

Beyond the orbit of Neptune lies the "trans-Neptunian region", which includes the Kuiper belt and scattered objects. The Kuiper belt is a ring of icy objects between 30 and 50 AU from the Sun. It has many small bodies, with the largest being dwarf planets like Pluto, Haumea, Makemake, and Quaoar. These objects can be moved by Neptune's gravity.

The scattered disc overlaps the Kuiper belt but goes much farther, up to near 500 AU. Objects here have been pushed into distant orbits by Neptune. Two notable dwarf planets here are Eris and Gonggong. Even farther out are extreme trans-Neptunian objects, such as Sedna, which have very long orbits. The Oort cloud is a theoretical shell of icy objects surrounding the Solar System, extending from about 2,000 AU to possibly 200,000 AU. It may be the source of long-period comets.

Main article: Kuiper belt

Main article: Scattered disc

Main article: Extreme trans-Neptunian object

Gravitationally unstable populations

Meteoroids, meteors and dust

Main articles: Meteoroid, Interplanetary dust cloud, and Cosmic dust

Meteoroids are tiny pieces of rock and metal in space. They are usually smaller than a meter. When they enter Earth's atmosphere, they make bright lines of light called meteors. Sometimes, many meteoroids enter at once and create a meteor shower. It looks like the lines of light come from one spot in the sky.

The inner Solar System has a dusty cloud called the zodiacal dust cloud. Sometimes, this can look like a soft glow called the zodiacal light. The outer Solar System also has a cloud of cosmic dust.

Comets

Main article: Comet

Comets are small objects in space made of ice and dust. When they get close to the Sun, the ice turns to gas. This creates a glowing area around the comet and often a tail that we can see from Earth. Some comets orbit the Sun quickly, in less than two hundred years. Others take thousands of years to orbit the Sun. A few comets come from far beyond our Solar System.

Boundary region and uncertainties

Much of the outer Solar System is still a mystery because we have not explored it much. We learn about this distant area by watching objects that come close to the Sun. These often appear as bright comets. Many more objects probably exist out there, waiting to be found.

The Sun's gravitational influence, called its Hill sphere, may reach about 230,000 AU. It might include a distant area known as the Oort cloud. The heliosphere, a bubble made by solar wind, ends at a place called the termination shock. This is about 80–100 AU from the Sun in one direction and about 200 AU in another. Beyond this, the heliosheath stretches far into space, shaped by the Sun’s movement through the galaxy.

Celestial neighborhood

Main articles: List of nearest stars, List of nearest exoplanets, and List of nearby stellar associations and moving groups

The Sun has neighbors in space. The closest star system is Alpha Centauri, about 4.4 light-years away. It has three stars: Alpha Centauri A, Alpha Centauri B, and a small star named Proxima Centauri. In 2016, scientists found a planet around Proxima Centauri called Proxima Centauri b. This planet might be able to support life.

Our Solar System is inside a region called the Local Interstellar Cloud. It is surrounded by a big, empty space known as the Local Bubble. This bubble has very hot gas and looks like an hourglass. It was probably made long ago by explosions from stars called supernovae.

Galactic position

See also: Location of Earth, Galactic year, and Orbit of the Sun

The Solar System is in the Milky Way, a very big spiral galaxy with many stars. The Sun is in one of the outer arms, called the Orion–Cygnus Arm. The Sun moves around the middle of the Milky Way at about 220 kilometers per second. It takes 240 million years to finish one full loop.

The Solar System's spot in the Milky Way helps keep Earth safe. By being in a quieter part of the galaxy, away from the busy areas, Earth has had long, peaceful times for life to grow.

Discovery and exploration

Main article: Discovery and exploration of the Solar System

For a long time, people believed Earth was the center of everything. But in the 1500s, Nicolaus Copernicus proposed that the Sun was at the center instead. Later, Johannes Kepler showed that planets move in oval-shaped paths called ellipses.

In the 1600s, Galileo used a telescope and found that Jupiter had moons, just like Earth has the Moon. This proved that not everything revolves around Earth. Since then, we have discovered many objects orbit the Sun, not just the planets.

In more recent times, we have sent spacecraft to explore our Solar System. These probes have visited every planet, flown through the Sun’s outer atmosphere, and brought back samples from asteroids and comets. We have also landed on the Moon and hope to return there soon.