Solar System

The Solar System is the gravitationally bound system of the Sun and the masses that orbit it, most prominently its eight planets, of which Earth is one. The system formed about 4.6 billion years ago when a dense region of a molecular cloud collapsed, creating the Sun and a protoplanetary disc from which the orbiting bodies assembled.

The Sun accounts for 99.86% of the Solar System's total mass. Inside the Sun's core, hydrogen is fused into helium, releasing energy that is emitted through the Sun's photosphere. This creates the heliosphere and a decreasing temperature gradient across the Solar System.

The next most massive objects of the system are the eight planets, which by definition dominate the orbits they occupy. Closest to the Sun in order of increasing distance are the terrestrial planets – Mercury, Venus, Earth and Mars. These are the planets of the inner Solar System. Earth and Mars are the only planets that orbit within the Sun's habitable zone, in which sunlight can keep surface water liquid under atmospheric pressure. Beyond the frost line at about five astronomical units (AU), are the planets of the outer Solar System: two gas giants (Jupiter and Saturn) and two ice giants (Uranus and Neptune). Jupiter and Saturn possess nearly 90% of the non-stellar mass of the Solar System.

Definition

The Solar System includes the Sun and all objects that are bound to it by gravity and orbit it. According to the International Astronomical Union, the Solar System consists of the Sun itself, its eight planets, and other celestial bodies that orbit the Sun. NASA also describes the Solar System as a planetary system, which includes the Sun and everything that orbits around it.

The way we write the name "Solar System" can change. When we use it as a proper 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 lowercase letters. The International Astronomical Union gives rules about how to name astronomical objects, and they use both forms 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 created the Sun and a flat, spinning disk of material. From this disk, the planets, moons, and smaller objects formed by clumping together.

The Sun now makes up most of the Solar System's mass. It will keep shining for about another 5 billion years, after which it will expand and eventually 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 uninhabitable.

General characteristics

Astronomers sometimes divide the Solar System into separate regions. The inner Solar System includes Mercury, Venus, Earth, Mars, and the bodies in the asteroid belt. The outer Solar System includes Jupiter, Saturn, Uranus, Neptune, and the bodies in the Kuiper belt. Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune.

The principal component of the Solar System is the Sun, a G-type main-sequence star that contains 99.86% of the system's known mass and dominates it gravitationally. The Sun's four largest orbiting bodies, the giant planets, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The Sun is composed of roughly 98% hydrogen and helium, as are Jupiter and Saturn. The planets and other large objects in orbit around the Sun lie near the invariable plane of the Solar System, as does Earth's orbit, known as the ecliptic, and most closely the orbit of Jupiter. Most of the planets in the Solar System have secondary systems of their own, being orbited by natural satellites called moons. The four giant planets have planetary rings, thin discs of tiny particles that orbit them in unison.

Sun

Main article: Sun

The Sun is the star at the center of our Solar System and is much more massive than any other object in the system. It has enough mass to create high temperatures and pressures in its core, allowing it to turn hydrogen into helium. This process, called nuclear fusion, releases a huge amount of energy that we feel as sunlight.

Because the Sun is a main-sequence star, it shines steadily and has a temperature that 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, which fills the space around it and creates a region called the heliosphere. Sometimes, the Sun has bursts of activity that can affect Earth, creating beautiful lights in the sky near the poles called aurorae.

Inner Solar System

Main article: Terrestrial planet

The inner Solar System includes the rocky planets and asteroids, all located close to the Sun. These objects are made mostly of silicates and metals, and they stay within a region smaller than the distance between Jupiter and Saturn. This area lies inside the frost line, which is about 5 astronomical units from the Sun.

The four inner planets—Mercury, Venus, Earth, and Mars—are dense and rocky, with few or no moons and no rings. They are made of minerals like silicates and metals such as iron and nickel. Three of these planets have atmospheres that create weather, and all show signs of past volcanic activity and other surface changes.

• Mercury is the smallest planet, with a gray surface and extreme temperature changes. It has no moons.
• Venus has a thick, hot atmosphere and an extremely hot surface. It also has no moons.
• Earth is the only known place with life and liquid water. It has a protective magnetic field and one moon, called the Moon, which 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 classified as a dwarf planet. Other notable asteroids include Vesta and Pallas, which have unique surface features. Some asteroids share similar orbits and are grouped into families, while 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 is home to the giant planets and their large moons. The centaurs and many short-period comets also orbit here. Because these objects are farther from the Sun, they contain 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, while 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 and has unusual weather patterns. Neptune is the farthest planet and has dark spots and mysterious weather, with its moon Triton having active geysers.

Centaurs

Main article: Centaur (small Solar System body)

Centaurs are icy bodies between Jupiter and Neptune that were pushed inward by the outer planets. Some, like 2060 Chiron, show comet-like activity when they get close to the Sun. The largest known centaur, 10199 Chariklo, even 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 extending between 30 and 50 AU from the Sun. It contains many small bodies, with the largest being dwarf planets like Pluto, Orcus, Haumea, Makemake, and Quaoar. These objects have orbits that can be influenced by Neptune in various ways.

The scattered disc overlaps the Kuiper belt but extends much further, up to near 500 AU. Objects here have been pushed into distant, eccentric orbits by Neptune's gravity. Two notable dwarf planets in this region are Eris and Gonggong. Farther out are extreme trans-Neptunian objects, such as Sedna, which have very long orbits and are less affected by the giant planets. 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, and is thought to 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 small solid objects in space, usually smaller than one meter. They can be made of silicates and metals like nickel and iron. When meteoroids enter Earth's atmosphere, they create bright streaks of light called meteors. If many meteoroids enter at the same time, they can create a meteor shower that looks like the streaks are coming from one point in the sky.

The inner Solar System has a cloud of dust called the zodiacal dust cloud, which can sometimes be seen as a hazy light called the zodiacal light. The outer Solar System also has a cloud of cosmic dust.

Comets

Main article: Comet

Comets are small bodies in the Solar System made mostly of icy materials. When they get close to the Sun, the ice turns directly into gas, creating a glowing coma and often a tail that can be seen from Earth. Short-period comets orbit the Sun in less than two hundred years, while long-period comets take thousands of years. Some comets come from far outside our Solar System.

Boundary region and uncertainties

Much of the outer Solar System remains a mystery because we have not explored it much. We learn about this distant area by studying objects that come close enough to the Sun for us to see them, often as bright comets. Many more objects likely exist out there, waiting to be discovered.

The Sun's gravitational influence, called its Hill sphere, may stretch out to about 230,000 AU and could include a distant region called the Oort cloud. The heliosphere, a bubble of solar wind, ends at a boundary called the termination shock, 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 close neighbors in space. The closest star system to us is Alpha Centauri, about 4.4 light-years away. It includes three stars: Alpha Centauri A, Alpha Centauri B, and a small star called Proxima Centauri. In 2016, scientists found a planet around Proxima Centauri named Proxima Centauri b, which might be able to support life.

Our Solar System is inside a region called the Local Interstellar Cloud, surrounded by a vast, empty space known as the Local Bubble. This bubble is filled with very hot gas and is thought to be shaped like an hourglass. It was likely created by explosions from stars called supernovae long ago.

Galactic position

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

The Solar System is located in the Milky Way, a huge spiral galaxy with many stars. The Sun is in one of the outer arms of this galaxy, called the Orion–Cygnus Arm. The Sun moves around the center of the Milky Way at a speed of about 220 kilometers per second, completing a full loop every 240 million years.

The Solar System's place in the Milky Way helps provide a stable environment for life on Earth. By staying in a quieter part of the galaxy, far from the busy spiral arms, Earth has had long periods of peace for life to develop.

Discovery and exploration

Main article: Discovery and exploration of the Solar System

For many years, people thought Earth was the center of the universe. But in the 1500s, Nicolaus Copernicus suggested that the Sun was at the center instead. Later, Johannes Kepler improved this idea and showed that planets move in oval-shaped paths called ellipses.

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

In the 1900s and 2000s, we began sending spacecraft to explore our Solar System. These probes have visited every planet, flown through the Sun’s outer atmosphere, and even brought back samples from asteroids and comets. We’ve also landed on the Moon and plan to return there soon.