Look up at the night sky, and you are seeing only a small part of the place we call home.
Earth is one planet orbiting the Sun, but it is part of a much larger system containing eight planets, hundreds of known moons, millions of asteroids and other small bodies, and countless icy objects. Far beyond the planets lies a vast region of small worlds and frozen debris that stretches into the outer Solar System.
The Solar System formed about 4.6 billion years ago from a large cloud of gas and dust. Gravity gradually gathered much of that material toward the center, where the Sun formed. The remaining material flattened into a rotating disk, and particles within it collided and joined together to build planets and other bodies.
Today, the Solar System is a complex collection of worlds with remarkably different environments. Earth has liquid water and life. Venus has a crushing, heat-trapping atmosphere. Mars preserves evidence of a wetter past. Jupiter is a giant ball of hydrogen and helium surrounded by dozens of moons. Saturn is famous for its spectacular rings. Uranus rotates on its side, while Neptune experiences some of the fastest winds known in the Solar System.
And beyond them are smaller worlds that preserve clues about how the Solar System began.
What is the Solar System?
The Solar System is the gravitationally bound system centered on the Sun.
It includes the Sun, the eight planets, their moons, dwarf planets, asteroids, comets, meteoroids, and other smaller objects. It also contains dust, gas, charged particles, and the magnetic environment produced by the Sun.
The Sun contains almost all of the Solar System’s mass. Its gravity therefore dominates the system and keeps the planets and most other objects in orbit around it.
The Solar System is also much larger than the region occupied by the eight planets. The outer boundary is difficult to define precisely because the Sun’s influence gradually fades rather than ending at a sharp edge.
How did the Solar System form?
The Solar System began forming about 4.6 billion years ago from a collapsing cloud of gas and dust called the solar nebula.
Gravity pulled material toward the cloud’s center. As the cloud collapsed, it began rotating faster and flattened into a disk.
Most of the material gathered in the center, where pressure and temperature eventually became high enough for nuclear fusion to begin. The Sun was born.
The remaining material in the disk gradually clumped together. Tiny particles collided and stuck together, producing progressively larger objects.
These objects eventually became planetesimals—small bodies that could attract additional material through gravity. Continued collisions and accretion produced larger planetary bodies called protoplanets.
Over millions of years, the planets took shape.
The planets did not all form in exactly the same environment. Close to the young Sun, temperatures were high enough that many volatile substances could not easily condense into solid material. Farther from the Sun, colder conditions allowed ices to accumulate alongside rock.
That difference helped produce two broad groups of planets: rocky worlds in the inner Solar System and giant planets farther out.
The Sun is the center of the Solar System
The Sun is a star and the central object of the Solar System.
It is a nearly spherical ball of extremely hot plasma composed primarily of hydrogen and helium. Energy produced in its core eventually reaches the surface and radiates into space as sunlight.
The Sun’s gravity controls the orbits of the planets and most other Solar System objects.
It also produces the solar wind, a continuous flow of charged particles streaming outward from the Sun. The solar wind creates a huge region of influence around the Solar System called the heliosphere.
Without the Sun’s energy, Earth would be a dark, frozen world. Sunlight drives Earth’s climate system and provides the energy that supports photosynthesis and, ultimately, nearly all ecosystems on the planet.
The layers of the Sun
The Sun has several major regions.
At its center is the core, where nuclear fusion takes place. Hydrogen nuclei combine to form helium, releasing energy.
Surrounding the core is the radiative zone, where energy moves outward primarily through the absorption and re-emission of radiation.
Farther out is the convection zone, where hot plasma rises and cooler plasma sinks, transporting energy through convection.
The visible surface is called the photosphere. Above it lie the Sun’s atmosphere, including the chromosphere and corona.
The corona is surprisingly hot, reaching temperatures much higher than the visible surface. Understanding why the corona is so hot is one of the longstanding questions in solar physics.
Solar activity
The Sun is not perfectly calm.
Its magnetic field changes over time and produces phenomena such as sunspots, solar flares, and coronal mass ejections.
Solar flares can release bursts of electromagnetic radiation, while coronal mass ejections can send enormous clouds of magnetized plasma into space.
When solar activity interacts with Earth’s magnetic field and upper atmosphere, it can produce auroras. Strong solar storms can also disturb satellites, radio communications, navigation systems, and electrical infrastructure.
The eight planets
The eight planets are commonly divided into two groups.
The terrestrial planets—Mercury, Venus, Earth, and Mars—are relatively small and rocky.
The outer planets—Jupiter, Saturn, Uranus, and Neptune—are much larger. Jupiter and Saturn are gas giants, while Uranus and Neptune are classified as ice giants.
The planets also differ enormously in temperature, atmosphere, geology, magnetic fields, moons, and other characteristics.
Mercury
Mercury is the smallest planet in the Solar System and the closest planet to the Sun.
Its surface is heavily covered with impact craters, giving it an appearance somewhat similar to Earth’s Moon. Mercury has a large metallic core relative to its overall size.
Because Mercury has only a very thin atmosphere-like exosphere, it cannot efficiently distribute heat around the planet. Surface temperatures therefore vary dramatically between the day and night sides.
Mercury completes an orbit around the Sun in only about 88 Earth days.
Despite being closest to the Sun, Mercury is not the hottest planet. That distinction belongs to Venus.
Venus
Venus is the second planet from the Sun and is similar to Earth in size and general composition.
But the similarities end quickly.
Venus is covered by a thick atmosphere dominated by carbon dioxide, with clouds containing sulfuric acid. Its atmosphere creates an intense greenhouse effect that makes the planet’s surface extraordinarily hot.
The pressure at Venus’s surface is also far greater than Earth’s surface pressure.
Venus rotates very slowly and in the opposite direction from most planets. A Venusian day, measured by its rotation relative to the stars, lasts longer than its year.
The planet’s surface contains mountains, plains, volcanic features, and large regions that have been reshaped by geological processes.
Earth
Earth is the third planet from the Sun and the largest of the four terrestrial planets.
It has a nitrogen- and oxygen-rich atmosphere, abundant surface water, an active water cycle, and a dynamic geological system.
Earth’s surface is divided into large tectonic plates that move over geological time. Their movements help build mountains, generate earthquakes, and contribute to volcanic activity.
Earth also has a strong global magnetic field generated largely by movement within its molten outer core. The magnetic field helps shield the planet from much of the solar wind and contributes to the formation of Earth’s magnetosphere.
Earth is currently the only world known to support life.
Earth’s Moon
Earth has one natural satellite: the Moon.
The Moon is much smaller than Earth but has a major influence on our planet. Its gravity is the primary cause of Earth’s ocean tides, while the Sun also contributes to tidal effects.
The Moon probably formed after a giant collision involving the young Earth and another planetary body, although details of its formation remain an area of scientific study.
Humans first landed on the Moon in 1969 during NASA’s Apollo 11 mission.
The Moon’s surface preserves an extensive record of impacts because it lacks the atmosphere, liquid water, and active surface processes that erase many ancient features on Earth.
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System.
Its surface is cold, dry, and covered by iron-rich dust that gives the planet its familiar reddish color.
Mars has enormous volcanoes, deep valleys, impact craters, polar ice caps, and evidence of ancient river channels and other water-related features.
Today, liquid water is not stable for long on most of the Martian surface, but water exists as ice, including significant amounts beneath the surface and in the polar regions.
Mars also has a very thin atmosphere dominated by carbon dioxide.
Robotic spacecraft have explored Mars for decades. Rovers and landers have examined its rocks and soil, while orbiters have mapped the planet from above.
A major scientific goal is to determine how Mars changed from a world that appears to have once had more persistent surface water into the cold, dry planet seen today.
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet in the Solar System.
It is a gas giant composed primarily of hydrogen and helium.
Jupiter has no solid surface like Earth. As you move deeper into its atmosphere, pressure and temperature rise enormously, and the hydrogen eventually takes on unusual physical properties.
The planet is famous for its colorful cloud bands and enormous storms.
The Great Red Spot is a giant storm that has been observed for centuries. Its exact appearance and behavior have changed over time, but it remains one of Jupiter’s most recognizable features.
Jupiter also has a powerful magnetic field and an extensive system of moons.
Jupiter’s major moons
Jupiter has many known moons, including four large ones discovered by Galileo Galilei in 1610: Io, Europa, Ganymede, and Callisto.
Each is a world in its own right.
Io is the most volcanically active world known in the Solar System. Its intense volcanic activity is driven by tidal heating caused by Jupiter’s gravity and interactions with other moons.
Europa has a surface dominated by water ice and is thought to contain a global ocean beneath that icy shell. Because of its potentially habitable environment, Europa is one of the most scientifically interesting worlds in the Solar System.
Ganymede is the largest moon in the Solar System. It is even larger in diameter than Mercury, although it has much less mass.
Callisto has an ancient, heavily cratered surface and may also contain a subsurface ocean.
Saturn
Saturn is the sixth planet from the Sun and the second-largest planet.
Like Jupiter, it is a gas giant composed mostly of hydrogen and helium.
Saturn is best known for its extensive ring system.
Saturn’s rings
Saturn’s rings are made mostly of countless particles of water ice, along with rocky material and dust.
The particles range from tiny grains to much larger pieces. The rings are divided into numerous sections and gaps shaped by gravitational interactions with Saturn’s moons.
Although the rings appear solid from a distance, they are actually a vast collection of individual particles orbiting the planet.
Saturn’s rings are also surprisingly thin compared with their enormous width.
Saturn’s moons
Saturn has a remarkable collection of moons.
Titan, its largest moon, has a thick atmosphere and lakes and seas of liquid methane and ethane on its surface. It is the only moon in the Solar System known to have stable bodies of liquid on its surface.
Titan also has an active cycle involving methane that resembles Earth’s water cycle in some broad physical ways.
Enceladus is much smaller but scientifically fascinating. It has an icy surface and shoots plumes of water-rich material into space from fractures near its south pole. Evidence indicates that a liquid ocean exists beneath its ice.
These moons demonstrate that planetary science is not limited to planets themselves. Some moons have environments that may be capable of supporting chemistry relevant to life.
Uranus
Uranus is the seventh planet from the Sun and an ice giant.
Its atmosphere contains hydrogen and helium along with methane. Methane absorbs some wavelengths of red light, contributing to Uranus’s blue-green appearance.
Uranus is unusual because its rotation axis is tilted by about 98 degrees relative to its orbital plane. In practical terms, the planet rotates almost on its side.
This produces extreme seasonal changes as Uranus travels around the Sun.
Uranus also has a faint ring system and numerous moons.
Neptune
Neptune is the eighth and most distant recognized planet from the Sun.
Like Uranus, it is an ice giant containing hydrogen, helium, and methane in its atmosphere.
Neptune is known for its deep blue appearance and extremely active atmosphere.
Its winds can reach extraordinary speeds, making Neptune one of the most dynamically active planetary atmospheres known in the Solar System.
Neptune has a faint ring system and several moons. Its largest moon, Triton, is particularly interesting because it appears to have been captured by Neptune rather than forming alongside the planet.
Triton has an icy surface and evidence of geological activity.
Why Pluto is a dwarf planet
For much of the 20th century, Pluto was taught as the ninth planet.
In 2006, the International Astronomical Union established a formal definition of a planet that requires an object to orbit the Sun, be massive enough for gravity to make it nearly round, and have cleared most other objects from its orbital neighborhood.
Pluto satisfies the first two conditions but not the third.
It therefore became classified as a dwarf planet rather than a full planet.
The decision did not make Pluto disappear or become less interesting. It helped scientists distinguish between the eight dominant planets and a much larger population of smaller planetary bodies.
Pluto is part of the Kuiper Belt, a distant region containing many icy objects.
Pluto and its moons
NASA’s New Horizons spacecraft flew past Pluto in 2015, providing the first close-up observations of the dwarf planet.
The spacecraft revealed a surprisingly diverse landscape, including mountains of water ice, plains of frozen nitrogen, glaciers, and other geological features.
Pluto’s largest moon, Charon, is unusually large compared with Pluto itself. The two bodies orbit a point in space outside Pluto’s physical center, making the system resemble a double world.
Pluto also has four smaller known moons.
What are dwarf planets?
A dwarf planet is a body that orbits the Sun and is massive enough for its own gravity to make it approximately round, but it has not cleared its orbital neighborhood of other objects and is not a satellite.
Several objects are recognized as dwarf planets, including Pluto, Ceres, Eris, Haumea, and Makemake.
Other distant objects may also qualify, but classification can depend on what is known about their size, shape, and orbital environment.
Ceres: the dwarf planet in the asteroid belt
Ceres is the largest object in the asteroid belt between Mars and Jupiter and the only dwarf planet located in the inner Solar System.
NASA’s Dawn spacecraft studied Ceres in detail and found a world with bright deposits, ice-rich material, mountains, craters, and evidence of geological activity.
Ceres is much smaller than Pluto but is important for understanding how planetary bodies formed in the early Solar System.
The asteroid belt
Between Mars and Jupiter lies the asteroid belt, a broad region containing millions of rocky and metallic objects.
Despite the way it is often depicted in movies, the asteroid belt is mostly empty space. A spacecraft traveling through it would not normally have to weave between giant rocks.
The objects in the belt range from tiny fragments to bodies hundreds of kilometers across.
Scientists think Jupiter’s strong gravity played a major role in preventing material in this region from accumulating into a planet.
Asteroids are leftovers from the Solar System’s formation. Studying them therefore gives scientists access to relatively primitive material from the early stages of planetary development.
What are asteroids?
Asteroids are rocky or metallic bodies that orbit the Sun.
Most known asteroids are concentrated in the asteroid belt, but many travel on other types of orbits.
Some asteroids cross Earth’s orbital path and are classified as near-Earth objects.
Astronomers closely monitor potentially hazardous asteroids because an impact by a sufficiently large object could cause serious regional or global consequences.
Space missions have also begun visiting asteroids directly. These missions allow scientists to examine their composition and structure and to test technologies for changing the trajectory of a small body.
What are comets?
Comets are small bodies containing significant amounts of ice mixed with dust and rocky material.
They generally spend most of their time in the cold outer Solar System.
When a comet approaches the Sun, solar heating causes some of its volatile material to turn directly into gas. The escaping gas carries dust away from the surface, producing a surrounding coma.
Solar radiation and the solar wind can then help create tails that extend away from the Sun.
A comet can have two prominent tails: a dust tail and an ion tail. Despite their appearance, the tails do not necessarily trail behind the comet along its orbital path. They are generally directed away from the Sun because of sunlight and the solar wind.
Where do comets come from?
Two major reservoirs contain many of the Solar System’s comets.
The Kuiper Belt lies beyond Neptune and contains numerous icy objects. Some comets with relatively short orbital periods originate from this region.
Farther away is the hypothetical Oort Cloud, a vast spherical reservoir of icy bodies thought to surround the Solar System.
The Oort Cloud has never been directly observed as a complete structure, but the orbits of some long-period comets provide evidence consistent with its existence.
The Kuiper Belt
The Kuiper Belt is a region beyond Neptune containing a large population of icy bodies.
Pluto is one of its best-known inhabitants.
The belt is a remnant of the Solar System’s formation and contains objects that did not become part of a larger planet.
Studying these distant bodies helps scientists understand the conditions and processes that existed in the outer Solar System billions of years ago.
The Kuiper Belt is also the source of many objects that can be gravitationally disturbed into new orbits.
The Oort Cloud
The Oort Cloud is thought to lie far beyond the Kuiper Belt.
Unlike the relatively flat Kuiper Belt, the Oort Cloud is expected to be roughly spherical, extending around the Solar System in all directions.
Scientists have not directly photographed the Oort Cloud. Its existence is inferred primarily from the unusual long-period orbits of comets.
If the Oort Cloud exists as expected, it represents one of the most distant regions influenced by the Sun’s gravity.
Meteoroids, meteors, and meteorites
These three terms describe related but different things.
A meteoroid is a relatively small piece of rock or metal traveling through space.
When a meteoroid enters Earth’s atmosphere and produces a streak of light, the phenomenon is called a meteor.
If part of the object survives the journey through the atmosphere and reaches the ground, that surviving material is called a meteorite.
Meteorites are valuable scientific samples because some come from asteroids and preserve material dating back to the early Solar System.
A few meteorites also come from the Moon or Mars, having been blasted from those worlds by impacts before eventually reaching Earth.
The Solar System is not perfectly flat
The planets generally orbit the Sun in roughly the same plane because they formed from a rotating disk of material.
This is why the planets tend to appear along a similar path across the sky.
But their orbits are not perfectly aligned. Each planet has its own orbital inclination, and small bodies can have much more tilted or eccentric orbits.
The Solar System is therefore better thought of as a broadly disk-shaped system than as a perfectly flat arrangement.
Why do planets orbit the Sun?
The planets are moving through space while the Sun’s gravity pulls them inward.
Their forward motion and the Sun’s gravitational attraction combine to produce orbital motion.
A planet is therefore not simply being held up by gravity or moving around the Sun because gravity is “pulling it sideways.” It is continuously falling toward the Sun while also moving forward fast enough to keep missing it.
This is the basic idea behind an orbit.
The same principle applies to moons orbiting planets and spacecraft orbiting worlds.
How long does it take planets to orbit the Sun?
The closer a planet is to the Sun, the shorter its orbital period generally is.
Mercury completes one orbit in about 88 Earth days.
Earth takes about 365.25 days.
Mars takes about 687 Earth days.
Jupiter takes nearly 12 Earth years.
Saturn takes about 29.5 Earth years.
Uranus takes about 84 Earth years.
Neptune takes about 165 Earth years.
These differences arise because planets farther from the Sun travel along larger orbits and generally move more slowly around the Sun.
Why do the planets have different environments?
The planets formed from the same broad cloud of material, but they ended up with very different compositions and histories.
Distance from the young Sun mattered. So did planetary mass, atmospheric composition, internal heat, impacts, volcanic activity, magnetic fields, and interactions with other objects.
A planet’s environment can also change dramatically over time.
Mars lost much of its atmosphere and surface water. Venus developed an extremely thick carbon-dioxide atmosphere and intense greenhouse heating. Earth retained abundant surface water and developed a chemically active atmosphere.
The giant planets followed different evolutionary paths because they formed in colder regions and accumulated much more material.
The outer Solar System is much more than the planets
Neptune does not mark the end of everything in the Solar System.
Beyond it are populations of icy bodies, including objects in the Kuiper Belt and farther-out scattered populations.
Some objects travel on highly elongated or inclined orbits that take them far beyond Neptune.
The gravitational influence of the Sun continues much farther still, although it becomes progressively weaker.
Eventually, the solar wind meets the surrounding interstellar environment, forming a boundary known as the heliopause.
The heliosphere and the edge of the Sun’s influence
The Sun continuously releases the solar wind, which creates a vast bubble around the Solar System called the heliosphere.
The heliosphere interacts with the material and magnetic fields between stars.
The heliopause is the boundary where the outward flow of the solar wind is no longer dominant compared with the surrounding interstellar environment.
NASA’s Voyager 1 and Voyager 2 spacecraft have crossed the heliopause and entered interstellar space, providing direct measurements of the environment beyond the heliosphere.
This does not mean the spacecraft have left the Solar System in every possible definition of that term. The Sun’s gravitationally bound region extends vastly farther than the heliosphere, potentially out to the distant Oort Cloud.
How do we explore the Solar System?
Most of what we know about the Solar System comes from a combination of telescopic observations, spacecraft, laboratory measurements, computer models, and analysis of samples such as meteorites.
Robotic spacecraft have flown past planets, entered orbit around them, landed on their surfaces, driven across them, collected samples, and studied their atmospheres and magnetic environments.
Different missions are designed for different scientific questions.
Orbiters can map an entire world over long periods. Landers can study local geology. Rovers can move across surfaces and investigate multiple locations. Flyby spacecraft can visit distant targets without entering orbit.
Sample-return missions provide something especially valuable: actual material from another world that scientists can analyze in laboratories on Earth.
The Solar System is still changing
The Solar System may look stable on human timescales, but its objects are constantly interacting.
Asteroids collide. Comets change as they approach the Sun. Moons exert gravitational forces on one another and their planets. Planets slowly perturb the orbits of smaller bodies.
The Sun itself changes as it moves through its stellar life cycle.
Over extremely long periods, gravitational interactions can alter the paths of objects throughout the Solar System. The system we observe today is therefore a snapshot of a much longer history that began billions of years ago and continues to unfold.
