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The Solar System

The Solar System

Notable objects in the solar system include the Sun, the eight recognized planets, the Asteroid belt, the Kuiper belt, and dwarf planets (including Pluto). The Solar System is the body bound by gravity containing all these objects.

Scale of the Solar System

The solar system is immense compared to an individual human being and the Earth itself. To measure distances in the solar system, astronomers frequently use the astronomical unit (AU), which is the average distance from the center of the Earth to the center of the Sun. One AU is equal to 149,597,871 kilometers or 92,955,807 miles. Measurements of the solar system's size vary depending on how it is defined.

Scale by Distance to Neptune

If you consider the solar system to be the distance from the Sun to the farthest currently recognized planet, Neptune, the distance is about 30 AU. By this definition, the solar system has a diameter of about 60AU from end to end.

Scale by Distance to the Edge of the Heliosphere

The heliosphere includes all the space around the Sun where the Sun's solar winds reach. Solar winds are particles given off by the Sun. You could think of the heliosphere like a gigantic bubble around the Sun.

The heliosphere ends about 90AU from the Sun. So the diameter of the solar system would be 180AU, by this definition.

Scale by Distance to the Farthest Object

The farthest known observable object orbiting the Sun is Sedna. It is a dwarf planet that reaches a practically incomprehensible 937 AU from the Sun at its farthest point in orbit. The orbit of Sedna is so large, it takes over 11 thousand Earth years to orbit the Sun.

By this definition, the radius of the solar system could be 937 AU in any direction from the Sun, so 1,874 AU in total diameter.

Regardless of which scale you use, the solar system is massive. Therefore, most of the illustrations in this guide are not to scale.

Major Objects

Here is a summary of all the major objects in the solar system, by average distance from the Sun.

solar system scale 720p

The Sun (0 AU)

The Sun is the most important and massive object in our solar system. It's classified as a G-type main sequence (yellow dwarf) star. 

Every other object in the solar system orbits the Sun. The Sun is a source of energy given off as heat, light, and radiation.

The Sun is enormous compared to everything else in the system, with a mass of about 1.989 x 1030 kg and a radius of 696,340 km. In terms of mass, the Sun is about 333,000 times the mass of Earth. In terms of volume, the Sun occupies about 1.3 million times more volume than the Earth. The Sun alone accounts for nearly all the mass in the entire solar system, about 99.8% of it.

The Sun is pure white, but appears yellow, red, or orange when viewed through Earth's atmosphere. Most images you see of the Sun are false color images which have been modified to show details that would otherwise be too difficult to see. A true color image of the Sun looks like a solid white sphere. It's far too bright to look at directly with the naked eye.

In about five billion years, the Sun will swell into a red giant hundreds of times larger than it is now. The Earth will become inhospitable, either too close to the Sun or swallowed by the Sun. After another billion years, it will contract into a white dwarf. Ending its life as a dim, white star about the size of Earth. 

paul stewart the sunThe Sun (False Color)

Mercury (0.39 AU)

Mercury is the closest planet to the Sun, with a distance ranging between 0.307AU and 0.467AU from center to center. It is named after the Roman god Mercurius.

Mercury is the smallest major planet in the solar system, with a radius of about 2440km. It's roughly 1/20 the size of Earth, both in terms of mass and volume.

The Sun facing side of Mercury is hot, with temperatures reaching 700K (800°F). The shaded side is much cooler, at an average of about 111K (-260°F).

Because of its proximity to the Sun, most of Mercury's outer shell may have been stripped away, leaving only a thin crust and a dense metallic iron core.

Mercury is one of the four terrestrial planets, meaning it's mostly made of rock and metal. All four inner planets are terrestrial.

nasa mercuryMercury (True Color)

Venus (0.72 AU)

The second planet from the Sun, Venus, is named after the Roman god of love and beauty. It's nearly the size of Earth, with a radius of 6,052km. Its mass is 81% that of the Earth and its volume is 87%.

Although Venus is farther from the Sun than Mercury, it is actually significantly hotter. This is because a day on Venus lasts an incredible 5,832 hours, or almost 117 Earth days. It rotates on its axis slower than any planet in the solar system, so the side facing the sun receives a lot of sunlight for a long time. The temperature of Venus's sun-facing side is 748K (900°F).

Venus is the brightest planet, appearing as a solid white orb in the sky. It's the third brightest object in Earth's sky after the Sun and Moon. From Earth, it looks like a bright star.

nasa venus false colorVenus (False Color)
nasa venus true colorVenus (True Color)

Earth (1 AU)

Earth is the third planet from the Sun. It is the densest planet in the solar system, with the largest moon relative to its size.

Earth possesses some of the widest variations of topographic features in the solar system because of its climate and active plate tectonics. It is the only known planet with stable bodies of liquid water on its surface, and the only planet we know of which can sustain life.

The radius of Earth is 6,378 km, making it the largest of the four terrestrial planets. It's the fourth smallest planet of the eight recognized in the solar system.

nasa earthEarth

Mars (1.52 AU)

The red planet, Mars, named after the Roman god of War, is the fourth planet from the sun and the last of the four terrestrial planets. It has unique topography, largely due to the flowing water on its surface several billion years ago. Canyons, valleys, and mountain ranges scatter the surface. It has volcanoes as well, though none remain active today.

Olympus Mons, the tallest mountain in the solar system, is found on Mars. It was formed from volcanic activity and towers 25km above the surrounding plains. That's roughly three times as tall as Mt. Everest on Earth.

Direct evidence of ice being present on Mars has been confirmed. Of all the planets in the solar system, Mars is the most Earth-like. It's roughly what Earth might look like if all the surface water vaporized. This makes it a key area of research in planetary studies. It has weather with clouds made of ice water and violent windstorms. A day on Mars is 24.7 hours, just slightly longer than a day on Earth.

It is significantly smaller than the Earth. In terms of size, Mars has a radius of 3,396 km, roughly 53% that of Earth. It's volume is 15% that of Earth and its mass is 11% Earth's. Mars has two moons, Phobos and Deimos, which are each much smaller than the Earth's moon.

Multiple robots or rovers have been sent to orbit and explore the surface of Mars. Manned missions to Mars are expected to occur in as little as the next decade. It is likely the only other planet in the solar system in which humanity could establish a permanent base.

nasa marsMars

Asteroid Belt (2.2 - 3.2 AU)

Asteroids are rocky bodies orbiting the sun, which are too small to be major planets. They differ from comets, which are composed of ice and gas, rather than rock.

The asteroid belt falls between Mars and Jupiter. It contains many asteroids in varying shapes and sizes. Researchers speculate it was formed from the remnants of several ancient planets during the early period of the solar system. The asteroids may collide with one another on a fairly regular basis and break apart into smaller pieces. Therefore, the asteroid belt is more fragmented today than it was earlier on the cosmic timeline.

The vast majority of meteorites found on Earth come from the asteroid belt.

Ceres, the largest object in the asteroid belt, is large enough to have formed a spherical shape and is considered a dwarf planet. Its rocky composition, lack of atmosphere, and impact craters make it appear quite similar to the Earth's Moon. NASA has sent spacecraft to orbit Ceres, so we have some lovely pictures of it.

When asteroids impact other bodies, they can form craters, bowl shaped cavities in the bodies they impact. Smaller asteroids are referred to as meteoroids. The bulk of meteoroids get burnt up in Earth's atmosphere, causing shooting stars. On the Moon, however, there is almost no atmosphere, which is part of the reason it's littered with visible craters.

asteroid belt map esahubbleLocation of Asteroid Belt (ESA)
nasa ceresCeres Asteroid/Dwarf Planet
4 vestaVesta Asteroid
nasa asteroid idaIda
az barringer crater shane torgersonBarringer Crater - Arizona


Jupiter is the first of the gas giants, and the largest planet in the solar system both in terms of mass and volume. Gas giants, also referred to as Jovian planets or failed stars, are massive planets mostly made of the gasses hydrogen and helium. If they were more massive, they could have possibly become stars.

The planet is named after Jupiter, the king of gods in Roman mythology. Compared to Earth, Jupiter is enormous. About 1,300 Earths could fit in the space Jupiter occupies. The radius is 71,492 km.

Jupiter's massive gravity hosts at least 80 moons. Its largest moon is Ganymede, which is also the largest moon in the solar system, and larger than the planet Mercury by volume.

The rotation of Jupiter is very rapid, at one complete rotation in just under 10 hours. The winds of Jupiter may move at speeds as fast as 1400 kilometers per hour (~900mph), faster than the speed of sound on Earth.

Jupiter is famous for its Great Red Spot (shown lower right of center in accompanying photo), which is a 16,000 km (10,000 mi) wide storm that has been raging on Jupiter for hundreds of years, if not longer. Since it's a storm, it's always changing. Recent observations show that the Great Red Spot is shrinking and may eventually disappear in the distant future.

Jupiter has a faint ring system. While humanity has known about Jupiter's existence for millennia, the ring system wasn't discovered until 1979, when NASA's Voyager I did a flew by.

jupiterJupiter (5.20 AU)

Saturn (9.57 AU)


The second gas giant, Saturn, is famous for its stunning, large ring system. It's not the only planet with rings, but it has the most vibrant rings. This planet is the second largest in the solar system.

It's named after the Roman god of agriculture. Being the last planet visible from Earth with the naked eye, it's the last planet whose name was directly recorded by the ancient Romans.

Saturn is 84% as wide as Jupiter, with a radius of 60,268km.

Its ring system is 270,000 km in diameter, making its ring system wider than the planet Jupiter. Saturn's rings are thought to come from various sources, including destroyed moons, icy comets, and rocky asteroids. Over time, with the help of gravity, they form a ring structure. The first reported observation of Saturn's rings comes from the 1600s, when the famous historical astronomer Galileo viewed Saturn through a telescope.

Saturn has 83 recognized moons that we currently know of, slightly more than Jupiter's 80. Most of these moons are tiny.


Uranus (19.16 AU)

Uranus, the third gas giant, is known for its distinctive cyan color, a result of the methane gas in its atmosphere. It's less than half the width of Saturn, with a radius of 25,559km. It's mostly made of ices and is sometimes referred to as an ice giant. The coldest natural temperature in the solar system was observed on Uranus, due in part to its unique rotation, atmosphere, and distance from the sun.

It was the first planet to be discovered using a telescope. An astronomer named William Herschel found it in 1781. He originally named it after his patron, King George III of England. Several decades later, it was given the name Uranus, after the god of the Sky from Greek mythology.

Uranus is significantly farther away from the sun than the other planets before it. At 19AU, it's about twice as far from the sun than Saturn. Uranus also has a ring system, which is fairly faint compared to Saturn's, but slightly more visible than Jupiter's.

Uranus from Voyager 2Uranus

Neptune (30.18 AU)

Neptune is the last of the gas giants, the second ice giant, the eighth and final recognized planet from the Sun. It's nearly the same size as Uranus, with slightly less volume and a bit more mass. Neptune's color is often described as azure blue, more dark/pure blue than Uranus. Following the naming conventions taken from mythology, it was named after the Roman god of the sea.

It was discovered a little over half a century after Uranus in 1846. Three different astronomers/mathematicians are credited with independently discovering it: Urbain Le Verrier (French), John Adams (British), and Johann Galle (German).

Like Uranus, Neptune is icy and cold. It can be categorized as an ice giant. At 30 AU from the sun, about 50% farther than Uranus, it's extremely distant.

On average, Neptune is slightly colder than Uranus by about 5°C. Its axis is more stable than the axis of Uranus, so even though it's colder than Uranus on average, it doesn't get as cold as the coldest parts of Uranus.

A faint ring was confirmed around Neptune a couple of years after the discovery of Uranus's ring.


Pluto (39.48 AU)

Pluto, first discovered in 1930, is an icy world deep within the Kuiper belt. Its name comes from the Roman god of the underworld.

With a radius of 1,188km, it's roughly 2/3 the radius of Earth's moon. It's distance from the sun and small size give it an average temperature of -225°C (48K), the lowest on this list so far.

Originally, Pluto was given the same major planet designation as the other eight planets before it. This changed in 2006, when the International Astronomical Union reclassified it as a dwarf planet.

The designation of dwarf planet is given to bodies which are large enough to appear spherical, but too small to have much gravitational effect on other nearby bodies. Their mass is not great enough to clear other nearby objects from the system, something all the eight major planets can do.

Despite being downgraded to a dwarf planet, Pluto is still a unique world in its own right. It's the largest object in the Kuiper belt it inhabits and even has several moons.

Charon, Pluto's largest moon, has a radius of 606km, a little more than half that of Pluto. It orbits Pluto at a close average distance of only 19,570km. That's slightly less than the width of the Pacific ocean, at its widest point.

Pluto has a much greater variation between its perihelion (closest distance to the Sun) and aphelion (farthest distance) than the major planets of the solar system. It's orbit is much more elliptical than circular. For a brief portion of its orbit around the sun, Pluto can actually be closer to the Sun than Neptune.

pluto charonPluto & Charon

Kuiper Belt (30-50 AU)

The Kuiper belt is a region of space beginning around Neptune (30 AU) and extending out to 50 AU. It can be thought of similarly to the asteroid belt, except far wider, far more massive, and far colder.

The Kuiper belt is home to trillions of icy bodies. Most of the comets humanity has observed come from the Kuiper belt.

Halley's comet, a famous bright comet visible to the naked eye which has been observed for millennia, orbits the solar system once every 75 years. It is thought to have originated in the Kuiper belt. Its last appearance was in 1986, and won't be returning until 2061, making witnessing it truly a once in a lifetime experience.

Pluto and several other dwarf planets exist within the Kuiper belt.

halleys cometHalley's Comet 1986

Planetary distances, radius, and other information sourced from: NASA - Planetary Facts Sheet

Orbits & Gravity

Gravity is the force which pulls masses together in space. When an object with a smaller mass enters the gravity of a larger mass, it is pulled towards the larger mass. Depending on the relative masses, velocities, trajectories, and distances, the smaller mass may either be pulled into an impact with the larger mass, it may orbit the larger mass, or it may pass by entirely. An orbit is a curved path an object takes around another object in space. When one object orbits another, it is considered a satellite of the object it orbits. Satellites may be natural, like the Moon, or man-made, like GPS or communication satellites.

One way to imagine an orbit is to picture a satellite that is always "falling" towards another object through gravity, while also moving at some velocity in another direction. As the object falls inward, its own velocity prevents it from colliding with the object it's falling towards. This way, the object is always falling towards the more massive object, but concurrently never impacts it.

Depending on velocities and relative gravities, some objects may only be satellites for a limited period. Some make a few orbits, only to be flung back off into space. Others may make a few orbits, then eventually fall to the surface of the larger mass. If an object's moving very fast, it may fly right by the larger mass before it can be captured in orbit.

Orbits are never perfect circles, but follow a slightly elliptical or stretched oval path.

orbitsBasic Orbits

The sun is the most massive object in the solar system, and all other bodies in the system orbit it. However, the center of mass for the entire solar system isn't an exact point in the center of the sun. While the sun's gravity pulls on every object in the solar system, the gravity of every other object pulls on the sun. A barycenter is the center of mass of two or more objects, and an invisible point around which they all orbit.

The barycenter of the entire solar system is hard to calculate exactly, given the number of objects in the solar system and their constantly changing positions. The location of the barycenter is always changing. The barycenter of the entire solar system is most heavily influenced by the Sun and Jupiter, the two largest masses. Sometimes it's within the sun itself, sometimes it's above the surface of the Sun.

jupiter sol barycenterBarycenter Example

The following diagram illustrates the orbits of the eight planets, and Pluto. Notice how different Pluto's orbit is from the other objects.

planet orbits nasaPlanetary Orbits

While everything in the solar system orbits the barycenter near the sun, the entire solar system is moving through space at roughly 200 km per second. The solar system orbits the galactic center, which contains a black hole called Sagittarius A. At this speed, it takes the solar system about 230 million years to complete one full orbit around the galaxy.

Interstellar Objects

Interstellar objects are objects that come from interstellar space (literally, the space between stars). They are not bound by the gravity of the sun or other star systems.

While humanity has been detecting interstellar objects for several decades, sightings of interstellar objects coming within our own solar system are rare and unpredictable. The first recorded interstellar object traversing our solar system, dubbed Oumuamua, was detected from Hawaii in October 2017. It was relatively small, reddish, and only 100 to 1000 meters long.

Oumuamua, having already passed Earth, was speeding away from the sun when it was detected. Its speed was measured at about 26km/sec, or 5.55 AU/year. This is very fast for such a small object. It was observed for eighty days, after which point it was too faint and fast for continued observation.

oumuamua renderArtistic Render of Oumuamua

The exact origins of Oumuamua remain unknown. It was originally thought to be a comet, but upon further analysis, was determined to not possess a coma, the defining characteristic of a comet. It could be the remnants of a comet. Oumuamua's officially classified as an asteroid.

Some astronomers have even suggested that it may be an alien spacecraft. Given its rotation, which is like an asteroid, this is unlikely. Of course, we have no way of definitively proving that it is or is not.

The second interstellar object detected in our solar system was 2I/Borisov, a comet found in 2019. It was traveling at 32 km/s, even faster than Oumuamua. This is the last interstellar object we have seen within our solar system, as of the time of writing this article.

image of comet 2i from nasa hubble space telescopeComet 2I/Borisov

In 2022, declassified data from the US Space Command revealed that a meteorite, which fell to Earth in 2014, had interstellar origins. This is the only interstellar object to have made it to Earth, that we know of. The meteorite itself landed somewhere in the Pacific ocean, making locating its fragments difficult, if not impossible.

More of these interstellar visitors likely exist. We just haven't found them yet.

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