Solar System
The Sun and planets of the Solar System. Sizes but not distances are to scale.
Age 4.568 billion years
Location Local Interstellar Cloud, Local Bubble, Orion–Cygnus Arm, Milky Way
System mass 1.0014 Solar masses
Nearest star Proxima Centauri (4.22 ly)
Alpha Centauri system (4.37 ly)
Nearest known planetary system Alpha Centauri system (4.37 ly)
Planetary system
Semi-major axis of outer planet (Neptune) 30.10 AU (4.503 billion km)
Distance to Kuiper cliff 50 AU
Stars 1 (Sun)
Planets 8 (Mercury · Venus · Earth · Mars · Jupiter · Saturn · Uranus · Neptune)
Known dwarf planets Possibly several hundred; five currently recognized by the IAU
(Ceres · Pluto · Haumea · Makemake · Eris)
Known natural satellites 431
(173 planetary · 258 minor planetary)
Known minor planets 659212 (as of 2014-09-25)
Known comets 3296 (as of 2014-09-25)
Identified rounded satellites 19
Orbit about Galactic Center
Invariable-to-galactic plane inclination 60.19° (ecliptic)
Distance to Galactic Center 27000 ± 1000 ly
Orbital speed 220 km/s
Orbital period 225–250 Myr
Star-related properties
Spectral type G2V
Frost line ≈5 AU
Distance to heliopause ≈120 AU
Hill sphere radius ≈1–2 ly
Ecliptic plane 3d view Solarsystem3DJupiter

The Solar System comprises the Sun and the many objects that orbit around it—eight planets, known dwarf planets, their 172 known moons, and countless asteroids and comets. The system occupies a disk-shaped volume of space more than 7.45 billion miles (12 billion kilometers) across. At the center is the Sun which contains more than 99 percent of the solar system's mass.[1]

The charted regions of the Solar System consists of the Sun, four relatively small inner planets surrounded by a belt of rocky asteroids, and four gas giants surrounded by the Kuiper belt of icy objects.

The hypothetical Oort cloud may also exist at a distance roughly a thousand times beyond these regions. The solar wind, a flow of plasma from the Sun, permeates the Solar System, creating a bubble in the interstellar medium known as the heliosphere, which extends out to the middle of the scattered disc.


Oort cloud Sedna orbit

The orbits of the bodies in the Solar System to scale (clockwise from top left).

An orbital period is the time it takes one object to travel around another in a complete circuit. The orbital period of a planet around the Sun is also the length of its year. The rotational period of a planet is how long it takes to make a complete turn on its axis.

Most of the planets, moons, and asteroids travel in almost circular orbits in the same direction (west to east) around the Sun. Most orbits also lie close to the plane of Earth's orbit, called the ecliptic. So if you looked at the solar system side-on, you would see most of the orbits are roughly on the same level. Mercury's and Pluto's orbits are not—they orbit at an angle.[2]



A diagram shows the formation of the Solar System.

The solar system is the name given to the planetary system of which the Earth is a part. It comprises planets, moons, comets, meteors and asteroids which are all held together by the gravitational pull of a star, named either the Sun or Sol. The solar system is believed to have formed from one nebula, the solar nebula.

As gravity forced the nebula to condense it became more dense and pressure inside it increased, resulting in the creation of a protostar, which began heating up to form the sun we see today. The proto-star would have been surrounded by interstellar dust and gases which began clumping together as a result of gravity. This process continued until, about 4.6 billion years ago, the clumps of rock and gas became much larger, and eventually gravity forced these irregular-shaped objects into the globular-shaped planets we see today. Many of the rocks did not become large enough to form planets and either remain today as asteroids, or they collided with the planets earlier in their history causing the large impact craters still visible throughout the solar system.[3]

The SunEdit

Sunrise over the Moon

Sunlight on the Moon is much brighter than that on Earth, due to the Moon's lack of an atmosphere. The seven Apollo landings all occurred during the early lunar morning. This allowed the astronauts to spot craters and boulders more easily, without the harsh sunlight during the day.
NASA/Science Photo Library

Planets and sun size comparison

The Sun compared to the planets

The Sun is our nearest star, located about 93 million miles (150 million km) from Earth. Even though it's made entirely of gas, its mass is 333,000 times greater than that of Earth and 750 times greater than that of all the planets in the solar system put together.[2]


White Dwarf Sun

Artwork of the sun forming a planetary nebula 5 billion years from now. Ejected gas (purple) is expanding towards the lifeless Earth (lower right) and moon (lower left) from the dying sun (upper center). This stage in the life of the sun follows its expansion during its red giant stage. When its nuclear fuel has been used up, its outer layers are ejected in this way. The remnant cools to form a white dwarf star.
Joe Tucciarone/Science Photo Library

In about 5 billion years, the hydrogen in the center of the Sun will start to run out. The helium will get squeezed. This will speed up the hydrogen burning. Our star will slowly puff into a red giant. It will eat all of the inner planets, even the Earth. As the helium gets squeezed, it will soon get hot enough to burn into carbon. At the same time, the carbon can also join helium to form oxygen. The Sun is not very big compared to some stars. It will never get hot enough in the center to burn carbon and oxygen. These elements will collect in the center of the star. Later it will shed most of its outer layers, creating a planetary nebula, and reveal a hot white dwarf star.

Nearly 99 percent of all stars in the galaxy will end their lives as white dwarfs. By studying the stars that have already changed, we can learn about the fate of our own Sun.


Size planets comparison

Planets of the Solar System to scale. Jupiter and Saturn (top row), Uranus and Neptune (top middle), Earth and Venus (bottom middle), Mars and Mercury.

The known planets in the solar system can be divided into two groups. The four planets closest to the sun, Mercury, Venus, Earth and Mars, are called the "terrestrial planets" after the Latin word for "land" because they all share similar surfaces comprising solid rock surrounding dense, metallic cores. The four outer planets, Jupiter, Saturn, Uranus and Neptune are known as the "Jovian planets," implying their similarities to the planet Jupiter—they are all much larger than the terrestrial planets, and do not share their rocky surfaces and metallic core. Instead they are giant balls of atmosphere, mainly comprising gases surrounding relatively small rocky cores. The terrestrial and the jovian planets are helpfully divided by a belt of asteroids, orbiting the sun between Mars and Jupiter. More recently Uranus and Neptune have been called Uranian rather than Jovian planets to highlight their differences to Jupiter and Saturn; mainly that they are appreciably smaller, are both bluish-greenish in color, comprise a significant amount of methane and have a thick coating of ice around their cores.[3]

Inner planetsEdit

Telluric planets size comparison

The inner planets. From left to right: Earth, Mars, Venus, and Mercury (sizes to scale, interplanetary distances not)


Mercury is the closest planet to the Sun has changed little in billions of years. It is a small, heavily cratered world with no atmosphere and no moons. Its year lasts 88 Earth days.[2]


Venus is the second planet from the Sun is similar in size to Earth, but the air pressure is 90 times greater than on Earth. It has no moon. Its year lasts 224 Earth days.[2]


Earth is the third planet from the Sun is the largest of the four rocky planets and the only planet with liquid water. Its year lasts 365 days.[2]


Mars is the fourth planet from the Sun has many craters as well as volcanoes, rift valleys, and winding canyons. It also has two moons. Its year lasts 687 Earth days.[2]

Asteroid beltEdit


Image of the asteroid belt (white), the Jupiter trojans (green), the Hildas (orange), and near-Earth asteroids.

Lying between Mars and Jupiter, the belt is around 112 million miles (180 million km) wide and contains thousands of asteroids.[2]

Outer planetsEdit

Gas giants in the solar system

From top to bottom: Neptune, Uranus, Saturn, and Jupiter (Montage with approximate colour and size)


Jupiter is the fifth planet from the Sun is also the largest. It has thin rings, 63 moons, and a cloud feature called the Great Red Spot. Its year lasts almost 12 Earth years.[2]


Saturn is the sixth planet from the Sun is the second largest, after Jupiter, but is light enough to float. It has 62 moons. Its year lasts 29.5 Earth years.[2]

Uranus öEdit

Uranus is the seventh planet from the Sun has a dark ring system and 27 moons. Its year lasts 84 Earth years.[2]


Neptune is the eighth planet from the Sun has a thin ring system and 13 moons. Its year lasts almost 165 Earth years.[2]


The centaurs are icy comet-like bodies whose orbits have semi-major axes greater than Jupiter's (5.5 AU) and less than Neptune's (30 AU). The largest known centaur, 10199 Chariklo, has a diameter of about 250 km. The first centaur discovered, 2060 Chiron, has also been classified as comet (95P) because it develops a coma just as comets do when they approach the Sun.


Pluto was once known as the ninth planet from the Sun, but it is now classified as a dwarf planet.[2]


Comet c1995o1

Comet Hale–Bopp

A comet is a celestial object consisting of a nucleus of ice and dust and, when near the sun, a "tail" of gas and dust particles pointing away from the sun.

Kuiper beltEdit

Outersolarsystem objectpositions labels comp

Plot of all Kuiper belt objects known in 2007, set against the four outer planets

The Kuiper belt, sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets, extending from the orbit of Neptune to approximately 50 AU from the Sun.

Oort cloudEdit

Kuiper oort-en

An artist's rendering of the Oort cloud, the Hills cloud, and the Kuiper belt (inset)

The Oort cloud is a spherical 'cloud' of frozen objects at the very edge of our solar system. You can imagine it a bit like a fishbowl, with the Solar System at the centre, and the spherical cloud of icy bodies on the edge.


  1. Carole Stott, Clint Twist [June 2003]. Space Facts - DK Pockets. DK Publishing. ISBN 9780789495938.
  2. 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 Peter Bond [2010]. Space: A Visual Encyclopedia. DK Publishing. ISBN 9780756662776.
  3. 3.0 3.1 Duncan John [2009]. A Pocket Guide to the Stars & Planets. ISBN 9781407579009.

External linksEdit

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