Solar System facts for kids

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Solar System
Solar System size to scale.svg
Planets and dwarf planets of the Solar System. Sizes are to scale. Distances from the Sun are not 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) 4.503 billion km (30.10 AU)
Distance to Kuiper cliff 50 AU
Populations
Stars 1
Sun
Planets 8
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
Known dwarf planets 5 (dozens more awaiting confirmation, possibly hundreds)
Ceres, Pluto, Haumea, Makemake, Eris
Known natural satellites 406 (176 of planets and 230 of minor planets)
Known minor planets 597,599
Known comets 3,175
Identified rounded satellites 19
Orbit about Galactic Center
Invariable-to-galactic plane inclination 60.19° (ecliptic)
Distance to Galactic Center 27,000±1,000 ly
Orbital speed 220 km/s
Orbital period 225–250 Myr
Star-related properties
Spectral type G2V
Frost line 2.7 AU
Distance to heliopause ~120 AU
Hill sphere radius ~1–2 ly

The Solar System is the gravitationally bound planetary system of the Sun and the objects that orbit it, either directly or indirectly. Of the objects that orbit the Sun directly, the largest are the eight planets, with the remainder being smaller objects, such as the five dwarf planets and small Solar System bodies. Of the objects that orbit the Sun indirectly - the moons - two are larger than the smallest planet, Mercury.

The Solar System formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system's mass is in the Sun, with the majority of the remaining mass contained in Jupiter. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being primarily composed of rock and metal.

The four outer planets are giant planets, being substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are ice giants, being composed mostly of substances with relatively high melting points compared with hydrogen and helium, called volatiles, such as water, ammonia and methane. All eight planets have almost circular orbits that lie within a nearly flat disc called the ecliptic.

The Solar System also contains smaller objects. According to IAU definitions, objects orbiting the Sun are classified dynamically and physically into three categories: planets, dwarf planets, and small Solar System bodies.

The Solar System is located in the Orion Arm, 26,000 light-years from the center of the Milky Way galaxy.

Evolution of the Solar System

TheNewSolarSystem-PIA02973
Planets within our Solar system montage

The formation and evolution of the Solar System began 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud.

Most of the collapsing mass collected in the centre, forming the Sun, while the rest flattened into a protoplanetary disk of loose dust, out of which the planets, moons, asteroids, and other Solar System bodies formed.

This widely accepted model, known as the nebular hypothesis, was first developed in the 18th (1700's) century by Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace. Its subsequent development has interwoven a variety of scientific disciplines including astronomy, physics, geology, and planetary science. As our knowledge of space has grown, the models have been changed to account for the new observations. Improvements in observational astronomy and the use of unmanned spacecraft have since enabled the detailed investigation of other bodies orbiting the Sun.

The Solar System has evolved considerably since its initial formation. Many moons have formed from circling discs of gas and dust around their parent planets, while other moons are believed to have formed independently and later been captured by their planets. Still others, as the Earth's Moon, may be the result of giant collisions.

The Solar System (37307579045)
The solar system

Many collisions between bodies have occurred, and have been important to the evolution of the Solar System. The positions of the planets often shifted, and planets have switched places. This planetary migration is thought to have been responsible for much of the Solar System's early evolution.

The Solar System will remain roughly as we know it today until the hydrogen in the core of the Sun has been entirely converted to helium, which will occur roughly 5 billion years from now. This will mark the end of the Sun's main-sequence life. At this time, the core of the Sun will contract with hydrogen fusion occurring along a shell surrounding the inert helium, and the energy output will be much greater than at present. The outer layers of the Sun will expand to roughly 260 times its current diameter, and the Sun will become a red giant. Because of its vastly increased surface area, the surface of the Sun will be considerably cooler (2,600 K at its coolest) than it is on the main sequence. The expanding Sun is expected to vaporize Mercury and render Earth uninhabitable.

Discovery and exploration

Cellarius Harmonia Macrocosmica - Scenographia Systematis Copernicani
Illustration of the Copernican system 1708

For thousands of years, people had no need for a name for the "Solar System". They thought the Earth stayed still at the center of everything (geocentrism). Although the Greek philosopher Aristarchus of Samos suggested that there was a special order in the sky, Nicolaus Copernicus was the first to develop a mathematical system that described what we now call the "solar system". This was called a new "system of the world".

In the 17th century, Galileo Galilei, Johannes Kepler and Isaac Newton began helping people understand physics more clearly. People began to accept the idea that the Earth is a planet and moves around the Sun, and that the planets are worlds with the same physical laws that control Earth. More recently, telescopes and space probes have led to discoveries of mountains and craters, and seasonal meteorological phenomena such as clouds, dust storms and ice caps on the other planets.

Structure and composition

Solar system a
Solar system

The principal component of the Solar System is the Sun, a G2 main-sequence star that contains 99.86% of the system's known mass and dominates it gravitation. 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 remaining objects of the Solar System including the four terrestrial planets, the dwarf planets, moons, asteroids, and comets) together comprise less than 0.002% of the Solar System's total mass.

All the planets, and most other objects, orbit the Sun in the same direction that the Sun is rotating (counter-clockwise, as viewed from above Earth's north pole). There are exceptions, such as Halley's Comet.

Lspn comet halley
Comet halley

The overall structure of the charted regions of the Solar System consists of the Sun, four relatively small inner planets surrounded by a belt of mostly rocky asteroids, and four giant planets surrounded by the Kuiper belt of mostly icy objects. Astronomers sometimes informally divide this structure into separate regions. The inner Solar System includes the four terrestrial planets and the asteroid belt. The outer Solar System is beyond the asteroids, including the four giant planets. 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.

Most of the planets in the Solar System have secondary systems of their own, being orbited by planetary objects called natural satellites, or moons, two of which, Titan and Ganymede, are larger than the planet Mercury and, in the case of the four giant planets, by planetary rings, thin bands of tiny particles that orbit them in unison.

The Sun

The Sun by the Atmospheric Imaging Assembly of NASA's Solar Dynamics Observatory - 20100819
The Sun

At the center of the Solar System is the Sun. It is a star, like the billions of other stars in the sky. The Sun is important to us because it gives us heat and energy that allows life. None of the life on Earth could exist without the Sun. The rest of the objects in the Solar System orbit (travel around) the Sun. The planets are the largest of these. The planets closest to the Sun are called the inner planets. These are Mercury, Venus, Earth, and Mars. Then comes a big ring of asteroids, chunks of rock much smaller than planets. This ring is called the asteroid belt. Within the asteroid belt, there is a dwarf planet (smaller than a normal planet) named Ceres. Then come the outer planets: Jupiter, Saturn, Uranus, and Neptune. Farther out there are two dwarf planets, Pluto and Eris.

469368main sun layers unlabeled full
Sun layers

Activity on the Sun's surface, such as solar flares and coronal mass ejections, disturbs the heliosphere, creating space weather and causing geomagnetic storms. Earth's magnetic field stops its atmosphere from being stripped away by the solar wind. Venus and Mars do not have magnetic fields, and as a result the solar wind is causing their atmospheres to gradually bleed away into space.

Coronal mass ejections and similar events blow a magnetic field and huge quantities of material from the surface of the Sun. The interaction of this magnetic field and material with Earth's magnetic field funnels charged particles into Earth's upper atmosphere, where its interactions create aurorae seen near the magnetic poles.

Inner planets

The first four planets closest to the Sun are called the inner planets. They are small and dense terrestrial planets, with solid surfaces. They are made up of mostly rock and metal with a distinct internal structure and a similar size. Three also have an atmosphere. The study of the four planets gives information about geology outside the Earth. Most asteroids are also often counted with the inner planets

Mercury

Mercury in color - Prockter07 centered
Mercury
Venus globe
Venus

Mercury (0.4 AU from the Sun) is the closest planet to the Sun and the smallest planet in the Solar System (0.055 M). Mercury has no natural satellites; besides impact craters, its only known geological features are lobed ridges or rupes that were probably produced by a period of contraction early in its history. Mercury's very tenuous atmosphere consists of atoms blasted off its surface by the solar wind. Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact; or, that it was prevented from fully forming by the young Sun's energy.

Venus

Venus (0.7 AU from the Sun) is close in size to Earth (0.815 M) and, like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere, and evidence of internal geological activity. It is much drier than Earth, and its atmosphere is ninety times as dense. Venus has no natural satellites. It is the hottest planet, with surface temperatures over 400 °C (752 °F), most likely due to the amount of greenhouse gases in the atmosphere. No definitive evidence of current geological activity has been detected on Venus, but it has no magnetic field that would prevent depletion of its substantial atmosphere, which suggests that its atmosphere is being replenished by volcanic eruptions.

Earth

The Earth seen from Apollo 17
Earth

Earth (1 AU from the Sun) is the largest and densest of the inner planets, the only one known to have current geological activity, and the only place where life is known to exist. Its liquid hydrosphere is unique among the terrestrial planets, and it is the only planet where plate tectonics has been observed. Earth's atmosphere is radically different from those of the other planets, having been altered by the presence of life to contain 21% free oxygen. It has one natural satellite, the Moon, the only large satellite of a terrestrial planet in the Solar System.

Mars

OSIRIS Mars true color
Mars

Mars (1.5 AU from the Sun) is smaller than Earth and Venus (0.107 M). It has an atmosphere of mostly carbon dioxide with a surface pressure of 6.1 millibars (roughly 0.6% of that of Earth). Its surface, peppered with vast volcanoes, such as Olympus Mons, and rift valleys, such as Valles Marineris, shows geological activity that may have persisted until as recently as 2 million years ago. Its red colour comes from iron oxide (rust) in its soil. Mars has two tiny natural satellites (Deimos and Phobos) thought to be either captured asteroids, or ejected debris from a massive impact early in Mars's history.

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 in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles, such as water, ammonia, and methane than those of the inner Solar System because the lower temperatures allow these compounds to remain solid.

Asteroid belt

Dawn spacecraft in asteroid belt
Artist's impression of the Dawn spacecraft in the main asteroid belt

Asteroids except for the largest, Ceres, are classified as small Solar System bodies and are composed mainly of rocky and metallic minerals, with some ice. They range from a few metres to hundreds of kilometres in size. Asteroids smaller than one meter are usually called meteoroids and micrometeoroids (grain-sized).

The asteroid belt occupies the orbit between Mars and Jupiter. It is thought to be remnants from the Solar System's formation that failed to form because of the gravitational interference of Jupiter. The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter. Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of Earth. The asteroid belt is very sparsely populated; spacecraft routinely pass through without incident.

Artist's impression dwarf planet Ceres
Ceres

Ceres

Ceres (2.77 AU) is the largest asteroid, a protoplanet, and a dwarf planet. Its mass is large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in 1801, and was reclassified to asteroid in the 1850s as further observations revealed additional asteroids. It was classified as a dwarf planet in 2006 when the definition of a planet was created.

Outer planets

Jupiter

Hubble Captures Vivid Auroras in Jupiter's Atmosphere
Jupiter
Saturn - March 2014 (16368126594)
Saturn

Jupiter (5.2 AU), at 318 M, is 2.5 times the mass of all the other planets put together. It is composed largely of hydrogen and helium. Jupiter's strong internal heat creates semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot. Jupiter has 79 known satellites. The four largest, Ganymede, Callisto, Io, and Europa, show similarities to the terrestrial planets, such as volcanic activity and internal heating. Ganymede, the largest satellite in the Solar System, is larger than Mercury.

Saturn

Saturn (9.5 AU), distinguished by its extensive ring system, has several similarities to Jupiter, such as its atmospheric composition and magnetosphere. Although Saturn has 60% of Jupiter's volume, it is less than a third as massive, at 95 M. Saturn is the only planet of the Solar System that is less dense than water. The rings of Saturn are made up of small ice and rock particles. Saturn has 62 confirmed satellites composed largely of ice. Two of these, Titan and Enceladus, show signs of geological activity. Titan, the second-largest moon in the Solar System, is larger than Mercury and the only satellite in the Solar System with a substantial atmosphere.

Uranus2
Uranus

Uranus

Uranus (19.2 AU), at 14 M, is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its axial tilt is over ninety degrees to the ecliptic. It has a much colder core than the other giant planets and radiates very little heat into space. Uranus has 27 known satellites, the largest ones being Titania, Oberon, Umbriel, Ariel, and Miranda.

Neptune Full
Neptune

Neptune

Neptune (30.1 AU), though slightly smaller than Uranus, is more massive (17 M) and hence more dense. It radiates more internal heat, but not as much as Jupiter or Saturn. Neptune has 14 known satellites. The largest, Triton, is geologically active, with geysers of liquid nitrogen. Triton is the only large satellite with a retrograde orbit. Neptune is accompanied in its orbit by several minor planets, termed Neptune trojans, that are in 1:1 resonance with it.

Dwarf planets

Pluto-enhanced-color-new-horizons
Pluto

Pluto had been called a planet since it was discovered in 1930, but in 2006 astronomers meeting at the International Astronomical Union decided on the definition of a planet, and Pluto did not fit. Instead they defined a new category of dwarf planet, into which Pluto did fit, along with some others. These small planets are sometimes called plutinos.

Trans-Neptunian region

2014MU69 MVIC
Ultima Thule

Beyond the orbit of Neptune lies the area of the "trans-Neptunian region", with the doughnut-shaped Kuiper belt, home of Pluto and several other dwarf planets, and an overlapping disc of scattered objects, which is tilted toward the plane of the Solar System and reaches much further out than the Kuiper belt. The entire region is still largely unexplored. It appears to consist overwhelmingly of many thousands of small worlds—the largest having a diameter only a fifth that of Earth and a mass far smaller than that of the Moon—composed mainly of rock and ice. This region is sometimes described as the "third zone of the Solar System", enclosing the inner and the outer Solar System.

(486958) 2014 MU, nicknamed Ultima Thule is a trans-Neptunian object located in the Kuiper belt. It is a contact binary 31 km (19 mi) long, composed of two joined bodies 19 km (12 mi) and 14 km (9 mi) across that are nicknamed "Ultima" and "Thule", respectively. With an orbital period of 298 years, it is classified as a classical Kuiper belt object.

Artist’s Impression of a Kuiper Belt Object
Artist’s Impression of a Kuiper belt object

With the New Horizons space probe's flyby at 05:33 on 1 January 2019 (UTC time), it became the farthest and most primitive object in the Solar System visited by a spacecraft, both bodies being planetesimal aggregates of much smaller building blocks.

It was discovered on 26 June 2014 by astronomer Marc Buie using the Hubble Space Telescope as part of a search for a Kuiper belt object for the New Horizons mission to target in its first extended mission; it was chosen over two other candidates to become the primary target of the mission. Its nickname, a Greco-Latin is a term for a place that is beyond the known world. The New Horizons team plans to submit a proper name to the International Astronomical Union when the nature of the object is better understood.

Farthest regions

Logarhitmic radial photo of the universe by pablo budassi 9MFK
Artist's scale of the observable universe with the Solar System at the center, inner and outer planets, Kuiper belt, Oort cloud, Alpha Centauri, Perseus Arm, Milky Way galaxy, Andromeda galaxy, nearby galaxies, Cosmic Web, Cosmic microwave radiation and the Big Bang on the edge

The point at which the Solar System ends and interstellar space begins is not precisely defined because its outer boundaries are shaped by two separate forces: the solar wind and the Sun's gravity. It is thought to extend up to a thousand times farther and contain the Oort cloud.

Oort cloud

The Oort cloud is a hypothetical spherical cloud of up to a trillion icy objects that is thought to be the source for all long-period comets and to surround the Solar System at roughly 50,000 AU (around 1 light-year (ly)), and possibly to as far as 100,000 AU (1.87 ly). It is thought to be composed of comets that were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be interrupted by infrequent events, such as collisions, the gravitational effects of a passing star, or the galactic tide, the tidal force exerted by the Milky Way.

Boundaries

See also: Vulcanoid, Planets beyond Neptune, and Planet Nine
Artist's conception of Sedna
Sedna

Much of the Solar System is still unknown. The Sun's gravitational field is estimated to dominate the gravitational forces of surrounding stars out to about two light years (125,000 AU). Lower estimates for the radius of the Oort cloud, by contrast, do not place it farther than 50,000 AU. Despite discoveries such as Sedna, the region between the Kuiper belt and the Oort cloud, an area tens of thousands of AU in radius, is still virtually un-mapped. There are also ongoing studies of the region between Mercury and the Sun. Objects may yet be discovered in the Solar System's uncharted regions.

Galactic context

ALMA and the centre of the Milky Way
The Milky Way

The Solar System is located in the Milky Way, a barred spiral galaxy with a diameter of about 100,000 light-years containing about 100 billion stars. The Sun resides in one of the Milky Way's outer spiral arms, known as the Orion–Cygnus Arm or Local Spur. The Sun lies between 25,000 and 28,000 light-years from the Galactic Centre, and its speed within the Milky Way is about 220 km/s, so that it completes one revolution every 225–250 million years. This revolution is known as the Solar System's galactic year.

The Solar System's location in the Milky Way is a factor in the evolutionary history of life on Earth. Its orbit is close to circular, and orbits near the Sun are at roughly the same speed as that of the spiral arms.

Milky Way Galaxy
Milky Way Galaxy

Therefore, the Sun passes through arms only rarely. Because spiral arms are home to a far larger concentration of supernovae, gravitational instabilities, and radiation that could disrupt the Solar System, this has given Earth long periods of stability for life to evolve.

The Solar System also lies well outside the star-crowded environs of the galactic centre. Near the centre, gravitational tugs from nearby stars could alter bodies in the Oort cloud and send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. The intense radiation of the galactic centre could also interfere with the development of complex life.

Exploration

Aldrin Apollo 11
Buzz Aldrin walks on the moon, July 20, 1969 - Apollo 11

Before the telescope, people explored the sky with their eyes. They saw how the planets seemed to "wander" through the sky. They learned to predict where the Sun, the moon, and planets would be in the sky. They built some observatories - places for watching the sky. They observed the Sun and stars to tell the time of year.

Telescopes were first made in the early 17th century and have vastly improved since. Astronomers saw that planets are not like stars. They are worlds, like the Earth. They could see that some planets have moons. They began to think about what these worlds were like. At first, some thought that the other planets and moons had life. Eventual progress sent spacecraft into space, and found that there life on the Moon or on Mars.

NASA Mars Rover
NASA Mars Rover

Twelve Astronauts walked on the Moon about 35 years ago. They brought rocks and dirt back to Earth. Spacecraft flew by Venus, Mars, and the outer planets. The pictures they took showed us a lot of what we know about these worlds. Robots landed on Mars in 1971, 1976, and 1997. They took thousands of pictures of the planets. Two robots, "Spirit" and "Opportunity" (on February 13, 2019, NASA officials declared that the Opportunity mission was complete, after the spacecraft failed to respond to repeated signals sent since August 2018). They send photos and movies back to Earth. They also check rocks to find out what the rocks are made of.

So far, we have not found any life except on Earth. Maybe tiny one-celled life once lived on Mars. Maybe there is life under the ice on Jupiter's moon Europa. New spacecraft are being planned to look for life on these worlds.

Visual summary

Solar System
Sun in February (black version).jpg
Jupiter and its shrunken Great Red Spot (cropped).jpg
Saturn closeup.jpg
Uranus2 (cropped)-1.jpg
Neptune Full (cropped).jpg
Africa and Europe from a Million Miles Away (cropped).png Venus-real color.jpg
Sun
(star)
Jupiter
(planet)
Saturn
(planet)
Uranus
(planet)
Neptune
(planet)
Earth
(planet)
Venus
(planet)
Mars 23 aug 2003 hubble (cropped).jpg
Ganymede g1 true-edit1.jpg
Two Halves of Titan.png
Mercury in color - Prockter07-edit1.jpg
Callisto (cropped)-1.jpg
Io highest resolution true color (non-edit version).jpg
FullMoon2010 (cropped)-1.jpg
Mars
(planet)
Ganymede
(moon of Jupiter)
Titan
(moon of Saturn)
Mercury
(planet)
Callisto
(moon of Jupiter)
Io
(moon of Jupiter)
Moon
(moon of Earth)
Europa-moon.jpg
Triton Voyager 2.jpg
Nh-pluto-in-true-color 2x JPEG-edit.jpg
Titania (moon) color cropped.jpg
PIA07763 Rhea full globe5.jpg
Voyager 2 picture of Oberon.jpg
Iapetus as seen by the Cassini probe - 20071008 (cropped).jpg
Europa
(moon of Jupiter)
Triton
(moon of Neptune)
Pluto
(Kuiper belt object)
Titania
(moon of Uranus)
Rhea
(moon of Saturn)
Oberon
(moon of Uranus)
Iapetus
(moon of Saturn)
Charon in Color (HQ).jpg
PIA00040 Umbrielx2.47.jpg
Color Image of Ariel as seen from Voyager 2.jpg
Dione color south.jpg
PIA18317-SaturnMoon-Tethys-Cassini-20150411.jpg
PIA19562-Ceres-DwarfPlanet-Dawn-RC3-image19-20150506.jpg
Vesta full mosaic.jpg
Charon
(moon of Pluto)
Umbriel
(moon of Uranus)
Ariel
(moon of Uranus)
Dione
(moon of Saturn)
Tethys
(moon of Saturn)
Ceres
(belt asteroid)
Vesta
(belt asteroid)
PIA17202-SaturnMoon-Enceladus-ApproachingFlyby-20151028-cropped.jpg
Miranda.jpg
Proteus Voyager 2 cropped.jpg
Mimas PIA12568.jpg
Hyperion in natural colours.jpg
Phoebe cassini.jpg
PIA12714 Janus crop.jpg
Enceladus
(moon of Saturn)
Miranda
(moon of Uranus)
Proteus
(moon of Neptune)
Mimas
(moon of Saturn)
Hyperion
(moon of Saturn)
Phoebe
(moon of Saturn)
Janus
(moon of Saturn)
PIA09813 Epimetheus S. polar region.jpg
Rosetta triumphs at asteroid Lutetia.jpg
Prometheus 12-26-09a.jpg
PIA21055 - Pandora Up Close (cropped).jpg
(253) mathilde crop.jpg
Leading hemisphere of Helene - 20110618.jpg
243 Ida large.jpg
Epimetheus
(moon of Saturn)
Lutetia
(belt asteroid)
Prometheus
(moon of Saturn)
Pandora
(moon of Saturn)
Mathilde
(belt asteroid)
Helene
(moon of Saturn)
Ida
(belt asteroid)
Phobos colour 2008.jpg
Deimos-MRO.jpg
First color image of Ultima Thule (LORRI crop, sharpened).jpg
Phobos
(moon of Mars)
Deimos
(moon of Mars)
2014 MU69
(Kuiper Belt object)

Images for kids


Solar System Facts for Kids. Kiddle Encyclopedia.