Our solar system

Our solar system consists of an average star we call the Sun, the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. It includes: the satellites of the planets; numerous comets, asteroids, and meteoroids; and the interplanetary medium. The Sun is the richest source of electromagnetic energy (mostly in the form of heat and light) in the solar system.

This picture shows the Sun and all nine planets of the solar system as seen by the space explorers. Starting at the top-left corner is the Sun followed by the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

The Sun is the most prominent feature in our solar system. It is the largest object and contains approximately 98% of the total solar system mass. One hundred and nine Earths would be required to fit across the Sun's disk, and its interior could hold over 1.3 million Earths.

http://www.kidsastronomy.com/our_sun.htm

The Sun is the closest star to Earth and is the center of our solar system. A giant, spinning ball of very hot gas, the Sun is fueled by nuclear fusion reactions. The light from the Sun heats our world and makes life possible. The Sun is also an active star that displays sunspots, solar flares, erupting prominences, and coronal mass ejections. These phenomena impact our near-Earth space environment and determine our "space weather."

Mercury, the planet nearest the Sun, is the second smallest planet in the solar system. Only slightly larger than the Earth's moon, Mercury's surface is covered with craters. This tiny planet does not have any rings or moons.

Earth, our home planet, is a beautiful blue and white ball when seen from space. The third planet from the Sun, it is the largest of the inner planets. Earth is the only planet known to support life and to have liquid water at the surface.

Venus, a "Sister" planet to the Earth that is very different from our home. Venus does not have any moons or rings.

Mars, Earth's outer neighbor, is the fourth planet from the Sun. Mars' bright appearance and reddish color stand out in the night sky. Impressive surface features such as enormous volcanoes and valleys are frequently obscured by huge dust storms.

Jupiter is the largest planet in the solar system. When approached from afar, its fantastic striped atmosphere gradually reveals intriguing clouds that move around the planet. Rich in historical and cultural connections, Jupiter is the site of recent comet impacts and continuing scientific discovery.

Saturn, the sixth planet from the Sun, has the most spectacular set of rings in the solar system. We now know that Saturn has 59 moons in addition to its complex ring system.

Uranus is the seventh planet from the Sun and the third-largest and fourth-most massive planet in the solar system. Uranus is similar in composition to Neptune, and both have different compositions from those of the larger gas giants Jupiter and Saturn. As such, astronomers sometimes place them in a separate category, the "ice giants".
Like the other giant planets, Uranus has a ring system, a magnetosphere, and numerous moons.

Neptune, which is usually the eighth planet from the Sun, is a very cold place. Occasionally, the ninth planet Pluto crosses Neptune's orbit and becomes the "eighth planet". Its bluish color comes from its atmosphere of methane gas. The planet has 13 moons and a very narrow, faint ring system.

In 2006 the International Astronomical Union (IAU) approved a new classification scheme for planets and smaller objects in our Solar System. Their scheme includes three classes of objects: "small solar system bodies" (including most asteroids and comets), the much larger planets (including Earth, Jupiter, and so on), and the new category of in-between sized "dwarf planets".

There are currently three official dwarf planets. Pluto, formerly the smallest of the nine "traditional" planets, was demoted to dwarf planet status. Ceres, the largest asteroid in the main asteroid belt between Mars and Jupiter, was also declared a dwarf planet. The third and final (for now!) dwarf planet is Eris, an icy body on the edge of our Solar System that was discovered recently in 2005. Eris was temporarily labeled 2003 UB313 when it was first discovered, and given the nickname "Xena", before astronomers settled on the official name of Eris.

Comets are lumps of ice and dust that periodically come into the center of the solar system from somewhere in its outer reaches, and that some comets make repeated trips. When comets get close enough to the Sun, heat makes them start to evaporate. Jets of gas and dust form long tails that we can see from Earth. These tails can sometimes be millions of miles long.

Asteroids are small bodies that are believed to be left over from the beginning of the solar system 4.6 billion years ago. They are rocky objects with round or irregular shapes up to several hundred km across, but most are much smaller.

Meteors are streaks of light, usually lasting just a few seconds, which people occasionally see in the night sky. They are sometimes called "shooting stars" or "falling stars", though they are not stars at all. Meteors are caused by the entry of small pieces of rock, dust, or metal from space into the atmosphere at extremely high speeds. These particles, called "meteoroids" when they are floating around in space (think of very small asteroids), are traveling at incredible speeds of tens of kilometers per second (tens of thousands of miles per hour) when they streak into the atmosphere. The incredible pressure meteoroids experience when they collide with Earth's atmosphere shatters them, transferring energy to atoms and molecules in the atmosphere, which then release the energy by glowing. This glow produces the bright trails of light in the sky we see as meteors.

Most meteoroid particles are quite small, ranging in size from a grain of sand to a pea-sized pebble. Almost all of them disintegrate in the atmosphere long before reaching the ground. Very rarely, a larger meteoroid actually survives to strike the ground, creating a meteor crater in a huge explosion. This explosion often vaporizes whatever solid material is left of the meteoroid after its fiery flight through the atmosphere. Sometimes, however, pieces of the meteoroid survive and are found in the crater or nearby. These chunks of rock or metal are called meteorites.

In common usage, a constellation is a group of stars that are connected together to form a figure or picture..

A galaxy is a massive, gravitationally bound system consisting of stars, an interstellar medium of gas and dust, and dark matter. The name is from the Greek root galaxias , meaning "milky," a reference to the Milky Way galaxy. Typical galaxies range from dwarfs with as few as ten million stars up to giants with one trillion stars, all orbiting a common center of mass. Galaxies can also contain many multiple star systems, star clusters, and various interstellar clouds. The Sun is one of the stars in the Milky Way galaxy; the Solar System includes the Earth and all the other objects that orbit the Sun.