THE SOLAR SYSTEM

A solar system is a star and all of the objects that travel around it—planets, moons, asteroids, comets and meteoroids. Most stars host their own planets, so there are likely tens of billions of other solar systems in the Milky Way galaxy alone. Solar systems can also have more than one star. These are called binary star systems if there are two stars, or multi-star systems if there are three or more stars.

The solar system consists of the Sun; the eight official planets, at least three "dwarf planets", more than 130 satellites of the planets, a large number of small bodies (the comets and asteroids), and the interplanetary medium. (There are probably also many more planetary satellites that have not yet been discovered.)

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. The Sun's nearest known stellar neighbor is a red dwarf star called Proxima Centauri, at a distance of 4.3 light years away. The whole solar system, together with the local stars visible on a clear night, orbits the center of our home galaxy, a spiral disk of 200 billion stars we call the Milky Way . The Milky Way has two small galaxies orbiting it nearby, which are visible from the southern hemisphere. They are called the Large Magellanic Cloud and the Small Magellanic Cloud. The nearest large galaxy is the Andromeda Galaxy . It is a spiral galaxy like the Milky Way but is 4 times as massive and is 2 million light years away. Our galaxy, one of billions of galaxies known, is traveling through intergalactic space.

Composition Of The Solar System
The Sun contains 99.85% of all the matter in the Solar System. The planets, which condensed out of the same disk of material that formed the Sun, contain only 0.135% of the mass of the solar system. Jupiter contains more than twice the matter of all the other planets combined. Satellites of the planets, comets, asteroids, meteoroids, and the interplanetary medium constitute the remaining 0.015%. The following table is a list of the mass distribution within our Solar System.
Sun: 99.85%
Planets: 0.135%
Comets: 0.01% ?
Satellites: 0.00005%
Minor Planets : 0.0000002% ?
Meteoroids: 0.0000001% ?
Interplanetary Medium: 0.0000001% ?

Formation
Many scientists think our solar system formed from a giant, rotating cloud of gas and dust known as the solar nebula . As the nebula collapsed because of its gravity, it spun faster and flattened into a disk. Most of the material was pulled toward the center to form the sun. Other particles within the disk collided and stuck together to form asteroid-sized objects named as planetesimals, some of which combined to become the asteroids, comets, moons and planets.

The solar wind from the sun was so powerful that it swept away most of the lighter elements, such as hydrogen and helium, from the innermost planets, leaving behind mostly small, rocky worlds. The solar wind was much weaker in the outer regions, however, resulting in gas giants made up mostly of hydrogen and helium.

THE SUN
The sun
The sun is by far the largest object in our solar system, containing 99.8 percent of the solar system's mass. It sheds most of the heat and light that makes life possible on Earth and possibly elsewhere. Planets orbit the sun in oval-shaped paths called ellipses, with the sun slightly off-center of each ellipse. NASA has a fleet of spacecraft observing the sun to learn more about its composition, and to make better predictions about solar activity and its effect on Earth.

Mercury

Mercury (0.4 AU from the Sun) is the closest planet to the Sun and the smallest planet in the Solar System (0.055 Earth masses). 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 accreting by the young Sun's energy.

Venus

Venus (0.7 AU from the Sun) is close in size to Earth (0.815 Earth masses) 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
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

Mars (1.5 AU from the Sun) is smaller than Earth and Venus (0.107 Earth masses). 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 captured asteroids .

Jupiter is the largest planet in the solar system. Fittingly, it was named after the king of the gods in Roman mythology. In a similar manner, the ancient Greeks named the planet after Zeus, the king of the Greek pantheon.
Jupiter helped revolutionize the way we saw the universe and ourselves in 1610, when Galileo discovered Jupiter's four large moons — Io, Europa, Ganymede and Callisto, now known as the Galilean moons. This was the first time that celestial bodies were seen circling an object other than Earth, major support of the Copernican view that Earth was not the center of the universe.

Saturn
Saturn is the sixth planet from the sun and the second largest planet in the solar system. Saturn was the Roman name for Cronus, the lord of the Titans in Greek mythology. Saturn is the root of the English word "Saturday."
Saturn is the farthest planet from Earth visible to the naked human eye, but it is through a telescope that the planet's most outstanding features can be seen: Saturn's rings. Although the other gas giants in the solar system — Jupiter, Uranus and Neptune — also have rings, those of Saturn are without a doubt the most extraordinary.

Uranus
Uranus is the seventh planet from the sun and the first to be discovered by scientists. Although Uranus is visible to the naked eye, it was long mistaken as a star because of the planet's dimness and slow orbit. The planet is also notable for its dramatic tilt, which causes its axis to point nearly directly at the sun.

Neptune is the eighth planet from the sun. It was the first planet to get its existence predicted by
mathematical calculations before it was actually seen through a telescope on Sept. 23, 1846. Irregularities in the orbit of Uranus led French astronomer Alexis Bouvard to suggest that the gravitational pull from another celestial body might be responsible. German astronomer Johann Galle then relied on subsequent calculations to help spot Neptune via telescope . Previously, astronomer Galileo Galilei sketched the planet, but he mistook it for a star due to its slow motion. In accordance with all the other planets seen in the sky, this new world was given a name from Greek and Roman mythology — Neptune, the Roman god of the sea.
Only one mission has flown by Neptune – Voyager 2 in 1989 – meaning that astronomers have done most studies using ground-based telescopes. Today, there are still many mysteries about the cool, blue planet, such as why its winds are so speedy and why its magnetic field is offset.

Pluto, once considered the ninth and most distant planet from the sun, is now the largest known dwarf planet in the solar system. It is also one of the largest known members of the Kuiper Belt , a shadowy zone beyond the orbit of Neptune thought to be populated by hundreds of thousands of rocky, icy bodies each larger than 62 miles (100 kilometers) across, along with 1 trillion or more comets.

Is there life elsewhere in the solar system? If not, why is Earth special? (Updated June 2014)
We do not yet know of life elsewhere. One of the things that makes Earth special of particular interest to the exoplanet search is our location with respect to our Sun -- the habitable or so-called “goldilocks zone”. The “goldilocks zone” the area around a star where water would be a liquid on the surface of a planet. The location and extent of which would depend on a number of criteria such as the parent star size and temperature. Once planets in these habitable zones are found the size of the planet is taken into account. The size is what may enable a suitable atmosphere for our familiar life forms. The Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo maintains the Habitable Exoplanet Catalog