Planets
Our solar system consists of a star -the sun- with nine planets under its gravitational influence. Each planet is spinning on its own axis as it orbits the Sun. The planets are different from each other and are in different orbits although they all move around the Sun in the same direction. The four inner planets (Mercury, Venus, Earth, Mars) are known as terrestrial planets. Of the remaining five planets (Jupiter, Saturn, Uranus, Neptune, Pluto) that spin in wider orbits further away from the Sun, four are known as gas giants because they are huge, dense balls of hydrogen and other gasses. Pluto, the furthest away from the sun is tiny and cold and perhaps is not truly a planet.
Mercury
Mercury is the closest planet to the sun. It completes an orbit of the Sun every 88 Earth days. It has virtually no atmosphere except traces of argon, helium, and neon, it has no water, and is heavily cratered. Mercury has a magnetic field and is so dense for its small size that scientists think that it is mostly iron. It is seen as either a "Morning Star" or an "Evening Star." Pluto is the only planet smaller than Mercury.
Venus
Venus is the second planet from the Sun. It completes an orbit of the Sun every 224.7 Earth days. The surface has mountains and plains. Its atmosphere is mostly carbon dioxide but clouds contain sulphuric acid and water. Venus is the planet that passes closest to Earth as they both orbit the Sun. The planet surface is shrouded by clouds which move around the planet very fast producing constant strong winds. Like Mercury, Venus is seen either as a "Morning Star" or an "Evening Star."
Earth
Earth is the third planet from the Sun. It completes an orbit of the Sun every 365.26 Earth days. It contains oceans, continents, and polar caps. It�s atmosphere is mostly nitrogen, but there is also a large amount of oxygen. There are also clouds of water vapor. 75 percent of the surface is water, and clouds cover 50 percent of the Earth at one time. All natural elements have been found in the ocean.
Mars
Mars is the fourth planet from the Sun. It completes an orbit of the Sun every 687 Earth days (1.9years). The surface is dry and cratered, and has inactive volcanoes and canyons. The atmosphere is thin, with mostly carbon dioxide with some nitrogen. Mars is the outermost of the four terrestrial planets. It has a reddish color ( and is sometimes called the Red Planet) because of the iron oxide (rust) in its soil. Olympus Mons is an inactive volcano as tall as Mount Everest. There are huge canyons one of which is four times deeper than Earth�s Grand Canyon. The planet is very cold and has dust storms whipped by hurricane force winds. Mars has two satellites, (moons) Deimos and Phobos.
Jupiter
Jupiter is the fifth planet from the Sun. It completes an orbit of the sun every 4,332.6 Earth days (11.89 years). The atmosphere is thick and is mostly hydrogen, with some helium and traces of methane, water, and ammonia. Jupiter is the largest planet in our solar system. As Jupiter spins its thick clouds tend to form bands and give the planet its red-brown-white banded appearance. Three Earths can fit in Jupiter�s Great Red Spot, which was first observed about 300 years ago and appears to be a permanent hurricane. This planet has 16 moons. Ganymede is the largest moon in our solar system and it is larger than Pluto and Mercury.
Saturn
Saturn is the sixth planet from the Sun. It completes an orbit of the Sun every 10,759.2 Earth days (29.5 years). The atmosphere is mostly hydrogen with some helium. Saturn is the second largest planet. Tremendously strong winds blow constantly at the equator. Saturn has at least 15 moons, and most of them are small and composed of rock and ice. Titan, the largest, appears to be the only moon in our solar system to have an atmosphere. Saturn also has many colorful rings made of ice, rocks, and dust.
Uranus
Uranus is the seventh planet from the Sun. It completes an orbit of the Sun every 30,685.4 Earth days (84 years). The surface probably liquid or slushy hydrogen "crust." The atmosphere is hydrogen, helium, and methane. Uranus, barely visible in Earth�s night sky, was not discovered until 1781. It has a faintly greenish color due perhaps to the methane in its atmosphere. It has faint rings and 5 moons.
Neptune
Neptune is the eighth planet from the Sun. It completes an orbit of the Sun every 60,189 Earth days (168.8 years). The atmosphere is composed of hydrogen and helium. Neptune was discovered in 1846. It is very cold, but probably has a hot core. It has at least eight moons. Two were first observed from Earth, and six others were observed by Voyager 2.
Pluto
Pluto is the ninth planet from the Sun the majority of the time, but it has an irregular orbit, so it is the eighth planet part of its orbit. It completes an orbit of the sun every 90,465 Earth days (2477.7 years). It is believed to be a ball of frozen of gasses. Frozen methane has been detected. Pluto was discovered in 1930 and is about half the size of Earth�s moon. It has one known moon, Charon, which was discovered in 1978. Pluto has an irregular orbit, so some scientists think that although we call Pluto a planet, it was perhaps once a moon of Neptune and was pulled out of its orbit by the pull from some other celestial body.