Our solar system consists of an average star we call theSun, the planetsMercury, Venus,Earth, Mars,Jupiter, Saturn,Uranus, Neptune, andPluto.
The Sun is our closest star. It is a member of the Milky Way galaxy. The Sun is a yellow dwarf star, which means it is a medium size star. It is believed to be over 4 billion years old. The Sun spins slowly on its axis as it revolves around the galaxy.
The center, or core, of the Sun is very hot. A process called "nuclear fusion" takes place there. Nuclear fusion produces a lot of energy. Some of this energy travels out into space as heat and light. Some of it arrives at Earth! Streams of gas particles known as the solar wind also flow out from the Sun.
On the Sun's surface, we can see storms. We call these storms "sunspots" because they look like dark spots on the Sun's surface. The Sun also produces big explosions of energy called solar flares. These flares shoot fast moving particles off the Sun's surface. These particles can hit the Earth's atmosphere and cause a glow called an aurora.
Mercury is the smallest planet in our solar system. Mercury is about the same size as our Moon. It is very close to the Sun. Mercury travels around the Sun faster than any other planet. That is how it got its name. It was named after Mercury, the swift messenger of the gods in ancient Roman mythology. Mercury can only be seen from Earth just before sunrise or just after sunset, but not in the middle of the night. That is because Mercury always appears near the Sun when viewed from Earth. Mercury has a very thin atmosphere. Humans would not be able to live there. The surface of Mercury has holes in it where objects such as meteorites and asteroids crashed into it.
Venus and Earth are almost the same size. Venus is the closest planet to Earth, but it does not have oceans or human life like Earth. Venus gets so hot during the day that it could melt a lead cannonball. The temperature rises to 484 degrees Celsius on the side facing the Sun. Venus has very thick, rapidly spinning clouds which cover its surface. These clouds hold heat in. That is why Venus gets so hot. These clouds also reflect sunlight. That is why Venus appears so bright to us here on Earth. There are constant thunderstorms in these clouds. Venus has several large inactive volcanoes. Much of the surface is covered by old lava flows from these volcanoes.
Venus is unusual because it rotates in a direction opposite that of all of the other planets. Venus spins very slowly as it orbits the Sun.
Earth is the third closest planet to the Sun. It has an atmosphere made up of many different gases, but mainly it is nitrogen and oxygen. The atmosphere gives us air to breathe. We live on the planet Earth.
The Earth orbits around the Sun. It takes one year to go around the Sun one complete time. The Earth also rotates, or spins, on its axis. It takes one day to spin around one complete time. The Earth's axis is not straight up and down, but tilted a little bit. This tilt is responsible for us having seasons. Otherwise, the temperature would be pretty much the same all year long.
The temperature on Mars can be very, very cold. On its warmest day, Mars can still be a very cold place. At the top and bottom of the planet are poles just like on Earth. During the Martian winter, ice caps can be seen at the poles.
Space probes have landed on Mars. These probes were sent on a fact-finding mission by the United States. They performed experiments on the Martian dirt and atmosphere. The dirt was found to contain clay which was rich in iron. The iron is what gives Mars its red color.
Mars has many craters which were formed by meteorites or asteroids hitting it. Mars also has some of the tallest volcanoes and some of the deepest valleys in our solar system. Mars has two moons, Phobos and Deimos which have unusual shapes. Scientists think these potato-shaped moons were once asteroids captured by Mars' gravitational pull .
Jupiter is a large gas planet whose clouds change colors daily. This planet is made mostly of hydrogen and helium gases. Jupiter gives off two times more heat than it gets from the Sun. It shines very brightly in the night sky for nine months of the year when it is closest to Earth. Huge areas of swirling gases can be found in Jupiter's atmosphere. The largest swirling area of gas is called the Great Red Spot. Scientists believe this is a large hurricane-like storm which has lasted for hundreds of years. Large bolts of lightning have also been seen in Jupiter's atmosphere. Pictures taken by space probes have shown thin rings around Jupiter. As of January 2011, Jupiter has 50 named moons. 13 more have been discovered but not given official status or names. One of Jupiter's moons, Io, has active volcanoes on it. Areas on Io that are near the volcanoes are very, very hot.
Saturn is a very large gas planet which spins very rapidly on its axis. It spins so fast that it flattens out the top and the bottom of the planet. The fast spin also causes Saturn to bulge at its equator. Saturn's atmosphere has winds which can blow at over 1800 kilometers per hour! The white spots on Saturn are believed to be powerful storms. Saturn is surrounded by over 1000 rings made of ice and dust. Some of the rings are very thin and some are very thick. The size of the particles in the rings range from pebble-size to house-size. Scientists believe that the particles came from the destruction of moons circling the planet. As comets and meteorites smashed the moons, Saturn's gravitational pull shaped the particles into rings. Saturn has at least 53 moons. Some of these moons orbit the planet within the rings, creating gaps in the rings.
Uranus tilts over so far on its axis that it rotates on its side. Because of this, its poles are sometimes pointed almost directly at the Sun. Uranus' atmosphere is made up of hydrogen, helium, and methane. The temperature in the upper atmosphere is very cold. The cold methane gas is what gives Uranus its blue-green color. The rapid rotation of Uranus causes winds up to 600 kilometers per hour to blow in its atmosphere. Uranus has eleven known rings which contain dark, boulder-sized particles. Uranus has 27 named moons. Some of these moons are less than 100 kilometers wide and black as coal.
Neptune and Uranus are very much alike. They are both large gas planets that look like big blue-green balls in the sky. Neptune has winds in its atmosphere which blow at over 2000 kilometers per hour! This planet has large, dark circles on its surface which astronomers believe to be storms. Neptune has two thick and two thin rings which surround it. Neptune also has thirteen known moons. Four of these moons orbit the planet within the rings. One of Neptune's moons, Triton, orbits the planet in a direction opposite to Neptune's other moons. Neptune is the farthest planet from the Sun.
For many years, Pluto was thought of as the farthest known planet from the Sun. It has a very unusual orbit. Once every 248 Earth years, Pluto swings inside the orbit of Neptune. It stays there for twenty years. During those twenty years, Pluto is closer to the Sun than Neptune. While it is closer to the Sun, Pluto has an atmosphere. The methane and nitrogen frozen at the poles thaw out, rise, and temporarily form an atmosphere. As it moves toward its farthest point from the Sun, Pluto's atmosphere freezes and falls back to the ground. Since the year 2000, astronomers realized that Pluto was not like the other eight planets but very much like a new group of objects found in the outer solar system. In 2006, astronomers re-classified Pluto to be a dwarf planet.
Pluto has three moons. Pluto's largest moon, Charon, is half the size of Pluto. In 2005, astronomers observed two more moons of Pluto. The moons were named Nix and Hydra.