Mercury created around 4.5 billion years ago when gravity drew whirling gas and dust together to form this tiny planet closest to the Sun. Mercury, like the other terrestrial planets, has a central core, a rocky mantle, and a solid crust. The core has a density similar to that of iron, the mantle is rich in iron and silicon, while the crust is very thin (only about 500 miles thick).
This image shows a near-Earth asteroid, which is any asteroid that comes within 0.05 AU of the Earth's orbit (55 million km or 34 million miles). As this image was taken in 2001, it shows the asteroid belt as most people know it from space movies - between 5 and 20 million objects larger than 100 meters (330 feet) across. But the actual size of the asteroid belt is unknown because many small objects are likely to have been missed by astronomers until now. It is estimated that there are at least 10 million such asteroids smaller than 100 meters across alone out there in the solar system.
In May 2009, astronomers using data from NASA's Wide-field Infrared Survey Explorer discovered an object that covers more than 1250 square miles and is five times wider than Pluto. This object is not a star nor a galaxy but instead appears to be a huge empty space dominated by dark matter.
Mercury has been recognized from ancient times and has been watched by people of many different civilizations for thousands of years. Around 3,000 BC, the Sumerians left one of the first accounts of Mercury. They called it gilgul - a natural red liquid - and they used it in rituals and medicine.
The Greeks adopted the name for their version, which was blackish-green. The Romans called it mercurius and used it in medicines and in ritual practices too. It was also used as an element in alchemy - the art of transforming lead into gold. Alchemy is more than 500 years old industry that developed around this mysterious substance.
Early scientists believed that mercury had elemental properties similar to those of iron or copper. It was not until 1580 that Anton van den Hoecke discovered its distinctive smell when it burned in a tube furnace. He named it mercuric oxide then later corrected himself and called it oxygen instead!
Its ability to burn cleanly in air made it useful for lighting lamps and fireplaces. It wasn't until 1848 that Thomas Edison improved on this technology by using electricity instead.
Today, mercury is used in a wide variety of products that we use every day.
When the solar system stabilized into its current configuration around 4.5 billion years ago, gravity drew spinning gas and dust in to create the third planet from the Sun. Earth, like the other terrestrial planets, has a central core, a rocky mantle, and a solid crust. It orbits the sun once every 24 hours and 37 minutes.
The Earth was created in a giant explosion called a supernova, which gave us the elements needed for life. This idea comes from scientists such as Hugh Everett III who proposed it as an explanation for some chemical elements that cannot be explained by existing science.
Earth's magnetic field is generated by its rotating iron core. Without this axis of rotation, the field would disappear completely.
Our planet is unique in many ways. It is the only one capable of supporting life as we know it, so it can be considered "living proof of life beyond Earth". It also shows how special our planet is because everything on it needs to work together to support life. For example, plants use the carbon dioxide we release into the atmosphere to grow their food and give off oxygen as a by-product. If they didn't have any oxygen, we would be living in an entirely different world.
Mercury is a rocky planet that is sometimes referred to as a terrestrial planet. Mercury, like the Earth's moon, has a solid, cratered surface. However, because it orbits so close to the sun, only about 38 million years old, the surface of Mercury is not as old as that of the Earth and no longer preserves evidence of its formation environment.
In addition to being covered in craters, Mercury has two large regions where rocks have been uplifted into great peaks, called tharsis. One of these, named after the Roman god of commerce, is found in the planet's western hemisphere. The other, named after the Egyptian god of war, is found in the eastern hemisphere. These uplifts are thought to be the results of giant volcanic eruptions many millions of years ago.
Like the Earth, Mercury goes through major changes as it orbits around the sun. The most dramatic example is when planets reach their points of maximum distance from the sun, which creates conditions suitable for life as we know it. At this time, they are also at their hottest. Mercury reaches its point of minimum distance from the sun in early June and returns closer than it has ever been before (but still more than 4 million miles away) in late September.