Where in the universe are we?

Scientists estimate that the edge of the universe may be 10 billion light-years away. (A light-year is the distance light travels in a year - 9.4 trillion km). But as vast as it is, the universe consists of just two simple things: matter, anything that contains atoms, and the energy that makes matter move. Some 10 percent of our universe's matter is clustered into 100 billion galaxies, very large groups of stars - some with orbiting planets. Scientists say the other 90 percent of our universe's matter is dark matter, invisible mass that scientists have not yet been able to detect.

A lot closer to home - and better understood - is our Milky Way galaxy. All the stars you see in the night sky, plus more - about 200 billion in all-belong to the Milky Way. Clouds of dust and gas make up the rest of the galaxy. Like all galaxies, our Milky Way is always in motion, rotating its spiral arms.

Zooming in, you'll find our star, the Sun, and the heavenly bodies that orbit around it: the nine planets (Mercury, Venus, Earth, Mars, and Jupiter [orbits shown above], and Saturn, Uranus, Neptune and Pluto); their 63 moons; and countless asteroids, meteoroids, and comets. Scientists say this solar system formed from clouds of gas and dust that surrounded the Sun after it formed 4.6 billion years ago. NASA has sent 20 robotic spacecraft to explore every planet in the solar system, except Pluto. As for as we know, Earth is the only planet capable of sustaining life.

Now we've zeroed in on the most familiar planet in the universe. What makes Earth such a home sweet home for life? First, Earth is the perfect distance from the Sun - 149.6 million km - so it's not too hot or too cold. Earth also has an atmosphere, a blanket of gases including nitrogen, oxygen, and carbon dioxide. This atmosphere gives us the oxygen we need to live, and helps shield us from the Sun's radiation. And Earth is the only planet that has liquid water, vital for life.

As we get closer to Earth we can see that water makes up much of our planet's surface - 70 percent. Billions of years ago, all Earth's landmasses were joined as one. Over time, this landmass broke apart to form seven large continents: North America and South America (shown here), Africa, Asia, Europe, Australia, and Antarctica. The continents are still moving today, up to 10 cm every year. This continuous shifting of landmasses, called continental drift, is caused by the movements of molten rock below Earth's crust. The same forces cause volcanic eruptions, earthquakes, and slower changes, such as the formation of mountains, lakes, and valleys.

Here we are in New York City, one of the largest cities in North America. Scientists keep a watchful eye on our planet's big cities and other areas with artificial satellites - spacecraft launched into Earth"s orbit. Satellites use different remote sensors to photograph and monitor weather, air and sea traffic, pollution, and more. This shot was taken by a satellite called Landsat 4.

When you get right down to ground level, you'll find the people who gather information about the universe and bring it to you here in Science World. We are (clockwise from bottom left): Sue Kass, Danny Cohen, Lynda Jones, Chana Freiman, Peter Fairley, and Karen McNulty.