You’ve probably heard of comets--they’re those balls of ice that circle around the Sun with a glowing tail streaming behind them. But do you know where they come from? For many centuries, even astronomers didn’t know. Now, they know many comets come from a place called the Oort Cloud. While they still know little about this place, but they have answered a few questions:

Q: What is the Oort Cloud?
A: The Oort Cloud is a circular region that surrounds our solar system. Scientists think it has very dense center, or core. Most of the cloud’s comets are in the core, and the rest are in the outside edges of the cloud. The comets found here are called long-period comets, meaning they take a long time to orbit the Sun. There are other kinds of comets, called short-period comets, and many of them come from a place called the Kuiper Belt.

Q: How far away is the Oort Cloud?
A: The Oort Cloud is nearly 30 trillion kilometers (about 18.75 trillion miles) away from the Sun.

Q: How do we know about the Oort Cloud?
A: A Dutch astronomer named Jan Oort predicted the existence of the Oort Cloud. He noticed that comets seemed to come from many different directions. This, he believed, proved that comets came from something that surrounded our Solar System. He used the orbits of 19 comets to determine where they probably came from, and additional information gathered by scientists has confirmed Oort’s theory.

Q: How many comets are in the Oort Cloud?
A: Scientists think there are about 6 trillion comets in the Oort Cloud. The total mass of these comets is believed to be about 40 times the mass of Earth.

Q: How did the Oort Cloud form?
A: Scientists aren’t sure. They don’t think the comets found there were formed there, though. They believe the comets were formed at many different times, and in many different places, but were thrown into the cloud by the huge gravitational field of planets like Jupiter, Saturn, Uranus, and Neptune.