Where does the Solar System end?

Have you ever looked up at a dark night sky full of twinkling lights? Our Sun, our Earth and the other planets all belong to one big family in space. That family is called the Solar System.

Your house has walls, and a playground has a fence. Out in space, there are no walls or fences anywhere. So where does the Solar System end?

Let's start in the middle, with the Sun. The Sun pulls on every planet, and that pull keeps them circling it instead of drifting away. Neptune, the farthest planet, circles a long, long way out. Yet the Solar System keeps going past Neptune.

The Sun is so hot that its outside keeps puffing tiny bits out into space. Scientists call this the solar wind, which means a stream of tiny, invisible bits rushing away from the Sun. The solar wind races out in every direction, whoosh, faster than any rocket.

Rushing past all the planets, the solar wind blows a giant bubble around the Sun. Picture a soap bubble, only this one is big enough to hold every planet inside. Earth, Neptune and all the others float along within it.

Space between the stars holds a very thin gas. Far beyond Neptune, the solar wind pushes into that gas, slows down and stops. That stopping place is the edge of the Sun's bubble. Is that the very end of the Solar System, or could the Sun reach even farther?

The Sun's pull reaches much farther than its solar wind. Far past the bubble, a huge cloud of icy comets circles the Sun. The Sun's pull keeps those comets from wandering off toward other stars.

Way out there, other stars begin to tug on those comets too. The Solar System ends where another star's pull grows stronger than our Sun's. Nobody has seen that faraway cloud yet, so scientists are still not sure exactly where that end lies.

Solar wind. The Sun's solar wind blows a bubble around the planets, but the Sun's pull reaches farther, so the Solar System ends where other stars pull harder than our Sun.

Sweet dreams, little explorer, snug and safe inside the Sun's big family tonight.