The Stars beyond our Solar System

Although it’s too early in our observational experience to give a percentage of the stars that have planets, that condition seems to be the rule rather than the exception. Most stars are probably also encircled by billons of small bodies like our asteroid and Kuiper belts that never coalesced into planets as the system evolved. And further out still is each star’s own Oort cloud, a sphere of trillions of frozen objects left over from that system’s primordial past. By this point we’ve come a few light years from the star and have reached the edge of it’s gravitational and magnetic influence.

The distance light travels in a year—a "light year," about six trillion miles—is the yardstick astronomers use for measuring large distances. Because other stars and galaxies are so far away, it takes time, and lots of it, for their "pictures" to get here. So as we look out into the universe, we are, by necessity, looking back in time. If, for example, the Sun exploded, we wouldn’t know it for 8.3 minutes, because that’s how long it takes light to get to us from the Sun. Pluto is five light hours away, and the next nearest visible star, Alpha Centauri, is 4.4 light years distant.

Everything else we see in the night sky that’s not in our solar system is older still. The nearest galaxy to us, the Andromeda galaxy, is just under three million light years away. That’s how old its picture is when we see a photograph of it. In fact, astronomers are now looking out so far into the universe (billions of light years) that they hope to see the universe begin, about 13.7 billion light years away and just that many years ago!