By Dr. Marc Rayman
Asked by one of our friends at the Discovery Museum Science & Space Center in Sacramento, CA.
I like this question because it may be based on someone's observation of nature. We all like looking at the sky and thinking about space, but this question is based on really noticing something about the way nature works. And that's how science often makes progress. The first step is observing the way things are, then we wonder why they are that way, and then we can try to find explanations.
In this case, people had seen for a long time that planets appear only along one narrow strip in the sky, and as our friend noticed, that's nowhere near the North Star. As scientists began to understand the beautiful architecture of the solar system, they understood why the planets occupy only a very small part of the sky. Let's think more about this.
The solar system is centered on the Sun, the vast bright star that conducts the solar system orchestra. There are 8 planets in orbit around it. You probably know their names: Mercury, Venus, Earth, Mars, Jupiter (that's my favorite!), Saturn, Uranus, and Neptune. The last 2 are so far away from Earth that the faint light from them shows up only with the help of binoculars or telescopes, so normally we see just 5 planets: Mercury, Venus, Mars, Jupiter, and Saturn. So now the question is why do these 5 planets appear only in a limited part of the sky.
Earth and these other planets orbit the Sun in one flat plane or disk -- like a CD. To help visualize this, make a simple drawing of the solar system with the Sun at the center and 6 circles surrounding it.
With your simple drawing, pretend the smallest circle is Mercury's orbit, as the planet lives out its life baking under the intense heat of the Sun. The largest circle then would be Saturn's orbit, far away in the deep cold of space. Comfortably in the middle is the third circle, representing the orbit of Earth. Now think about being there on Earth, when it is anywhere on that circle, as it makes its one-year loop around the Sun. No matter where the other planets are in their orbits, the planets would always appear to be somewhere in a ring around you, but never above the ring and never below it. Astronomers call the flat surface in which the planets spend their entire lives the ecliptic. You only see planets along the ecliptic because they are never anywhere else! When you go outside, the ecliptic is an imaginary band across the sky, and planets are always somewhere on that band. If a planet had a tilted orbit that took it out of the ecliptic, then you could see it in a different part of the sky.
Unlike the planets, stars can appear anywhere. So we don't see planets near the North Star or any of the other stars that are above or below the flat ecliptic.
Of course, sometimes our views of the planets are blocked by the light of the Sun, which makes the daytime sky a bright blue. Visit your local planetarium or science museum or ask for help at your library to find out when and where to look so you can spot the planets and begin to see where in the sky the ecliptic lies. In fact, the planets aren't the only objects that move along the ecliptic. The Moon and Sun do as well, so if you imagine the paths they take across the sky, that will give you an idea of where to look for planets.
From the observation that planets show up only in a narrow zone of the sky, scientists began to understand something significant about the arrangement of the solar system. Your observations of nature may lead to other new scientific insights, because understanding begins with questioning.