November 19, 2003
A: FROM MENTOR MOLLY WILLIAMS IN MI
Hi Crystal,
You have two questions here. First, the planets stay in orbit because the gravitational pull of the Sun keeps them from traveling in a straight line. Their path gets bent into a curve. Similarly, the Moon stays in its orbit around the Earth, because of the pull of the Earth. Most of the planets in our solar system are in orbits that are nearly circular. We suspect that they coalesced out of a cloud of spinning dust (imagine a big, spinning cookie), so their orbits just preserved the shape of the original circular mass. However, sometimes orbits can be elliptical. Comets, for example, have very "eccentric" orbits, meaning that their distance from the sun varies greatly throughout their orbital year. Some comet orbits get as close to the sun as the inner planets (Earth or Mars) and then get so far away that they are outside Pluto. Pluto has a much more eccentric orbit than the other planets. Its orbit may have been disturbed by another large object that passed by, or it might have been captured rather than forming at the same time as the other planets. So, the answer to the second question is that sometimes Pluto can be closer to the Sun than Neptune, even though on average Neptune is closer to the Sun. But Mars and Earth are in nearly circular orbits; they keep almost the same distance from the Sun at all times, so they can't switch places.
NASA has a really good website about the Solar System: http://sse.jpl.nasa.gov/planets/index.cfm