Big Questions for New Horizons
Many of the other solar systems have massive Jupiter like planets close to their Sun, closer even than Mercury. Many scientists now believe that these gas giants could not have formed there. Rather, they must have began out where our Jupiter is, and moved inwards, scattering the smaller planets with their powerful gravity as they went. Why is it that our Jupiter and Saturn did not migrate inward? We are trying to learn more about our outer solar system by sending probes there. We sent Galileo to Jupiter, Cassini is at Saturn right now, and New Horizons is on its way to Pluto even as you read this.
For the first time in human history we know of planets around other stars and many of those other planetary systems look quite different from our own. Many have a planet like Jupiter, or even bigger, nearest to the Sun. If we are to understand why this is the case, and how likely it is that there are Earth-like planets elsewhere, we need to better understand how planets form.