Positions with the government pay a little better, and jobs at private institutions are the most lucrative. Undergraduate degrees or concentrations in astronomy, geology, chemistry or physics are helpful. Planetary geologists get to be pioneers, who thrill at being among the first to explore new worlds. Each time there's a spacecraft mission, there is an opportunity to make new discoveries. It's about as glamorous work as geologists can do.
Most careers are highly dependent on whether the U.S. government continues to fund space missions.
If NASA's budget is cut, almost everybody suffers. A few speculators in private industry have talked about mining Mars and other planets, but it's uncertain whether such ventures will ever get off the ground. If excitement builds for a new series of space probes, like the planned mission to Pluto, or if private contractors enter the field of planetary exploration, the field could expand rapidly. Otherwise, there will be stiff competition for the jobs, which number about 700 currently. The discipline is popular at universities which offer it, so there's a good chance teaching positions could increase.