About Planetary Systems 

What is so special about the number nine? All 2nd graders learn that there are nine planets in our solar system. They then spend the rest of their life thinking nine must be the cosmically ordained number of members in the Sun’s family. Such thinking might have made sense in the years before we knew of other planetary systems but it does not make sense anymore. In the mid 1990s astronomers discovered the first "extra-solar" planet orbiting the star 51 Pegasi. The discovery was made using super-sensitive equipment that could track the wobble induced in the star by the planet’s gravitational attraction. In the years that followed the list of extra-solar planets has grown and currently is greater than 70. This momentous discovery answered more than 2000 years of questioning about the existence of other worlds. 

The discovery of new planets beyond our Sun would be news enough but Nature always manages to surprise us by going one step further. The new planetary systems look nothing like our own. Our solar system has lower mass planets orbiting in the inner regions and then a bunch of massive planets orbiting further out. The first extra-solar planets discovered were so-called "Hot Jupiters", massive bodies orbiting very close to their star. In our solar system, all the planets move on fairly circular orbits. In many of the extra-solar planetary systems we have discovered the planetary orbits are much more elliptical (cigar-shaped). Given the solar systems in the census so far, it looks like our planetary family may be very unusual. How did these other planetary systems end up looking the way they do? Did the "Hot Jupiters" form close to the parent star or do they "migrate" inward from more distant beginnings? Did the extra-solar planets begin with elliptical orbits or did they evolve away from circular ones? 

Of course, the way we are doing our searches (the star-wobble technique) means we are more likely to find big planets in close to the parent star (why is this so?). We may find later that this is an important bias. One thing is certain however, planetary systems come in lots of flavors and we do not know very much yet about how this diversity forms or evolves.