What We Have Found
What we have found has been quite a surprise for astronomers. The planets that have been found so far did not fit the theories of solar system formation held up to that point.
Solar System Formation
Theories of how other solar systems formed have been based on how we believe our own solar system formed. The most basic idea in this theory is that the material the planets are made of depended on how far from the sun the planets formed. At distances closer to the sun, temperatures were too high for anything but rocky material to begin clumping together to form planets. Rocky material is relatively scarce as far as material for planet building goes, so the inner planets are typically fairly small. It is not until more remote distances from the sun are reached that it becomes possible for gasses such as hydrogen to begin collapsing into a planet, but since there is so much hydrogen gas, planets whose major composition consisted of hydrogen could grow to enormous sizes.
What we expected when searching for extra-solar planets was to find solar systems that fit this theory of solar system formation, but what we found was quite a different story.
The very first planets that were found turned out to be very large planets, (similar in size to Jupiter), that orbited very close the their parent star. The very first planet found orbiting a main sequence star, (51 Pegasi), had an orbital period of only 4.2 days, placing it very close to its parent star. As a comparison, the closest planet to the sun in our solar system, Mercury, has an orbital period of 88 days. A planet orbiting so close to its parent star would have a very high surface temperature, (on the order of 1000 K). Because of the size of these planets, and their hot surface temperatures, these planets were dubbed, "hot jupiters".
The discovery of hot jupiters raised serious questions about our theories of solar system formation. Either our theories were wrong, or there was something else going on that had been overlooked. Careful consideration of these questions has led most astronomers to believe that some process must be taking place that causes giant gaseous planets like Jupiter that form far from their parent star, to somehow wander into an orbit closer to their parent star than the one where they were formed.
The most recent theories of solar system formation continue to assume that planets form much as described above, but now it is believed that planets don't necessarily remain in the same orbits in which they formed. In fact, our own solar system may be the exception rather than the rule. It appears more likely that planets form in nearly circular orbits such as in our solar system, with size and composition distributions similar to that of our solar system, but if too many massive jupiter size planets form, or if they form too close together, the gavitational tugs between the planets will wreak havoc on the initial orbits. Some planets will get pulled in towards the sun, other planets will get flung out into space, and some planets may have resonances with each other that reinforce the stability of their orbits, or cause them to be quite unstable. All of these things add up to a very dynamic solar system in which the arrangement of a solar system when it first forms probably will not last, possibly making our own solar system more unique than we once thought; however, it is still expected that we will find many solar systems similar to our own, so the search goes on.Next --> Summary