Is our solar system unique?

It's looking more and more like our solar system is fairly unique in its composition. While the detection of extrasolar planets is still very much in its infancy, and the results thus subject to selectional effects, these early discoveries are indicating that other solar systems don't look like ours.

Take the latest discovery, for example. This new class of planets are about 10 to 20 times the size of Earth (about the size of Neptune) and far smaller than any of the 140+ extrasolar planets so far detected. They're probably too small to hold a gas giant type atmosphere. And they're also very close to their sun, whipping around them in a matter of days. Gliese 436 spins around its M dwarf sun every 2.5 days. 55 Cancri spins around its sun in just under 3 days. Three larger planets also revolve around the star every 15, 44 and 4,520 days, respectively.

One of the things that astronomers are discovering -- and again this might be a selectional effect due to our primitive equipment -- is that most solar systems have large objects, whether they be gas giants or giant rock planets, that are quite close to the sun. In fact, many of the solar systems thus far discovered have gas giants in the inner solar system.

This is not good as far as the advent of life is concerned. It's been argued that the Earth resides in a very sensitive habitable zone in our solar system, and that the gas giants in the outer solar system act as crucial vacuum cleaners which ensure that little debris makes its way to Earth (ie preventing too many catastrophic impact events that would stunt life or prevent it from happening altogether).

Much of this speculation adds fuel to the Rare Earth hypothesis, which is one particular explanation for the Fermi Paradox. Who knows, perhaps we reside in a very atypical solar system -- and not just because there's life in it.