The torrent of discoveries that has come out of the exo-planet research community lately has brought up more new questions than it has answered. Some of the more interesting ones have to deal with solar system formation. Up until the discovery of the first exo- planet in 1988, scientists had a sample size of one system to use in the development their models of how planets form around a star. Today they have a sample size of 726.

One hundred and fifty of those samples were used in a study on planetary system formation published by a team led by Lars Buchhave at the University of Copenhagen's Niels Bohr Institute.

The team was attempting to understand the distribution of different types of planets around different types of stars. It had previously been found that Jupiter- like gas giants form around metal-rich "heavy" stars but there was no such understanding of the types of stars that rocky, terrestrial planets form around.

The reason for the bias of the gas giants, the team explains, has to do with a time limit on their formation imposed by the solar wind. In the core-accretion model of solar system formation, a newly born star is surrounded by a disk of gas and dust that is gravitationally pulled together to form planets.

However, the stellar wind of the star is also trying to push the gas and dust out of the system and will eventually sweep almost all the remaining dust away.

To counteract that push, the solar system needs large, metallic planetary cores whose gravitational pull is stronger than that of the solar wind's push.

Stars with high metal concentrations are more likely to be able to form those types of cores early in their lives, when more dust and gas is present in the system. Those metal cores are then able to collect the gas and dust remaining in the system, and thus a gas giant is born.

If the star does not have enough metal content, the rocky cores don't form early enough and the gas is gone with the wind forever. The team also pointed out that eventually even low metallicity stars will form metal cores, which answers the question of what kind of stars the terrestrial planets prefer.

The good news for extra-solar Earth seekers is that terrestrial planets don't seem to have any bias towards the "heavier" stars and can be found orbiting stars with all different levels of metal content. This means there are even more stars that might potentially harbor rocky Earth-sized planets than are likely to harbor Jupiter-sized gas giants.

Additionally, terrestrial extra-solar planets might have formed earlier in the progression of the universe than their gas giant cousins. The metal concentrations in stars needed to form gas giants were not present at the beginning of the universe and took multiple iterations of stellar formation and death to accumulate.

Since terrestrial planets don't have the same metal content limitations, they were freer to form much earlier in the history of the universe without multiple solar life cycles.

Much work has to be done to prove these theories. With current technology it is much easier to find gas giants orbiting stars than the normally smaller terrestrial worlds, so there are still many questions about whether rocky planets do in fact significantly outnumber their larger gas counterparts.

With more research and better technology, scientists might be able to answer one of the questions extra-solar planetary research has opened up.