[AgentZero]

Mayday, I quite like your ideas on planetary conditions. The idea of a habitable band within the solar system where the planets most suited to your race would be found would make a great addition to SE5. The only thing is that we would need more than just a little green star over a planet to let us know it's habitable if things as complex as atmospheric composition were taken into account.
I can think of two ways of adding this level of detail without making it too complex for new players.
1) A slight rework of the Conditions Category. Assuming a planet is roughly habitable, the Conditions will be compared against your homeworld to determine it's habitability. So a planet with a bit more sulpher in the atmosphere than Earth's might be Unpleasent, and so forth. If a planet doesn't match your race's requirements (ie: Methane atmosphere for a Oxygen race), the conditions will read Uninhabitable, though that wouldn't stop you from putting down a dome colony.
2) Underneath 'Planet Size' add 'Available Landmass Size' (ALS). This expressed as a percentage of the planet's 'normal' available space. So you would have 50%, 75%, 100%, 125% and 150%. For simplicities sake we could use Small, Medium, Average, Large and Huge, with the percentage in brackets. ie:

'Planet Size: Large'
'Available Landmass Size:' Small

This would describle, for a terrestrial race a large planet made mostly of water, for example. This would of course require that a player pick whether they will be a Terrestrial or Aquatic race when creating their empire. One also have technologies that would eventually eliminate the penalties and increase the bonuses from ALS.

Just for an example of the Large/Huge ALS, imagine a planet the size of Earth (considered to be Medium size), but covered mostly by land. That planet would have a lot more space for terrestrial races to build stuff, even though it was the same size as Earth.