Real planets come in all sizes, geographies, climates, geologies, and probably ecologies. They circle a wide variety of stars. But they do all follow certain physical laws, and these laws constrain how the various aspects of a planet work together. Orbital mechanics and the physics of how stars work do provide some limits. In inventing a planet, you need first to consider what you require of that planet. Is it to be a carefully chosen object for colonization by human beings? The site of an accidental colonization? An uninhabitable world with something worth exploiting? The home of a non-human and totally alien species? A base for scientific exploration?

For Homecoming I invented three planets. Two are intended to be fully livable, Earthlike planets, the results of long-ago modification. Both circle sun-like stars and have climates, year lengths, day lengths and axial inclinations similar to Earth’s. The ecology of Central is assumed to be a mixture of species imported from Earth and the home planet of the R’il’nai, and differences from Earth ecology are unimportant for this story, though they do exist. That of Riya is a combination of a native ecology with that from R’il’n. In the native Riyan ecology, land animals with internal skeletons have six, rather than four, limbs. This allows for four-legged animals with wings, like the little lizard-like animals that act as pollinators for the native vegetation. Photosynthesizers also differ from Earth’s, with a growth structure based on expanding sheets (with holes) rather than branches. Both planets have components—like the tinerals on Riya—brought in from planets other than the primary sources.

On the third planet, Mirror, I let my imagination go. This is not a comfortable planet for human beings, nor is it intended to be. It is, however, modeled on the early stages of evolution on Earth, with an interesting twist—both left-handed and right-handed biochemistry co-exist. Is this possible? I don’t know, and neither do my characters. But every scientist I know would agree that this is a planet to be preserved for study, not colonized. Mirror circles a star much like our sun at a distance that allows water to be liquid, so as real planets go it’s pretty tame. Its atmospheric pressure is high—like that of early Earth—with a significant fraction of the pressure coming from carbon dioxide, which contributes to its hot climate. There is nothing surprising about a planet having lots of water and carbon dioxide. Hydrogen, carbon, oxygen and nitrogen are the four most common reactive elements in the universe, so we expect their compounds to be common. Water is just hydrogen and oxygen, while carbon dioxide is carbon and oxygen. What is unusual is to have free oxygen, as on Earth, and pressure and temperature that allow water to be liquid.

On Mirror the process of combining carbon dioxide and water to produce carbohydrates and free oxygen, using sunlight as an energy source, is just getting started. There is oxygen in the air, but not enough to support much in the way of land dwellers. My castaways are quite reasonable in wanting to get away as soon as possible—they’re not properly equipped for the scientific study they recognize the planet needs. Besides, they want to get home! I’ll have more to say on planet building later. For now, welcome to my blog and I hope you surf on over to my website and find out what Homecoming is all about (