Maps

Here follow various properties of the planet Tau in the story. The real system of Tau Ceti is unknown, except that it does indeed have a dust disk at large radii. I have set up the fictional system to be below the detection thresholds of 2009 instruments. Obviously, any planets Tau Ceti actually has will be very different from the below (and Geoff Marcy of Berkeley has already noted a tentative Doppler signature of a hot Neptune, which would eliminate any habitable planets in the system). I am not concerned by this. All that matters is that the system and planet I have constructed is plausible.

Tau

Tau Ceti has six major planets near the star and a debris disc in the outer portion of the system. The largest planet, Skana, (with a period of ~50 years) is ~50 Earth masses, sufficient to block many incoming cometary bodies but nowhere near as efficient as Jupiter is in the Sol system. The second largest is somewhat smaller than Neptune and has a mass of 10 Me. The four inner planets are terrestrial. Three are quite small.

The second planet is now called Tau by the human inhabitants. Some physical data:

Density: 5.3 g/cm^3
Radius: ~0.82 Re
Gravity: 0.8 g
Mass: 0.52 Me
Teq: 245 K (top of atmosphere)
Surface Pressure: 1.1 bar, 19% oxygen
Rotation Period: 26.5 h
Year: 385 d, 349.8 local days.

There is a small solid core and a ~0.2 gauss global dynamo field. The planet has no large natural satellites, and the resulting oscillations in the planetary obliquity and hence the global climate have dominated the evolution of local lifeforms. Currently, the planet is five hundred years into a four-millennium warm period. There is a high impact rate of ~1 m particles from the debris disc, which causes impressive meteor activity but no major cratering (usually).

Tectonic activity on the planet is normal by Earth standards, with several major continental landmasses. Even in the warm periods, however, much of the planet’s surface is covered by ice: the permanent ice caps are at about 60º latitude. During winter, the northern ice cap moves as far south as 35º, aided by mountains. The southern cap advances over ocean, to about 50º.

Like any planet with a significant biosphere, Tau II has a dramatic variety of microbial lifeforms. However, the large plant and animal forms are the most important for the history of human colonization on the planet. The plant forms consist of tree analogues, ground cover, and various small plants. There are two major classes of large animals: the zards and the pedes. The zards are six-limbed, tailless, with pleated, scaly skin. They are most different from Earth’s vertebrates in that they have no spine as such, but rather two parallel lines of somewhat longer bones. The pedes are invertebrates, with multiply-segmented bodies. They can be very large: one phylum has evolved lungs.