M Fnord Wrote:Zet Tuc IV was named Thule by the discoverers for the very impressive extent of its polar caps. While not completely frozen over like New Alaska or similar iceworlds discovered around red and brown dwarf stars, Thule is covered with ice all the way down to the tropical latitudes. Based on core samples taken during the first and second Zeta Tucanae expeditions, the Thulian ice age has been underway for the last six million years. Radar and ice-penetrating lidar surveys indicated that the planet has two supercontinents situated over the poles with Thule's ocean Okeanos lying between them. The fact that Okeanos has uninterrupted currents running through the equator is likely key to the planet's habitability--the heat transfer keeps the ice at bay and allows the tropical coastlines a reasonably temperate climate.
Back in the 1960s, Russian climatologist Mikhail Budyko devised a model of Earth's climate that predicted that, among other things, should the Earth's climate cool enough that the ice sheets reach the tropics, the growth of the sheets would enter an unstoppable runaway effect and cover the Earth down to the Equator. There's also evidence that Earth may have indeed completely frozen over at one point or another (called "Snowball/Slushball Earth"; the terms "snowball" and "slushball" differentiating interpretations suggesting either a completely frozen "snowball" Earth or a "slushball" Earth with areas of still-liquid water at the surface).
From the looks of it, it sounds as though Thule may be one decently-sized climatological upset/disaster away from becoming a snowball world itself.