Tebríth is the fourth major celestial object orbiting Lumenoferos, and the second once-habitable planet. In spite of, or perhaps due to being colder and harsher than Etéra, it gave rise to the great Tebríthan civilisation in its equatorial regions. However, said civilisation ultimately became the makers of a catastrophe that made the planet uninhabitable.
mean distance 1.1 au perihelion 1.1 au aphelion 1.1 au axial tilt 4° annual period 447 Etéran days, 1012 local days. local day 10.6 hours
mass 1.81 (1.081e25 kg) diameter 1.3 (1.6e7 m) density 0.8 (4.4 g/cm³) surface gravity 1.1 (10 m/s²) escape velocity 13 km/s
pre- / post-catastrophe vulcanism moderate / extreme tectonics moderate / extreme climate very cold / frozen
temperature pre- / post-catastrophe average -23°C (250 K) / -39°C (234 K) periphelion same as above aphelion same as above illumination 0.59 / 0.60
pre- / post-catastrophe hydrographics 20% (water) / 0% (all lakes frozen) tidal range 0.52m atmosphere composition N2, O2 pressure 53 kPa / 5 kPa
While Tebríth is a large planet with a lot of landmass, relatively little of it could be inhabited due to a combination of extreme cold away from the equator, and a spotty hydrographic coverage. Unlike Etéra, Tebríth's dynamic weather patterns were caused primarily by geological activity, not by seasonal changes in illumination. In general, only the tropical region has enough lakes and wetlands to support comfortable life, and the inhabitable area was slowly shrinking on a geological scale. That being said, what life did exist was quite robust, efficient, and well-adapted to the harsh but stable environment.
At the height of its development, Tebríth boasted a population in the hundreds of millions. Most of the population was concentrated around equatorial lakes, while temperate and polar regions have only been sparsely settled after industrialisation, as they offered better mineral resources.
Needs for relatively large-scale transportation between the habitable tropical regions and the mining-heavy temperate regions has always been a pressure towards development and improvement of better flying craft and maintaining relatively high numbers thereof. While never able to quite match the Khæn, and mostly focused on 'working horse' craft, they always had some tricks up their sleeves in the field of aerospace engineering.
While details are lost, one of the guilds was involved in a prospective breakthrough. Specifically, research into FTL transpontation methods, something along the lines of teleportation, gates or hyperspace. It wasn't a very well-known project until the year JR 60 / GR 1520 / KR 50,309. That was when portals began spontaneously appearing, originally near one of the research centres, but eventually all over the planet. Anything going into the portals was never seen again. The sheer number and size of them meant that the already-modest atmosphere was being lost at a rapid pace, while also generating storms through pressure differentials. Furthermore, portals moving relative to the planet would destroy buildings, leave trenches and tunnels in the ground, and even uncork sleeping volcanoes. There have been reports of other, less easily identifiable destructive phenomena at the time, and it's unclear how much those are caused by skewed perceptions during panic.
The event was meant with doubt, then realisation and panic, then attempts at evacuation. Despite the unpredictable dangers on the ground, the first phase of the retreat - picking up people near the most devastated settlements and taking them either to safer ones or off-world, went better than expected. It's the second phase that was the bottleneck: for all the town-to-town and town-to-orbit throughput, Tebríthans did not have enough accommodations to keep many people off-world or in interplanetary transit. In time, Etérans - first the Khæn, then the continentals, joined in with interplanetary rescue missions, but of course that was still insufficient.
There have been cases of desperate pilots doing late pickups of people who managed to survive for slightly more than a month since the start of the catastrophe, but by then these were singular miracles, not things to rely on, and often such late attempts ended in losses of craft and pilots.
By then, the bottom line was more-or-less drawn: by different accounts, there have been between half a million and one million survivors, which is less than 1% of the original population.
In a couple of months, the remaining atmosphere stabilised at approximately 5 kPa. Remote scans have shown some other anomalous readings, but the many of the remaining Tebríthans insisted on staying away from the planet. The most fanatical ones formed the Graveyard Shift - a fleet made of people who vowed to forever patrol the space surrounding the now-dead planet, and ensure than nobody would desecrate it nor attempt to repeat what lead to the catastrophe in the first place. This put an end to attempts to further research the remaining anomalies on or around Tebríth, or to salvage what technological objects could've survived.
L3 (counterpoint): Counter-Tebríth Hub
In opposition to Tebríth, on the other side of the sun, there's a constellation of space habitats. They were originally built by Tebríthan refugees who wanted to just stay away from the place of the Catastrophe, but didn't want to go planetside. Initially there was some participation from the Khæn in terms of both helping construct the habitats, and actually inhabiting the place. Nowadays, all the Khæn are gone, but many memebers of other races came in.
Currently, the cluster serves as an alternative trade hub for exchanges between the inner and outer system (for times when a direct delivery to the endpoint isn't appropriate).
L4 (front): Hermit Asteroids
The leading Trojan point of Tebríth holds a handful of asteroids. They're of relatively little value for mining, but much like Etéra's L4, are home to groups that are on the uncongenial side.
This point includes the Radiance asteroid, whose Asylum habitat has been destroyed by the Graveyard Shift in the beginning of GR1,550 / JR90 / KR 50,339.