The Great Collapse
"Billions of years ago, our solar system was once very close to being a dual-star system. If Jupiter was only 5% more massive, it would have spontaneously began to fuse like the old sun once did." A young professor began to inscribe a picture of the old system on the blackboard while her class listened. The room was illuminated by an LED lamp, but otherwise had no furnishings outside of the desks for the students. There were no windows, and there was only a single door at the back of the room to an unlit hallway.
"At the current rate, Jupiter should gain the necessary mass in the next 100 years; however, we have a greater problem. Since the Great Collapse, none of the planets had any anchoring point and began moving tangential to the direction of their orbit when the Sun disappeared. We were lucky enough to be sent off with Jupiter, but so were several other planets, many of which will hit us before Jupiter's fusion. Simply put, we are very likely to hit Mercury in 10 years. If we cannot find a way to stop it, that will be the end of the Earth and humanity."
While talking, the professor drew the trajectories of several planets from the old solar model, emphasizing key impacts that would happen in the near future, and some minor impacts that had already happened.
"We have obviously lost a majority of the population after Earth lost its atmosphere and began to live underground, channeling energy from the core. Even with our limited resources, it is entirely possible to change either our own motion or Mercury's in a few ways. The most promising method seems to be to sacrifice our moon to send mercury off-course. This is, of course, only possible due to the Earth's weakened atmosphere allowing us to launch satellites and spaceships at a fraction of their cost before the collapse."
She continued the lecture for another 30 minutes before dismissing the students with a reminder of their test on Tuesday.
It had been 300 years since the Great Collapse. Though students are still taught the history of the world, it might as well have been fiction to the generations of people who have been forced to live underground. There were no countries. No divisions. It felt like every day brought another cataclysmic event. Humanity had to work together to survive.
No matter the case, humanity had survived. Only years before the Great Collapse, researchers had developed synthetic trees, capable of transforming carbon dioxide into oxygen. These trees kept humans alive for the first decade until the underground forests began to take root. As most of the water existed in the form of ice on the planet's surface, the trees were grown on the ceiling of the new cave system, extending towards the lights from the city below, which were kept warm through molten lava. Eventually, farming became common-place, allowing for sustainable, vegetarian living.
Most technology was maintained from the old world: computers, phones, internet, etc. Several new technologies were also developed to allow humans to wander the surface for brief periods of time. Though the Sun no longer existed, time still continued day by day, year by year.
Even in the Earth's darkest hour, humanity maintained their most defining trait: hope. Everyone knew they would live their entire lives in the caves, but they worked for a brighter future. One where their children could once again enjoy life on the surface.