History of Ru'un
Long before humans existed on Ru'un or Earth, the two planets constituted a single planet, known simply as Ru. The Ru'Harem existed on Ru for millennia before predicting a terrible collision that could potentially destroy their beloved homeworld. The resulting collision, caused by a huge asteroid known as Lua, did not destroy Ru but instead broke it into two pieces. The Ru'Harem watched from the darkness of space as their home planet was cracked open by the huge asteroid. Knowing full well that the remains of Ru would be too dangerous to revisit, they left the system, leaving nothing behind. The two pieces of Ru were split apart, tumbling together through space. As their surfaces bubbled with molten rock, the remnants of the great planet began to reform. They were remoulded by gravity into two new planets, one slightly larger than the other. Some parts of Lua, the rock that had caused the collision, had been pulled in by the gravity of the reformation, and remoulded into what would become moons.
Return of the Ru-Harem
After travelling the galaxy, the Ru'Harem revisited their home after millions of years. By this time the Ru'Harem had ascended beyond the capabilities of any other sentient life in the universe, possessing incredible spiritual powers and foresight. After seeing the parts of their homeworld orbiting the sun in tandem, they immediately got to work preserving the planets for life to evolve.
Construction began immediately on Ru'un. As it was the larger of the two planets and had kept most of the spiritual energy from the original planet. The Ru'Harem constructed a vast city on the site of the impact, building for miles across and deep into the planet's crust. After completion the city was locked down and ready for the Ru'Harem's long awaited return, but it never happened.
Humans, the cosmic successor of the Ru'Harem, had begun to evolve on Ru'un and Earth. Plans to colonise Ru'un were cancelled and construction team fleets left the system once again. This time, a council of elite Ru'Harem specialists were left to monitor the progress of human life on both planets. It was during this time that the four cities of Ru'un were constructed. Some time after this the transfer of Earth humans to Ru'un, such as Arthur, began.
Similarities to Earth
Despite having two seperate masses (Ru'un being the larger of the two at 1.4 times the size) both planets have an equal density and thus equal gravitational pull. This allows them to orbit on virtually the same trajectory, at almost the exact same speed. Mathematically the two planets must eventually collide, but the balance is so miraculous that the planets will continue without a collision for hundreds of thousands of years.
The atmosphere and landscape is very similar to that of Earth, however the increased size of Ru'un means much of its topography is more exaggerated than landscapes of Earth. Mountain ranges, rivers, forests and volcanoes are usually much larger and vary more in shape. Weather systems are also different in a similar fashion.
The range of lifeforms which evolved to live on Ru'un and Earth were incredibly diverse. Between the two planets the basic biology is the same, allowing humans to naturally exist on both worlds. Most animals native to Ru'un have similarities to Earth animals, the main differences being size and lifespan.