During what is supposed to be their last adventure in ‘Mummy on the Orient Express the Doctor mentions various alien worlds. Clara isn’t very interested but they could serve as inspiration for your own adventures.
Magellan black hole
The Orient Express offers passengers a majestic view of the Magellan black hole. The dark centre is surrounded by a swirling cloud, which could be gas or the remains of planets. The Doctor remembers that region of space as having planets as far as the eye could see and calls the black hole a beast.
The Doctor could just making a joke but a solar system with a high number of planets that were eventually all destroyed by a black hole would make a good setting for several adventures, exploring each of the worlds and charting their history as they approach oblivion.
The black hole could have occurred naturally or it could have been artificially created. It might have been created as a weapon by one of the planets, dooming everyone else or it could have been used by an outside force. It could be that the planets created a strong alliance that had to be ended.
If we take the the Doctor’s comments literally then the black hole could be a creature that only resembles a black hole. Once it had consumed all that remains of the planets might it move on to other star systems?
All the planets the Doctor mentions could be the worlds that were destroyed. Certainly he describes them in the past tense and Maisie confirms that Thedian Four was destroyed thousands of years ago (although that would suggest that it orbited a star called Thedian).
Just as likely is that the Doctor has just gone on a tangent, just talking about random planets to either silence Clara or to entice her into continue travelling with him.
A planet of perpetual darkness. The lack of light could be due to a lack of bright star in the planet’s solar system or it could be that the sky is constantly covered by cloud. Obsidian is produced by lava flows so it could be that volcanic ash blotted out the sun.
It could be that the planet itself has a unique property to absorb light. This would mean that no matter what illumination is brought to its surface there would still be constant darkness.
If such a world was capable of sustaining life there several races might see the appeal of using Obsidian to make their home. The Vashta Nerada would find it an ideal environment to thrive in. It could be that Obsidian was covered in forests, the air thick with the Vashta Nerada swarm.
A lack of light would mean an inability to see, which would suit the Weeping Angels. If others were unable to see them then there would be no risk of being quantum locked. Even if other races brought artificial light sources the Angels have the ability to neutralise them.
The Silence might eventually decide that they don’t want to be forgotten any more. Only those who see the Silence forget them, so on a world where no one can see anything they might be remembered.
A world of perpetual darkness would be a good location for covert or military installations. It could be that the world not only absorbs light but other signals that could give its presence away.
It might also serve as a haven for those who are blind or who have disfigurements. Obsidian would allow them to exist in an environment where everyone is on equal footing, unable to see.
In the Doctor Who universe there could be diseases that make it dangerous to view the infected subject. This might be a disease that turns victims into gorgons, changing anyone who gazes upon them into stone. Obsidian might be the best place to hide such afflicted people away.
Shrubs are similar to trees, distinguished by their multiple stems and short height, so a world of shrubs is no stranger than the forest moon of Endor. Some shrubs are capable of growing into trees so the Doctor could just be amused that none of these plants do.
The constant acid rain didn’t stop the Doctor from having a nice picnic here, although he did have to wear a gas mask. According to Maisie it was destroyed thousands of years ago and the Doctor doesn’t dispute this.
Acid rain is produced by emissions of sulfur dioxide and nitrogen oxide, combing with water particles in the atmosphere. This can occur naturally through a combination of lightening strikes and volcanic eruptions. If this is the case on Thedian Four constant acid rain would such a planet with constant electrical storms and volcanic activity.
Pollution is much more commonly the cause of acid rain. This could mean that Thedian Four’s environment was ravaged by its native population or colonists. The environmental damage caused could be the basis for an adventure concentrating on the consequences of pollution.
Acid rain can poison and kill plant and animal life. It can wipe out fish and insect life which could further damage the planets eco-system. It can damage the composition of soil, making it more difficult for plants to grow, especially food crops.
PCs might have to save a race from extinction or help those who are starving. When their very environment is killing them what do the PCs do? They might attempt terraforming or move who they can to other worlds.
Acid rain can erode metal and stone structure, which means that Thedian Four could be a good place to oppose robots, cyborgs and beings made of stone (such as the Weeping Angels).