Bobby, a human space traveler from Theta, and has been in space for a couple of centuries. He's been kept alive either through his own biology or some feat of technology. Having no contact with his people for most of those years and losing his bearings in space, he crash lands on an alien planet. He concludes that the planet's similar to his own, but distinct enough that he concludes it must be alien. Little does he know, this is planet Theta, altered by the aftermath of some apocalypse caused by his people.
After a few days of exploration, Bobby is able to conclude some of the following:
- The flora and fauna are different form Theta's but not by much
- The atmospheric makeup is largely similar to Theta's
- There was some ruined civilization here
- A few pieces of technology he picked up is unrecognizable to him
- Ultimately, there are sufficient reason to believe that this is not theta
Bonus points if
- There are inhabitants here might be similar to humans in biological makeup, but not exactly human (as in Sapiens vs Neanderthals), but they don't have to be if that makes sense
- The brief view of the planet that Bobby got before he crash landed was dissimilar to his knowledge of the geography of theta
and keeping in mind that:
- Bobby has yet to make contact with inhabitants
- Bobby has yet to come across any recognizable landmark that might give the identity of the planet away (A la Planet of the Apes)
- Theta's technology is space-faring but not vastly more advanced than earth's, Bobby was on a pioneering mission and his ship is the first of its kind.
- The Theatans, and therefore Bobby, are confident there are planets similar to their own with possible alien life forms similar to them within a 100-200 years travel time
- Given the scientific progress of the Thetans, it is entirely likely for them to have genetically modified life on their planet in Bobby's absence
- The apocalypse doesn't have to have happened immediately after Bobby's departure/loss of contact with Theta
- The apocalypse can occur due to a technology we on earth think is science fiction, but still should be grounded in some scientific face
- Bobby's ship is not fast enough such that a planet within 600 years of travel will have a noticeably different set of constellations
- He is aware that possible Goldilocks planets reside in solar systems much like his own, such that it is possible for these planets to have the same night sky as Thetas without the use of advanced instruments, which Bobby no longer possesses
So, with all that in mind, What kind of apocalypse might make a planet unrecognizable within a few centuries?
I am aware that without drastic change, life does not adapt quickly enough to evolve to an unrecognizable degree. However, there seems to be evidence of insects like mosquitoes evolving enough to greatly diverge from the original species within two centuries. Still, I have left open the option for genetic alteration via science. Similarly, Land masses do not change that drastically within such a short period of time, but with sufficient advances in technology this too should be possible. Of course, bonus points if the above is caused by the apocalypse.
I am also aware that Nuclear Armageddon is certainly a possibility, but find the option rather cliche. I'm also skeptical that radiation can cause such drastic changes in the fauna as depicted in series such as Fallout. That said, I am open to anything.
EDIT although a couple of centuries is preferable, if it suits the explanation, Bobby can be gone as long as 600 years.