What kind of Apocalypse might make a planet unrecognizable within a few centuries?

Question

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.

Answer 1

A noticeable change in global temperature + quick advance in genetic engineering

For some reason, your planet went through a brief and intense volcanic period. Maybe a Yellowstone-like supervolcano erupted. This caused a several years long nuclear winter, followed by a noticeable increase of the global temperature caused by all of the newly release greenhouse gases. That may be quite a stretch on Earth, but it's your planet, and you can adjust the composition of gases expelled by your volcano(s) to suit your needs. As a result, polar ice caps melt, oceans rise and the coasts are completely different (with a big portion of landmass simply disappearing).

The brief view of the planet that Bobby got before he crash landed was dissimilar to his knowledge of the geography of theta

Check

The atmospheric makeup is largely similar to Theta's

Hotter but still similar, check

Parallel to that, your civilisation gets really good at genetic engineering, and starts modifying the fauna and flora (and themselves !) to cope with these brutal climate changes.

The flora and fauna are different form Theta's but not by much

Check

There are inhabitants here might be similar to humans in biological makeup, but not exactly human (as in Sapiens vs Neanderthals)

Check

Other technologies advance as well during these times, effectively rendering some devices looking alien (think of what a rusty mobile phone or computer would look like to someone living in the 1910s).

A few pieces of technology he picked up is unrecognizable to him

A bit of a stretch, but could be, semi-check

Unfortunately they realized, but too late, that the genetic engineering had some dire side effects on the long term upon their brain development, making each generation dumber that the previous one. They tried to fix the issue, but with less and less intellectually able people, and more and more dumb brutes having access to some nasty weapons...

There was some ruined civilization here

Check

Answer 2

Give Bob about 3 centuries on his trip

Bobby Leaves for his little jaunt and on his way out of the Solar system he bumps a couple of asteroids out of their orbit into a collision course with his home planet. This is because of the experimental nature of the drive in question.

Meanwhile, Bob's relatives planetside turn their gazes inward and begin experimenting with genetic body modifications. A few groups that are most heavily into the mods seek out places to live in isolation and start their own city

Civilization advances for a hundred years, with Moore's law rendering advances we can't even conceive of until the Hammer of the Gods falls. Remember those asteroids? One strikes the moon, possibly hard enough to crack it or break it. Certainly hard enough to mark the surface of the moon and make it hard to recognize. The second, larger asteroid misses the moon and hits the planet directly.

By an astonishingly horrible coincidence, it hits a fault line near to a Yellowstone sized super volcano. This triggers all sort of catastrophes. Tectonic plates shift in unexpected ways, colliding to create new mountain ranges, subsiding in other places to create vast inland seas where there weren't any before. All of this shifts the coastlines around enough to not be easily recognizable from orbit.

All of the former, major cities get destroyed outright or suffer massive problems right after impact. Lack of maintenance after 200 years will leave enough remnants that you can tell it was a built landscape, but he wouldn't be able to tell what city it was.

Tast we come to the genetic body mod folks. they were on the opposite side of the planet when it all went down. Their skills at genetic manipulation give them just enough of an edge that they survive, mainly by forcing adaptations that will allow them to take on the new, incredibly harsh environment. They don't just modify themselves, they change a few livestock animals to survive as well.

Bobby gets back home around 200 years after the Hammer of the Gods. What he will be able to observe is that the atmosphere is similar, but slightly different. The landmasses aren't exactly right and there are no surviving cities for him to explore, just ruins.

I suspect that the climate will be almost done settling in to the new normal, and the tectonics should have settled down some. That is not to say that the surface of the planet is going to be hospitable. probably far from it.

The Body mod folk are going to put the confusion capper on it for Bobby. when he comes across them, he is going to see something that looks like a case of parallel evolution. Language should have shifted far enough to make them hard to understand. He won't know that they are genetically just like him until he gets a chance to pull some of their DNA and test it. That will be a while because just plain survival is going to be a full time job on the surface.

This is of coursed based on nothing scientific. It's totally made up, but I'd buy it.

Answer 3

Aliens

The main problem I could see is that 200 years might not be long enough to have technology advance to unrecognizable, have an apocalypse, and then have plant and animal life altered/mutated/evolved enough to also become unrecognizable and the landmarks and world change enough too except if there was a massive acceleration in technological development, the only answer is aliens.

Shortly after Bobby left on his journey a far more advanced alien race landed on Theta. They brought with them technology unlike anything humans have seen or could even understand. They taught some of us their language which made understanding easier over time. Some devices were quite mundane to these aliens such as near infinite energy production and teleportation. But to humans it changed everything, within weeks Theta was in a renascence the likes of which have never been seen. The most important was a device similar to what we know as CRISPR. Allowing fast, painless, and error free editing of genomes. They could create plants that fixed atmosphere in only a couple years and could feed millions. They could create animals that produce meat that tasted like it came straight from heaven. Some people even changed themselves just for the fun of it like putting on a new outfit.

But with all these changes many panicked, some feared what these aliens wanted from us in return or why they would ever even want to help us, it was all too good to be true. And with tie people started to see the problems. Those that mutated themselves too often began undergoing strange changes that they didn't do. Horrible disabilities and painful sickness arose after a couple decades of use. Those unchanged enough to be unaffected became angry, riots and attacks against the aliens rose faster and faster. Governments were in shambles trying to decide what to do and who to protect; it's own people or it's saviors that tried to perfect the world. A section of a military didn't want to wait anymore for orders and went rouge, using one of the near infinite energy batteries the aliens brought they created a "small" anti-matter bomb and detonated at the alien headquarters on the planet.

War has begun. The aliens didn't take too kindly to trying to help these primitive humans and be attacked for it. With their advancements we were no match. They returned to their ships and laid waste to most of the planet deeming Theta a lost cause and not worth trying to help anymore. Most of the cities are turned to rubble, destroying the gifts they gave to us and sending mankind far far back in technology. Once the fight was over they simply left, the planet burning, humans almost all dead save for those that hid well enough and nothing salvageable or recognizable to find.

Over a century later after they left nature has regrown, albeit mostly mutated and strange now. The people are not quite human as we see them but something more hideous and weaker from their deformations. Their language was different too now, changed from talking with the aliens it wouldn't be understandable to an outsider. The vast cities swept away with the rain and winds, only rusted skeletons of some buildings stand as evidence that anything was once here. Any lasting small bits of technology would be mangled and foreign. Mountain ranges and entire continents were destroyed by the attacks as a hope to kill anything that may try to survive.

Even to a man who spent his whole life growing up here, this planet would be as alien to him as the aliens that landed there long ago.

Answer 4

All it takes is a big enough cosmic collision and the the planet will be unrecognizable.

For example, the oceans could be almost entirely boiled away into outer space. Thus the outlines of continents and oceans will not be immediately recognizable from other space. But if Bobby is able to make a radar scan and a record of the heights of the surface, he may later recognize the outlines of the former continental shelves.

I fear that boiling away the oceans is already getting into extinction of all life on the planet territory. So an even bigger catastrophe that alters the outlines of the continental shelves would probably exterminate all life.

So you should assume that Bobby is really, really, really bad at geography. And also that your planet doesn't have large continents, just hundreds and thousands of really large islands with a total land area similar to that of Earth. Thus making it almost impossible for people with better geographic skills than Bobby to remember the shapes of the islands and their shelves.

Or maybe the opposite. The planet had large low lying continents and vast ice caps when Bobby left. Something melted all the ice caps and the sea level rose many thousands of feet and now the continents are all under shallow seas except for many small islands in their higher regions. Bobby doesn't have enough geographic sense to recognize that the islands are shaped like the highlands of his planet.

That avoids a catastrophe that boils away most of the oceans.

Answer 5

An Ice age would take care of a a lot of geography and make rubble of most signs of previous civilization in the affected area and you'd get an extinction level event that creates a lot of empty niches once the ice draws back.

There would of course be survivors in the equatorial belt unless you had a completely snowball planet, which I'm not sure is actually feasible (Edit: Earth has probably had a snowball glaciation back during the Cryogenian Ice Age, the Sturtian Glaciation, though that lasted for aproximately 60 million years.). However a nuclear war for diminishing space and resources would likely sort that out and set technology level back appropriately for the technologically dominant species.

Now, could an ice age come and go in just over half a millennium? The big one we had, and most usually refer to as "the ice age" was several thousand years long.

I'm lifting a bit from History Channel here:

Scientists have recorded five significant ice ages throughout the Earth’s history: the Huronian (2.4-2.1 billion years ago), Cryogenian (850-635 million years ago), Andean-Saharan (460-430 mya), Karoo (360-260 mya) and Quaternary (2.6 mya-present). Approximately a dozen major glaciations have occurred over the past 1 million years, the largest of which peaked 650,000 years ago and lasted for 50,000 years. The most recent glaciation period, often known simply as the “Ice Age,” reached peak conditions some 18,000 years ago before giving way to the interglacial Holocene epoch 11,700 years ago.

At the height of the recent glaciation, the ice grew to more than 12,000 feet thick as sheets spread across Canada, Scandinavia, Russia and South America. Corresponding sea levels plunged more than 400 feet, while global temperatures dipped around 10 degrees Fahrenheit on average and up to 40 degrees in some areas. In North America, the region of the Gulf Coast states was dotted with the pine forests and prairie grasses that are today associated with the northern states and Canada.

As I mentioned the time scales don't really work with your original premise so you'd either have to extend the time Bobby is away or find a way to accelerate the process of glaciation-deglaciation.

Answer 6

When Physics goes horribly wrong.

There was an experiment that opened a wormhole and earth went through it. It found itself in relatively close proximity to a neutron star. Oh dear, many people and animals died pretty quickly. The people lucky enough to be on the other side of the planet for example did their best in the limited amount of time to open another wormhole. They ended near a sun-like star in a reasonable distance. Being the lucky guys that they were to randomly end up where they wouldn't die, they decided to call it a day and live with the result. But the ecosystem was changed forever. The survivors then had to adapt earth to their new position in space and remodeled earth afterwards so that the climate and ecosystem stabilized. While they were at it, they overdid it a bit maybe - you know those guys from the other side of the planet. Well, some centuries later, anything could happen technologically. You would not recognize many pieces of technology from 1700 and those even exist on the same planet at the same time as you.

I get the impression this is pure story and not really a world building question, but it's fun enough.