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How can I make it so that an otherwise colonizable planet (or moon) can (entirely naturally) seem uninhabitable from a certain point on its surface?

Background: There are a series of immovable gates scattered across the galaxy by an unknown race. Each gate has access to thousands (at minimum) of other gates; enough that there is no danger of running out of new planets anytime soon. A majority of these gates are on planets, moons, and asteroids. These gates are used by other races to colonize and exploit planets. Just about any environment that a human in a space suit could survive in is a good candidate for colonization. Exciting or valuable planets are snapped up by corporations or polities, while less interesting planets are noted and eventually sold to other groups. Planets with extreme conditions or that present obvious hazards are passed over and blacklisted, and unless there is reason to do so, are generally not explored far beyond the gate. A robotic civilization examines planets on this blacklist in greater detail, looking to colonize planets that not as uninhabitable as they seem to be. Planets like this would seem to be bad candidates for colonization or mining within a ~100-kilometer radius from the gate, but would be more habitable beyond that. Ideally no more than a quarter of the planet would be completely infeasible to colonize.

I'm looking for almost any sort of hazard that would make a planet (incorrectly) seem dangerous or hostile enough to someone stepping through one of these gates that it's inadvisable or not worth the effort to exploit, with three stipulations:

1. Nothing interesting: There can't be anything that would make mining or research seem to be worth the risk. Obvious signs of abundant rare elements, scientific anomalies, alien civilization (no matter how primitive), or native life would all be too interesting. I want anyone who discovers that the planet is inhabited to wonder why the inhabitants bothered. Planets with really unusual dangers are cool, but I want explorers to be saying "Wow, this place sucks!" rather than "Wow, I wonder what's up with this place!"

2. Gate access: The inhabitants need to be able to occasionally access the gate, both for initial colonization and for import/export purposes. The gate being inaccessible a majority of the time would work, as would having gate conditions be lethal within a matter of hours. Sticking the gate in the middle of an ocean of lava would not.

3. Novel: I already have a tidally-locked planet with the gate on the terminator, which discourages colonization with high winds, ash storms, and the occasional rain of semi-molten rock carried over from the hot side. Weather-related answers are fine as long as they're dissimilar enough from this. I also have a planet in a white dwarf system where the gate receives enough ultraviolet radiation to destroy even hardened electronics in a matter of hours.

I have a few ideas of how this could work.

  • Dangerous system: A planetary system that appears to be on the verge of suddenly planetary catastrophe would not be worth doing anything with unless there was something worth the risk. A star that looks like it's about to go nova, a large asteroid belt likely to send asteroids at the planet, or an x-ray binary likely to sterilize the planet would all make potential colonists turn and run. The difficulty would be how to make it quickly obvious that the planetary system would probably kill you, while allowing further research to conclude that it won't.

  • Volcanic activity: Frequent volcanism can rearrange landscapes and kill without warning. Given time, it could be studied and predicted, but on an otherwise unremarkable planet, that generally wouldn't be worth the effort. Unfortunately, it would be hard to limit the volcanism such that it's intense enough in certain areas to scare off explorers, but mild enough elsewhere that permanent structures can be built. Additionally, major volcanism would be likely to bury the gate in rock, as well as potentially making surface conditions too similar to those of Venus.

  • Planetary purge: Some frequent event that, while apparently unpredictable, can be predicted and allow the inhabitants to bunker down to survive it without much issue.

An ideal answer would include an explanation that passes a reality check, but if necessary I can just make a follow-up question to ask how it could be made to work.

Edit: Made it more obvious that I'm looking for an apparently dangerous planet, not just an uninteresting one.

Edit2: To clear up confusion, 'colonizable' means the following:

  • Temperatures between -250° and 200° C.
  • Surface pressure ranging from vacuum to 3 atms.
  • Gravity at or below 2.5g.
  • No conditions that would frequently destroy buildings dug into the crust.
  • At least 75% of the time, external conditions wouldn't cripple or kill a human in a hardened space suit.
  • Flooding, tides, or underwater land aren't an issue unless the liquid would be hazardous long-term to a deep-sea submersible.
  • The presence of enough metals and carbon for at least low-scale industry, and enough power options (solar, geothermal, volatiles, fusion, etc.) to support a colony.

Edit3: Again, 'colonizable' is a relative term. Habitability for unaugmented humans without extensive technological support isn't a concern. If a Mars rover can trundle around on the surface of the planet without summary destruction, consider it to be well within the bounds of colonizable.

Edit4: In response to a question. The gates are two-way and have been demonstrated to be able to reassemble in a matter of weeks from anything short of the detonation of an antimatter bomb. There have even been (inaccessible) gates that appear to be located deep in the atmospheres of gas giants. More information on the gates can be found here and here.

Edit5: Please do not have your answer involve aliens in any way. Please do not have your answer involve life in any way. Please do not have your answer make the planet unusual enough that either native life or alien intervention are the most likely explanation for how it came to be that way. The only exception is if it would take extended and in-depth exploration and research on the planet to figure out that that the conditions didn't arise naturally

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  • $\begingroup$ I want to make sure I understand: you want scenarios causing human explorers to go through a gate, look around a bit and then blacklist the planet. But, the planet needs to later be evaluated by the robots as useable by them or by the humans? Who is going to be the final colonists of this planet--robots or people? Because things that seem unihabitable to humans could easily be inhabitable by robots. $\endgroup$ – Thom Blair III Dec 7 '16 at 19:13
  • $\begingroup$ And, another thought I just had was why would humans not send robot probes to evaluate blacklisted planets, after seeing the robots do it? Do the humans not know about the robots profiting from the incorrect human evaluations? $\endgroup$ – Thom Blair III Dec 7 '16 at 19:19
  • $\begingroup$ @Thom Blair III - Yes. The humans explore the planet with the help of probes, but give up quickly since it seems like exploiting the planet would be too much hassle for not enough payoff. They could look into it further, but there are so many other planets available that it's more efficient to just dial up another planet to explore. Later on, the robots secretly explore the blacklisted planets to see if they might be colonizable after all. If one is, they colonize it and set up outposts in the hazardous zone, so that anyone who visits the planet just sees some idiots who are barely... $\endgroup$ – emo bob Dec 7 '16 at 19:26
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    $\begingroup$ To make a planet uninhabitable? Stick the 4chan banner all over it. Most people will keep a really safe distance from it. $\endgroup$ – T. Sar - Reinstate Monica Dec 7 '16 at 19:50
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    $\begingroup$ @Thales Pereira - True, but the people it doesn't keep away are exactly the type of people you don't want to have visiting your brand new colony! $\endgroup$ – emo bob Dec 7 '16 at 19:54

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The amount of resources expended to determine the habitability of a new planet is inversely proportional to the number of gates/planets they have access to. If there are thousands of options then the criteria for exclusion will be low, but if there are only 4-5 options, then they will be very thorough in evaluating each planet, especially if they have competition for colonization.

In light of this, I'm assuming there are many planets, several hundred at least, so human explorers will not spend more than a few days at most surveying the local area around the gate and do not have the ability (budget) to bring in rockets to launch satellites into space or drones that can do more than patrol a few dozen km around. I'm also assuming that the robotic explorers may visit this list some time afterwards, so conditions could have changed. And they do have the ability to launch satellites and durable drones for a wider recon of the system.

  1. The gate opens into an area underneath a hole in the ozone (or ozone equivalent) such that there is an unacceptably high level of radiation. Without journeying outside of that area (which could be hundreds of km in diameter) the explorers wouldn't know that the rest of the planet is more protected and safer. The radiation exposed area would be devoid of life.

In order to jack up the levels of radiation penetrating the atmosphere, the planet wouldn't have a spinning iron core able to generate a protective magnetic field. So the atmosphere is very thick and dense, able to shield the surface in most places, but a naturally occurring ozone hole above the gate leads the humans to assume the entire planet is bombarded.

In addition the planet (moon, in this case) could normally be protected by the magnetic field of a nearby large planet, but it has an elliptical orbit that periodically takes it out of the field. This could explain an abnormally high level of solar exposure through the ozone hole (particularly if there is a Van Allen belt equivalent around the larger planet and the moon is currently travelling through it, but normally the moon isn't exposed to much solar radiation, allowing it to maintain an atmosphere and better conditions elsewhere.

  1. The planet is currently passing through a dense asteroid field or comet tail (like our annual Perseid meteor shower but much worse). So the planet is being bombarded with impacts and it looks like there will be more coming. Without sticking around to monitor the solar system there would be no way to know how often such showers occur.

Now normally this would make the planet VERY attractive, as all these impacts deliver all sorts of cool heavy elements to the planet. But obviously nearby impacts would destroy any mining/colonization attempts. So humans don't bother to wait around, but robots could spend the resources to determine the frequency of impacts and determine that there are significant periods of time without impacts such that they could conduct operations and perhaps erect a meteor defense system before the planet passes through the comet tail again.

  1. The local star just happens to be undergoing a rare super flare. Again, this is dangerous to the explorers and they have no way of determining the length and frequency of such solar outbursts.

To spice this up, perhaps this is a binary star system, with one star producing all the flares. The planet in question is part of it's own paired orbit with another planet that frequently shields it from the flares from the star, but it happens to be out of the shadow of this planet when the humans visit. So normally the gate planet is livable, with light coming from a less active star (so solar power is available) while it is in the shadow of the other planet to block solar flares from the other star.

  1. The planet is exiting an ice age with melting glaciers. The gate site is frequently submerged by meltwater floods that remold the surface, making construction difficult. It is possible that if the robots survey the planet a few decades later, the glaciers and flooding could have withdrawn from the area around the gate, making it a more inhabitable spot. The amount of flooding could have also been transiently increased due to a meteoric impact that precipitated flash flooding (much like some folks think occurred to Earth 12,000 years ago), such that conditions improve just a few years later.

  2. The planet is close to a black hole and experiences significant time dilation (like the planet in Interstellar). This makes colonization by humans very difficult as there is effectively no two way communication, humans on the planet can't come back through, there is no way for them to build up facilities in any kind of time line that would allow for mass immigration through the gate or to bring anything back. Robots may not fear such an isolating phenomenon though, since they have a much longer lifespan.

  3. The gate connection is very erratic, with significant periods of disrupted connections that either prevent gate travel or destroy whatever is going through. Psychologically this would be devastating for humans, but robots may not mind.

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  • $\begingroup$ 3. and 5. violate the restrictions that I listed. But 1. shows real promise, especially if combined with something else. For instance, the star could be unstable and high in UV and x-rays. The atmosphere would block a decent portion of it, but the hole area would end up scorched and highly irradiated. The difficulty, of course, is in figuring out how to make that work realistically, but that's a matter for another question. $\endgroup$ – emo bob Dec 7 '16 at 21:51
  • $\begingroup$ 6.: more likely not inhabitable by robots and habitable by humans becouse of corrosion. Depends heavily on the tech level and kind of your robots (gold or platinum circuit doesnt rust, but whole robot? :)) Also, most of "natural" O from the big bang made compound with H, (and there is so much more H then there is O (1000:1) in our universe) so "too hight O2 planet" will be jungle planet or extremely exceptional planet worth of planet-scale resource gathering. $\endgroup$ – Jan 'splite' K. Dec 8 '16 at 12:16
  • $\begingroup$ 1 doesn't seem really feasible, unless the whole planet has too much radiation. A spacesuit or some kind of vehicle or building would be able to block the UV quite easily. #6 might be possible, as it is thought that Earth used to have higher O2 making the large dinos possible, and lots more fires. Fire loves O2. $\endgroup$ – MikeP Dec 8 '16 at 15:49
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    $\begingroup$ You're allowed to post multiple answers. So please, for the love of the site, don't post six skeleton answers that masquerade as a single fleshed out answer. $\endgroup$ – Samuel Dec 8 '16 at 18:20
  • $\begingroup$ After much deliberation, I've decided to mark this as the answer. I'll probably end up using parts of multiple answers, but this one was the most thorough and had the largest number of ideas that I'm interested in. $\endgroup$ – emo bob Dec 9 '16 at 15:44
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The gate is at one of the magnetic poles of the planet, the field is very powerful. There is a source of extremely energetic particles in the system, probably a neutron star or black hole. The magnetic field is sufficient to deflect these particles down onto the poles--the gate area is subject to a very high radiation dose (say, 10Sv/min.) An explorer that doesn't immediately about face and leave is dead, most probes will be fried.

What explorers don't know is that this isn't a planet, but rather a moon. Occasionally it goes behind it's parent planet and the radiation stops for a short period. The inhabitants can quickly approach and use the gate during this interval. They have a maglev line that serves the gate--but the track is buried under an inch of rock and it's only energized when it's going to be used. A detailed search would find it but nobody's going to be doing that.

Note that the moon is not in an equatorial orbit, most orbits it passes above or below it's parent and there's no eclipse. (Think of the moon--we don't get eclipses on most orbits of the moon.)

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Expanding on the previous warzone ideas, make the planet ideal for colonisation with fertile valleys and rich ores right in front of the gate (maybe that's why the gate was there).

At some point in the distant past, some race found this and colonised it. At some other point in the distant past, another race found this and wanted it. The subsequent war created an area uninhabitable for the entirety of the first race's colonised territory which is as big or small as you want it to be. The actual reasons for it being uninhabitable are as varied as weapons are but could be anything:

  • nano-phages
  • radiation
  • bio-engineered, hyper-evolving/mutating beasts (Xenomorphs)
  • a fleet of AI drone carriers producing aggressive self-repairing sentinel drones
  • gravity warping (don't detonate hyperspacial refolding weapons in a gravity well fool, it creates flux fields)
  • a continent sized crater with a hostile micro-climate

Whatever the results of the alien weapons, the warzone is no longer habitable and because the colony was centered on the gate, the warzone is also centered on the gate. Go far enough from the gate and it's a normal planet again.

The only caveat is that the warzone shouldn't be scientifically interesting, so whatever it is, it should be some technology that is more or less understood by everyone rather than having traces of exotic technology that could be scavenged. On the other hand; dealing with bottom-feeding scavengers risking death to loot the battlefield for technology is an interesting story too.

EDIT: Expanding on the idea slightly, the point is that the gate is by definition, the most interesting part of the planet. Therefore it's naturally the centre of any activity on the planet, it is the natural place for an outpost and the epicentre of any conflict. Rather than attempting to subvert this conclusion, exploit it by having the strategic value of the gate be the reason that the area appears uninhabitable.

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    $\begingroup$ This has enough detail and reasoned argument to pass as an answer and not a comment. Even though, it expands on other answers. Welcome to Worldbuilding, bp, I think you're going to have fun here. $\endgroup$ – a4android Dec 8 '16 at 3:22
  • $\begingroup$ I think that answer gave me the rep to comment now but anyway... $\endgroup$ – bp. Dec 9 '16 at 7:24
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The planet has constant electrical storms. At all times there are massive planet-wide electrical storms. The inhabitants have adapted their very way of life to counteract the constant electrical activity. All electronics must be shielded. Anyone outdoors needs modified clothing or suits to survive.Any housing or vehicle on the surface need to be properly grounded to avoid electrocution and fire. Most inhabitants live underground. Anyone coming through the gate unprepared and ungrounded, would be greeted by thousands of volts within a few minutes. Any scouting technology sent ahead without proper precautions would be fried.

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    $\begingroup$ "Anyone outdoors needs modified clothing or suits to survive." - how about a hand-held lightning rod with a hefty insulating handle that you drag across the ground? $\endgroup$ – John Dvorak Dec 7 '16 at 20:50
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    $\begingroup$ @JanDvorak: lightning instantly heats up and expands a band of atmosphere, which is in all practical senses an explosion. You don't want to go around holding an explosion attractor. $\endgroup$ – sampathsris Dec 8 '16 at 4:13
  • $\begingroup$ @Krumia. You are holding an explosion diffuser. Lightning is current trying to force it's way though an insulator (air), not an explosion per se. Giving it a conductive path to flow through is the best way to remove the danger of excessive charge buildup, which would cause the explosion. $\endgroup$ – Mad Physicist Dec 8 '16 at 17:41
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The gate could be deep in a frozen arctic wilderness, like it is near the north pole, where there is just snow and ice and nothing else. No minerals, no land, no plants or animals. Natives could still trek there if they are properly supplied, but anyone popping through from the other side would come through, think they are in an ice age, and leave again. Why bother if there are other places to choose from.

It could also be in an underwater cavern. If the natives have a way to drain out the water when they need it, or hoist it up and into the light, but leave it flooded most of the time as a security precaution then that would be a deterrent. The water could also be chemically active, such as highly acidic or alkaline, so that it would attack anyone coming through even if they are suited up. Especially if the water goes pouring through the gate when it's opened.

An impenetrable marshy bog like the Sudd has promise too.

The whole idea seems to be like locks on your doors. Locks are not hard to get around. Pick them, bump them, card them, smash them. You put locks on your doors to make your house look less attractive than the neighbors house. So a world that looks like it is going to take more work to get anything out of will probably be passed over for greener worlds.

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  • $\begingroup$ Less locks and more booby traps. Sure, you could get past it, but it wouldn't be easy or pleasant. The house with a good lock is annoying and difficult, but you might try to break in if it's a slow day. The house with bear traps in the front yard is dangerous and to be avoided, even if it means you have to go back to the locked house. I do like the idea of an under-liquid cavern (water, even chemically active water, wouldn't be enough). The interesting part, then, is figuring out how to get a hazardous liquid like that without making the planet either interesting or completely uninhabitable. $\endgroup$ – emo bob Dec 7 '16 at 19:53
  • $\begingroup$ Suspended above a volcano. Or channel the runoff from an acid like like the one on Indonesia's Kawah Ijen volcano. If it keeps going down into a low area it could become super concentrated like the dead sea is without having a mineral rich volcano right there. $\endgroup$ – AndyD273 Dec 7 '16 at 20:57
  • $\begingroup$ I'd always heard that the presence of strong acids over geological timescales was a sure sign of either life or tampering. Is that not the case? $\endgroup$ – emo bob Dec 7 '16 at 21:57
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    $\begingroup$ @Miral - True. There will always be thrill-seekers who are attracted to danger. If for whatever reason they decide that this planet looks fun, they'll show up, run around whooping and hollering, and no one will question their disappearance. Things would get a whole lot more complicated if a well-equipped expedition showed up intent on exploiting the planet. Their disappearance would almost certainly be missed. Fortunately a group like that would need a good reason to brave the dangers in the first place - there is unlikely to be much overlap between them and the thrill-seekers. $\endgroup$ – emo bob Dec 8 '16 at 13:21
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    $\begingroup$ To continue the metaphor, from what you can see through the windows the house is full of junk and doesn't have much worth stealing. No one in their right mind is going to think that the potential haul is worth the risk of breaking in. $\endgroup$ – emo bob Dec 8 '16 at 13:22
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I like the idea of a binary star system. The humans could have visited during a central transit time. When the planet was between the two stars causing an extreme surface temperatures. Then when the robots come back its in an outer track with one star eclipsing the other. All ecology could be adapted to "hibernate" during the extreme periods.

Adding details per request.

Planets can move in a variety of ways in a binary star system. They can orbit a single star, orbit both stars in an oval or perform a figure 8 track between the stars. The planet I'm imagining would be on the figure 8 track. When the planet was between the stars there would be 2 suns in the sky and double the thermal energy arriving at the planet. This would shoot up the surface temperature for "months" at a time while its in the central transit period. When it exits this period the stars would eclipse each other lowering the thermal energy and allowing life to flourish.

You could even make water enter the atmosphere during the hot times and fall back to earth in the cool times. Planetary fog with sweltering heat would make me not consider living somewhere. Animal and plant life would need to be underground during the hot periods and could emerge for the cooler periods. They would likely bury themselves in mud before the water evaporation then hibernate.

The robots could really like the available solar energy and perhaps high mineral contents of the planet.

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  • $\begingroup$ There are some intriguing ideas here mentioned only in brief. Perhaps you would like to expand them into a more detailed answer? $\endgroup$ – kingledion Dec 7 '16 at 21:45
  • $\begingroup$ Added some details. As its life and complex eco systems I would help to have a direction to expand in. $\endgroup$ – Kaiju Dec 7 '16 at 22:47
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Bacteria

Though you asked for an answer involving no life, this method is explainable using nothing but the bacteria brought in by explorers.

As it turns out, the planet is actually really inhabitable. It has the perfect operating standards for all manner of... bacteria. Just the right minerals, heat, water, and various chemicals to allow all kinds of microorganisms to thrive. Of course, this means that the gate and everything nearby is coated with bacteria that will eat right through anything from rubber to metal, and the bacteria in the air will kill anyone not in an airtight suit. Worse, these aren't "local" bacteria; they are all the little microbes from the original expedition. Of course, with the right plastics, the bacteria can't get in, and a bleach bath washes them right off.

Permastorm

With the right landscape, a permanent storm can form; high winds, lots of rain, and other hazards like hail, lightning, and tornadoes or micro-bursts can be deadly. The portal is located on an island, surrounded by water, heated by geothermal vents. The area is blanketed with steam, and lashed by acid rain and high winds day in and day out. The storm covers miles around the gate, blocking technology with constant lightning strikes, blocking vision with steam and rain, and blocking travel with a boiling sea (did I mention it's filled with dangerous chemicals from the acid rain? No? Well, it is!).

Of course, the area around the portal is a dormant volcano, and the storm is trapped inside. Outside the storm, it's beautiful. And inside the storm, if you go around to the back of the portal and go into where the boiling steam is thickest, it leads straight out. Granted, you have to know where it is - the path is hidden under a foot of water - but any acid-proof vehicle could make it through without trouble.

Time and Tides

The portal opens into an underwater cavern; a bit of space to move about, but attempting to leave will meet a watery doom! The rushing water will pummel anything to a fine silt, be it living creature or well-crafted machine. There is a tremendous undertow, rushing through sharp rocks. Tunneling through the cave walls would only flood the cavern; the only way out is through the rushing mayhem.

Which isn't so bad, once a month or so. While the tunnel is overwhelmed by crashing tidal waves most days, when the planet's moons align, there is a couple good days of nearly placid water - plenty of time to get a shipment. The rest of the planet is just as chaotic, tidally, but nobody cares about tides underground.

Needle/Haystack

The portal opens onto a... rock. A boring asteroid, in a poorly-populated asteroid belt, in a dead system that seems to have nothing but a single red dwarf. It's where boredom goes to die. It's not dangerous; it's just... boring. It's not even worth mining minerals, because there's nothing there but iron and carbon, and you can get that anywhere. There's hardly enough enough gravity to hold you onto the asteroid. Oh, man this place is boring.

...Except for a strange phenomena with the various asteroids. As it turns out, 99% of the material in the asteroid belt is "cloaked" by scanner-jamming crystal formations. While there is no atmosphere, several larger asteroids would make excellent homes, and the force required to reach them is in "big jump" range. With a basic jetpack, it's simple to shuttle over to any of the cloaked asteroids and set up shop, and the crystals and mineral deposits make these hunks of rocks incredibly lucrative. The first step is a doozy, but after that, you'll have an amazing system to call home!

Inside Out

No green people here; rather, the surface of the planet is a living hell. High temperatures, high gravity, constant dust-storms, toxic air, radiation, it's got it all. It's not a cycle, either; it is always hell. The whole planet might as well be on the back of the sun. Worse, the various dangers absolutely wreck any scanning equipment.

Inside, though, is a network of tunnels, hollowed out by countless eons of volcanic activity. As the toxic air filters through the rock, the worst is filtered out; the iron and other metals near the surface quiet most of the radio noise, and the thick rock insulates from the surface radiation. There are a few natural resources - water, oxygen, etc. - that have filtered down through the rocks, and plenty of minerals to exploit.

MY EARS

This planet is LOUD. Constant winds over high canyons have created the universe's largest pan-flute orchestra, and constantly blow it 24/7, at 200 db. It's like a heavy-metal bagpipe convention. It doesn't matter how much insulation you put between you and the outside world, that multi-pitched scream cuts through like a clarinet in the mouth of a four-year-old. Beethoven could hear this "music."

Other than that, it's fine, if a bit boring. And robots can just turn off their audio input, right?

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An unknown civilization has dumped all of their nuclear waste right outside the gate.

Time has hidden all signs of the intellectual source of the radiation. Instead, human explorers determine that the planet has had a high natural concentration of uranium in its crust, and at some time in the past has developed into a huge natural nuclear reactor, leaving so much waste as to likely make the whole planet uninhabitable.

But in reality the unknown civilization has been smart enough to bury the waste so that it stays in a single place, leaving rest of the planet habitable.

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  • $\begingroup$ When I saw your answer in my notifications list, I'd thought that it was another answer that relied on obvious alien intervention. But now that I read it in its entirety, I see that it involves alien intervention that is indistinguishable from a natural occurrence without significant study, which is in keeping with my original intent after all! Consider yourself exempt from my newly-added Edit5. $\endgroup$ – emo bob Dec 8 '16 at 13:32
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Let me add an idea that doesn't quite follow the letter of your question, but might follow the intent.

The gate is on a totally useless moon. Let's say the gate opens up onto a completely barren moon of a gas giant. It's surveyed, there's nothing in the crust, it's heavily radioactive, can't be terraformed. Maybe it is actively dangerous, moonquakes, volcanoes etc.

The bureaucrats put it on the "don't bother" list, because there's much more interesting (and easier) places to see.

However, another moon of the gas giant is completely habitable, but it takes a couple of days to get from the gate to Habitable Moon. In an era of instantaneous travel, that would be highly unacceptable.

No one's going to go colonize Habitable Moon if there's thousands of easier-to-access planets. I'm thinking of the Hyperion Cantos, where planets in the Web (linked with man-made gates) are culturally very linked. Giving up easy access to a gate is a huge cultural thing.

So Shitty Moon is hard to deal with, and only the best pilots used to landing on it can get people in and out, which effectively isolates Habitable Moon in the same way you're thinking.

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    $\begingroup$ A neat piece of reasoning. Nice to see some lateral thinking applied to the question. $\endgroup$ – a4android Dec 9 '16 at 11:46
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The planet has no atmosphere or liquid on its surface; it's all subterranean.

One cause could be this planet has a tight crust (either somewhat naturally, or as a side effect of a superweapon having been used on it some time in the past, or intentionally by those trying to "hide" the planet from gate-travelers) which keeps gases from leaking outward much. (Small quantities of gases leaked to the surface may get stripped away by solar wind. Lack of protective atmosphere, and apparently magnetosphere as well, discourages travelers.) The crust's contents are whatever boring-onium you find making up 65% of every other planet anywhere.

Another cause may be the planet is insufficiently warmed by its nearby star, so the atmosphere (and oceans) are both solid. But nuclear geologic processes warm the planet deeper inside, and due to geology / chemistry / inhabitant history, there are quite large pockets with liveable arrangements of resources in there. (See Vernor Vinge's "A Deepness in the Sky" for a planet whose inhabitants hibernate right before the atmosphere freezes on a cyclic schedule due to an on/off star.)

Anyone passing through the gate who is willing to drill deep enough through the planet's crust could figure this out. Detailed scanning of an obviously deserted, crater-pocked cold dry rock may never occur to any visitors. If they notice gravitational variations, they could chalk it up to varying ores far too deep to be worth mining for. Seismic detection for a few hours after a meteor makes a new crater on the far side of the planet might reveal the extreme density changes deep underground.

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  • $\begingroup$ It's a good answer, but unfortunately it doesn't really answer the question. Lack of atmosphere does not equate to danger, or else we would never be considering colonization of the moon. A majority of planets would probably have similar surface conditions, and wouldn't present much of an obstacle to someone inclined to set up an outpost. $\endgroup$ – emo bob Dec 7 '16 at 23:04
  • $\begingroup$ Sorry, I assumed a planet with zero atmosphere would be "uninhabitable". I failed to retain the question's information that "Just about any environment that a human in a space suit could survive in is a good candidate for colonization." :/ $\endgroup$ – X Goodrich Dec 9 '16 at 0:28
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Considering Earth as an example, a gate that opened to the abyssal plain far from anything other than flat ocean bottom would make arriving explorers wonder if the whole planet was that way, and be thousands of miles from land so hard to access even if they did do a thourough job of exploring.

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  • $\begingroup$ While this planet would indeed not provide much reason for colonization, it doesn't have much to actively prevent it either. And if the gate is deep enough to pose a serious hazard to explorers, it would also inconvenience the colonists enough that it wouldn't satisfy 2) gate access. I've edited the question to make the danger more clear. Sorry for the ambiguity! $\endgroup$ – emo bob Dec 7 '16 at 18:02
  • $\begingroup$ This would only delay and deter a small amount. The potential explorers might very well send out an expedition or a probe to explore. Once the probe got to the surface, it would find land. $\endgroup$ – MikeP Dec 7 '16 at 18:43
  • $\begingroup$ «Planets with extreme conditions or that present obvious hazards are passed over and blacklisted, and unless there is reason to do so, are generally not explored far beyond the gate.» so having the gate in a hostile location will suffice. «A robotic civilization examines planets on this blacklist in greater detail, looking to colonize planets that not as uninhabitable as they seem to be. » that contradicts the previous. Unless you mean that this is what is done when there is a reason to do so. My answer satisfies your point #1, “nothing interesting”, difficult to proceed. $\endgroup$ – JDługosz Dec 8 '16 at 5:31
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Many Micro Moons

I read a story once where a planet had a baseball-sized moon orbiting it really quickly about 6 inches above the planet surface. The moon would punch a hole through anything placed in it's way. If your planet had many micro moons like this orbiting at supersonic speed at various altitudes, if people came out of the gate and suddenly had one or more moons zip through them, the rest of the team might just mark the planet as too dangerous because they can't even reliably send a standard probe through to explore.

However, perhaps the robots have seen that standard human probes are a certain size, say car sized, and so they send:

  • clouds of nanobot swarm probes that can assume any cloud shape, and thus have enough of the probes survive long enough to map the moon orbits, and thus be able to avoid them during colonization

  • quantum probes that super-position themselves with all points of the planet's surface and then probe each point simultaneously.

This obviously requires the humans to have never thought of these options, and the robots to be secretive enough and clever enough to have discovered this loophole in human logic and exploit it.

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  • $\begingroup$ Sorry Thom, I think that a planet that so completely challenges known laws of physics and planetary formation would be well worth the cost of developing tiny robots to explore even if such robots didn't already exist. $\endgroup$ – emo bob Dec 8 '16 at 13:25
  • $\begingroup$ Oh ok, I see another facet of what you're looking for/not looking for. $\endgroup$ – Thom Blair III Dec 8 '16 at 13:36
  • $\begingroup$ To be honest, the difficulty we're having in even imagining a planet that fits this scenario seems to suggest, after a moderately in depth look, that there won't be too many of these kinds of planets (or at least too many different kinds). Does that seem to be what you're thinking? $\endgroup$ – Thom Blair III Dec 8 '16 at 13:37
  • $\begingroup$ If there's anything else you're unclear about, don't hesitate to ask. It's become apparent that I didn't communicate my intent nearly as well as I originally thought I had. $\endgroup$ – emo bob Dec 8 '16 at 13:38
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The scenario bears considerable resemblance to the Stargate storyline. Just as the primitive Egyptians did, bury the gate when not needed. Anything that comes through "becomes one" with the surrounding rock. As Ra Sun God figured out, there wasn't a whole lot he or anyone on the other side could do about it. Even sending a bomb through would fail as only part of the devices would emerge and fuse with the rock before the rest of the device got through. Depending how suspicious your robots are, they might conclude that the gate on that planet was somehow buried - landslide, planet-quake, meteor-strike, the gate just fell over on its face, whatever - and simply strike that planet off the list as unreachable.

Now here's how to permit the locals to use the gate while still protecting themselves from probes and other mischief. Put the gate on wheels no wider than depth of the gate and carve a slot in the side of a mountain just wide enough to accommodate the narrow dimensions of the gate (and wheels). When not in use, slide the gate into its tomb. When needed, roll it out of the mountain, use it and shove it back in. This would also have the benefit of protecting the world from unannounced/unscheduled visits from near neighbors. Said neighbors and the source-world would have to work out a signaling systems to assure that the gate was available.

For your story - I assume you want the persistent robots to somehow figure out that they're being duped. In the above scenario, they send a really tiny hovering probe through the gate to determine just how badly buried it is. While the little probe is realizing "Hey, I'm still alive!", the natives pull the gate out of its tomb for a scheduled use, pulling the little probe out of the mountain along with it. Then follows the frantic but obligatory "What is that?!? / Oh sh!t!" moment and the crisis is on!

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  • $\begingroup$ In Stargate, T'elc rescued RDA through a burried gate because the connection disintegrates a cavity, which can then be tunneled out of. $\endgroup$ – JDługosz Dec 7 '16 at 17:33
  • $\begingroup$ Unfortunately this universe has established rules regarding gates, which include the gates being immobile, and the gate connection pushing passengers into objects rather than melding them. So probes could tunnel through the rock to see what the planet is like. Even if the extra work wouldn't make the planet an immediate priority, it would still be on the "we'll get to it" list rather than the "don't bother" list. $\endgroup$ – emo bob Dec 7 '16 at 17:38
  • $\begingroup$ @JDługosz - I think that was a liberty taken by the TV series. In the original movie, particularly the uncut version, there were a few petrified "gods" found just below (the gate was buried face-down) the gate. In the original-release version, as the Egyptian workers are raising the gate with ropes, you can see other workers staring at the rock-bed the gate was resting on. $\endgroup$ – Jym Dec 7 '16 at 17:41
  • $\begingroup$ @emobob - two questions. 1) Why do your robots need "habitable" planets? Planets without an atmosphere would be easier to mine. 2) How far along, technologically, are your inhabitants? $\endgroup$ – Jym Dec 7 '16 at 17:48
  • $\begingroup$ @Jym - by "semi-habitable" I'd meant a planet that could be colonized without a huge amount of trouble (and, as mentioned, probably wouldn't kill a suited human). Technology includes nanofacturing, metamaterials, carbon nanotubes, and limited Von Neumann machines/smart materials. This is a good example of what's available for colonization purposes. $\endgroup$ – emo bob Dec 7 '16 at 17:58
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Possible Scenarios

  • Gate area and/or the rest of the planet is so rocky it would take too much effort to make it suitable for humans. One extreme example of this would be a planet covered with huge rocks that humans could squeeze between & walk around under, but that would be porous enough for nanoscale robots to colonize through and around all the rocks. Scoria-like rocks would be good candidates for these enormous, porous rocks.
    Possible flaw: humans could create floating cities above the rocks.

  • The planet could be covered with Vantablack-like material, and also not have enough sunlight to light the environment. With no sunlight and a planet surface so dark that all light gets immediately absorbed, humans might consider it too difficult to illuminate for widespread exploration.

  • The planet surface could be so extremely convoluted and reflective (and possibly transparent) that human vision and artificial sensors cannot easily determine safe walking paths.

  • Perhaps the planet could have several stars lighting it, making it so extremely brightly lit that it is blinding to human vision. In addition, a thick cloud of extremely rapidly orbiting asteroids block out the light and then move to allow the light back to the planet that the lighting is essentially like a permanent blinding irregular strobe light. The strobe effect could be so irregular and so bright that even automatically correcting brightness technology cannot adequately make the planet attractive.
    Possible flaw: Computers might be able to adjust faceplate light regulation enough so as to counteract the strobing.

  • Perhaps most of the interesting things about the planet are not detectable with human senses or technology, but are to robots calibrated to easy perception of higher dimensions. Requirement: Human probes would have to not be calibrated to extra dimensional sensing, yet the robot civilization would be.

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  • $\begingroup$ (Ostensibly) naturally-occurring vantablack seems like it would be unusual ant interesting enough to warrant further examination. It is a cool idea though, and I might steal it for something else. And anything that makes the planet hard to navigate visibly (including reflection, transparency, etc.) could be mitigated by blocking visible light and switching to sensors outside the visible spectrum that are unaffected. A porous, unstable surface might be workable in conjunction with something that makes flight difficult. $\endgroup$ – emo bob Dec 7 '16 at 20:14
  • $\begingroup$ Indeed it is! You can see why I had to turn to stackexchange. $\endgroup$ – emo bob Dec 7 '16 at 20:27
  • $\begingroup$ Possibly the visibility scenarios could be further rendered overly difficult by randomizing of environmental obstructions. Perhaps sandstorms, falling and bouncing hail, or frequently moving rocks could make it too complicated for sensors to get a stable reading via non-visible sensors. $\endgroup$ – Thom Blair III Dec 7 '16 at 20:28
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You could do a reverse Potemkin Essentially build a giant, barren desert set around the gate. Bury everything in the area underground, cover the whole lot with sand of some boring, low value material. Probe comes out, sees nothing but a darned large amount of worthless dust, and leaves.

Anything of value would be further out, or in extremely deep, underground monitoring stations.

Spreading word of how damn inhospitable the planet was, and stories of expeditions dying of boredom, heat or going broke would further discourage travellers, as would any expedition going out claiming to be unsucessful expeditions going home after trying to find something there.

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How about this: It opens up to what's like the Sahara Desert on Earth. It's huge, but it's finite, if you take the time to get past the hot sand, you'll get to places like Siberia, or America. And, possibly really hot, and with sandstorms. It seems uninhabitable, but you can get past it.

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    $\begingroup$ Welcome to Worldbuilding.SE, ATA! It seems that your answer would be well within what the OP considered "habitable", actually. The OP also made several edits to the question to clarify various things, so remember you can edit your answer anytime to improve it, add detail, and make sure it answers the question. Good luck! $\endgroup$ – type_outcast Dec 7 '16 at 22:58
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Plague. Vicious creature attacks - not like large creatures, but small ones that are impossible to defend against, like poisonous mosquitos or fleas.

Maybe it is already inhabited by another intelligent race who does not like outsiders.

Maybe the gate is in space or on a moon of a nice planet.

Arctic/antarctic/desert area. These could easily deter settlers near the gate. Speaking of arctic, it is cold there, really cold, like -40. The desert could be too hot, like really, really hot, like 190F hot or even hotter.

Top of a mountain like Everest or even higher. Air is too thin to live, no animals, no trees, no crops.

Tidal region. Maybe the moon is REALLY large/dense and causes huge tides on a huge scale so an area like Florida would be completely submerged at high tide. That would not be settleable. (Though this doesn't fit your criteria #2.)

Extreme gravity - like Jupiter scale. Settlers couldn't even be there long without being crushed (though this applies to the whole planet, unless the gravity is moon-caused.

Gravitation lensing from a near-ish star going supernova or a black hole creates a path of radiation and destruction in the orbital path of the planet so 2x per year, it is quite deadly.

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  • $\begingroup$ The first 2 violate 1) Nothing interesting, and the second one in particular would mean that the planet probably isn't colonizable. A gate in space or on a moon don't really present hazards, and you could just send a rocket through to get to the planet. The fourth and fifth ones are both easily survivable in a space suit. Gravity sufficient to impede colonization would necessarily affect the entire planet; a moon close enough to significantly affect the gravity would be torn apart. The radiation has potential, but seems like it would only happen once rather than being a recurring danger. $\endgroup$ – emo bob Dec 7 '16 at 19:12
  • $\begingroup$ Maybe I'm missing what you are looking for. Is it something like - Probes/explorers find an outcrop of unobtanium 200' from the gate. But, _____ is a problem, and thus stops the settlement - like that? $\endgroup$ – MikeP Dec 7 '16 at 19:29
  • $\begingroup$ Not quite. More like probes/explorers find a hazardous planet with nothing of much interest, and no real reason to believe that further searching would reveal something of interest, so they decide that it's not worth it and leave. Put something obscenely valuable in plain sight, and no matter the danger you can all but guarantee that some entrepreneur will undertake a near-suicidal mission to exploit it and get rich. $\endgroup$ – emo bob Dec 7 '16 at 19:34
  • $\begingroup$ The tides does not reject #2. So long as they can get to it during low tide they can still use it. Prehaps small islands at high tide would provide a path to increase the area. And it does not need a large moon, just a large flat area a couple of meters below high tide level. $\endgroup$ – Catprog Dec 7 '16 at 22:41
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Since these gates were build by some kind of ANCIENT RACE™, they're pretty likely to be real old. One specific one was build tens of thousands of years ago on a planet in the habitable zone. Since then, however, the local sun has burned through much of its mass and turned into a red giant, scorching this particular planet. It's not quite inside the sun but it's significantly closer than is healthy for organic life. However, the size increase of this sun moved the habitable zone a ways off and a planet that was an iceball before is now smack in the middle of that sweet spot for life.

When probes arive through the gate, all they'll find is scorched earth and deadly radiation without realising that paradise is just a short trip through space away.

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Turn up the core activity on your planet. This will create a larger magnetic field (we want enough to affect electronic computers) on your planet and increase the volcanic activity. Then place a star that is very active and frequently emits solar waves and radiation.

Your planet's magnetic properties should protect humans, but increasing the magnetic field enough should disable non-hardened computers making advanced species highly susceptible to the environment. Not to mention the solar activity in your system should be high enough that external searches would not know you're there from the "noise".

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TLDR; Make it wet and/or cold

To make a planet uninhabitable, yet still livable, the question of why they would stay needs to be answered. You've placed your colony on the terminator, so presumably there's something on this planet of value to the greater populations of people elsewhere. This is beside the point and presumably you've generated an idea of why they're located on this planet.

The best answer I could think of would be an large moon that causes tides on the hot side of the planet as it rotates around the planet, creating a water cycle. As it travels over the terminator in the form of clouds, it solidifies to a liquid (or solid if you want), and pours down in mass amounts on the gate area. Given either enough water or incredibly consistent water, flooding could be a serious problem for any would-be settlers.

Given enough people, requiring large area for crops could also pose an issue. Hydroponics requires indoor controlled spaces. With a sufficiently large colony, there would be a need for increasingly more space to house the food supply. Having constant rains or floods would not be good for crops.

There's also water damage to housing and terrain. If this sort of thing would be common to the planet, erosion will be an engineering nightmare. In some cases, the supports for any potential buildings would need to be meters deep, and possibly replaced every so often depending on the soil type you use. This holds true especially for softer rock like sandstone or commonly used limestone, especially if these are the prevalent building or foundation materials on your world. If you're looking for an even more common building material, concrete has its issues with moisture as well.

In general, lots of rain wreaks havoc on buildings and agriculture, both of which your colony needs to survive. Given enough of it, any colony may see the difficulties of surviving in such a wet environment where although it's possible to live, it comes with incredible difficulty.

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