# How can I have a habitable world on which life can not develop?

For my story, I need there to be a planet close to the home world of my species (in the same star system) that the species could easily get to and easily colonize, but that does not have its own life already on it. Assume that this species is very similar to humans.

I figured that the easiest way to do this would be to have a world in which intelligent life can survive easily (as in, humans could grow plants, walk around outside, breathe the air, etc. without any terraforming that can't be done in a few years) but on which life without intelligence will die out within a few thousand years if there is no intelligent life there to sustain it. Is there a way this can happen?

If necessary, it need not technically be a planet but rather an equally sized body (maybe a planet-sized moon around a gas giant).

• Your problem here is that intelligent life cannot survive without other life. There are more bacterial cells in your body than human cells! – Timpanus Mar 8 '17 at 2:20
• @Timpanus I meant that non-intelligent life without the presence of intelligent life will die out. I should have been more clear in my original question. – Jarred Allen Mar 8 '17 at 2:30
• Does life not have to develop, or just within a practical time period? The earth had a period of around a billion years where all that was going on was photosynthesis bounding molecules in the water. Your civilization could come and become extinct in that time period. So although life would eventually exist, it certainly wouldn't have to during the duration of your story. Even if you arrived in the last 100,000,000 years, so you had oxygen, it would still be long after your people arrived. – cybernard Mar 8 '17 at 14:01
• @FumbleFingers Thats not necessarily a great comparison. The human genome contains a lot more genetic information than any bacteria. I'd wager when it comes to total information, humans come out on top. – kingledion Mar 9 '17 at 0:37
• Your words don't work together. "Habitable World" in your context could be directly interpreted as meaning "Life Can Develop", so in that case your question contradicts itself. If you mean an uninhabited world that people could just land on, it's impossible. Free oxygen cannot survive on a planet without life, it's too reactive. It took a LONG TIME for plant life on earth to create enough oxygen to oxidize all the exposed surfaces and actually create "Extra" oxygen and make the planet truly habitable. – Bill K Mar 9 '17 at 1:43

# No...

Earth's environment has been shaped by the creatures that live on it. In particular, the oxygen in our atmosphere (and ocean) comes from and is maintained by photosynthetic life. If Earth had not developed life on it, for some reason, and humans came to visit it, it would have a nitrogen-carbon dioxide atmosphere, perhaps with some methane sprinkled in. Without breathable air, I doubt you would consider it habitable.

# ...unless you are terraforming

On the other hand, a lifeless planet in the habitable zone of a star would be pretty ideal for terraforming. Mars is in a pretty good condition to be made Earth-like, and it isn't even that close to Earth. An ideal planet for terraforming would have a strong magnetic field, like Earth, and a pre-existing dense atmosphere, preferably of nitrogen. If we found such a planet, terraforming would be easy (on the scale of terraforming).

A way to make this terraforming scenario possible is having multiple moons of about the same size in the orbit of the same gas giant. For example, the four Galilean moons of Jupiter all have a surface gravity of 0.13-0.18g and masses within an order of magnitude. If only one developed life; then colonizing (and terraforming) the other three might be the scenario you are looking for.

• Even then, with the scientific curiousity like humans, they'll find a way to change it anyways(e.g.: sciencealert.com/…). You have to find a solution for that too, which might be done be suggesting there is an extremely sensitive atmosphere, change it too much, bye bye life – Martijn Mar 8 '17 at 10:47
• You'd probably want a bunch of CO2 in the atmosphere, too, to have some source for oxygen. But then you'd have a ferocious greenhouse effect (quasi-Venus) so it wouldn't really be habitable. And if it was far enough from its star to be comfortable with the CO2 atmosphere, then terraforming it would likely make it too cold. – jamesqf Mar 8 '17 at 18:24
• Exactly, you don't need a planet that already has complex life, only a planet that is extremely suitable to terraforming. Though I personally would like to eat new food made from plants and animals on a different planet, killing the dream. – Necessity Mar 8 '17 at 22:09
• @jamesqf: Has the Earth ever been anywhere near as warm as Venus is? I think you might be overestimating the effects of greenhouse gases. (Anthropogenic climate change is projected to be very damaging to our planet and ourselves, but mostly because of how quickly it's happening (so that ecosystems can't adapt fast enough) and how much we have invested in soon-to-be-inundated locations. Those would not be issues for a hypothetical habitable-but-uninhabited planet.) – ruakh Mar 9 '17 at 0:10
• @ruakh: No, but the sun was only about 70% as bright back when the Earth had a reducing atmosphere with a lot of CO2. (See "faint young sun" for more.) As for AGW estimations, even notice that the predictions only seem to go out about a hundred years or so? For what Earth might be like after a few more centuries, look up the Permian-Triassic Extinction. – jamesqf Mar 9 '17 at 5:39

If you're willing to have a world without complex life but the possibility to support it, a fairly easy way to do this would be to have the other planet be in its equivalent of the Precambrian Supereon.

Have the evolution of life occur 500 million to 1 billion years after the planet from which your intelligent life originates, and there will be very little complex native life. Time it right, and you'll have explorers arriving on a world with little to no life on land, and simple multicellular organisms as the most complex form of life. Have your explorers only bring domesticated animals that would die out without their constant support, and you have a world where complex life only exists alongside intelligence.

With water in the oceans, oxygen in the air, and no complex native life for another few hundred million years, you'll have exactly what you want!

• +1 Fortuitous timing is the simplest, most likely reason. It's also why it's so difficult to have multiple technological civilizations at the same time. – Spencer Mar 8 '17 at 16:48
• It doesn't even need to be 500 million years if your intelligent planet never had a large lizard era (that took up 200-300 million years on earth where intelligent life could have developed). – ratchet freak Mar 9 '17 at 13:14

The planet has a highly eccentric elliptical orbit.

It's a paradise when it's far from the star, which is most of the time. But once a year (planet's year, not necessarily similar in length to Earth's year), it will pass close to the star and become hot enough to kill all life. Those passes will be very quick compared to the far sides of the orbit, that's how eccentric elliptical orbits work. But they will be long enough that no life can survive it.

Unless a technological civilization runs an air-conditioned Noah's Dome for the duration.

Optional: artificially hardened plants (genetically engineered perhaps) actually can survive outside the dome, but are not indefinitely viable without tending, due to some other biological limitation. Maybe they can't reproduce on their own, similarly to our consumer-grade bananas.

• @ L.Dutch: hum, I'm pretty sure airco works when the outside is hotter than the inside, otherwise what would be the point of using airco instead of a window here on Earth? If the outside is really hot, keeping the inside cool would take a big machine and lots of energy, but no new physics. – Emilio M Bumachar Mar 8 '17 at 14:41
• @L.Dutch: Indeed I made a poor choice with "blazing inferno" . "Hot enough to kill all life" will be better, I'll edit it in. – Emilio M Bumachar Mar 8 '17 at 16:40
• Air Conditioning needs to take air in from the outside and make it hotter, and in the process it also takes air from the inside and makes it colder. As long as your air conditioner is capable of operating in "blazing hell" temperatures, and can still raise the temperature of the outside air it brings in, it will still be able to cool the dome. – Cort Ammon Mar 8 '17 at 17:29
• Given the huge difference in scale between time as perceived by individuals and by evolving species, you can potentially make the disasters much worse and also much further apart. Ten thousand years between massive bursts of sterilizing radiation (as the parent gas giant periodically un-knots its magnetic fields, perhaps?) would count as perfectly habitable to an established civilization, but still render the world inappropriate for evolution of large life forms. Lengthening the period also gives them time to fix up the atmosphere, and maybe just wait out the disaster on the home planet. – Leushenko Mar 8 '17 at 18:21
• @L.Dutch No, that isn't true. That would make A/C quite useless, not to mention simple home fridges. All you need is to have the working fluid be hotter than the ambient temperature at some point in the process - and there's many ways to do that. Both fridges and A/Cs get much hotter on their radiatiors/convectors than the ambient air, allowing them to pump the heat against the temperature gradient. The same mechanism allows A/Cs (heat pumps) to heat the insides of a house for less energy than is actually added to the system - they "steal" the heat from the cold ambient air. – Luaan Mar 9 '17 at 9:37

The only way I can come up with: habitable world used to have life, but it developed a sentient species that created a weapon as part of a war that killed off all of that life. When your species arrives, they have the technical chops to stop the life-killer, but without their constant vigilance, the killer comes back online. I'm thinking some sort of actively adapting mechanized system that your species cannot completely shut down (maybe they cannot find its automated manufacturing base).

• Or a trillion or two of killer-nanites. You might be able to clean a small area using strong electromagnetic pulses, but you can't feasibly clean the whole planet, and there is always a chance of recontamination of previously cleared areas. – Philipp Mar 8 '17 at 10:01

Life doesn't just die without a reason. So maybe there's a rolling cataclysm that happens every couple of hundred years which wipes out most life on the planet. And the only thing stopping it might be your humans. But, as Ian Malcolm was wont to say "Life finds a way."

For conditions to be right to sustain humans, there has to be the right mixture of oxygen in the air. Before life on this planet, the air was not breathable for us. Plant life is what changed things and made it sustainable. For a rock in space to have an atmosphere, the right mixture of oxygen and all that sort of thing...you really do need life, it doesn't spontaneously happen.

If your colonists are growing things, there has to be, to put it bluntly, poop in the soil. And, if humans are living there, they will be contributing life and organisms to the planet.

If they are raising animals, those animals will also be contributing to organisms on the planet on a microscopic level.

That doesn't mean that conditions aren't harsh, and that there won't be very little life.

Terraforming HAS to happen in your scenario, but life is not going to just give up and die, unless what they brought with them isn't enough to sustain a population. Even then, life will develop. It could look to the naked eye to be devoid of life, but...so did the dust bowl at one point...

What you might do is look at the Precambrian era--there's life, but it isn't complicated, and will give settlers what they need in terms of oxygen. However, that life will not disappear once your settlers are dead or gone. It will continue to adapt and eventually become complex, but that will take...oh...billions of years.

But, happy news in this article you can make it earth. That's right. Earth.

There's a little thing called The Lomagundi Event that happened here on earth. Prior to that the oxygen levels were pretty low, but suddenly, on a massive scale, they spiked. (Could it be your humans? Using a plankton soup in the oceans to push the oxygen levels up to a level they could use?)

“The Lomagundi Event has recently been proposed as an interval of rather high oxygen levels, perhaps even nearing modern values,” says Michael Kipp at the University of Washington in Seattle.

There is no consensus on why our planet briefly gained and then lost an oxygen-rich atmosphere.

And during this time, evolution may have been bumped a little, but it soon dipped back down again. Because so too did the oxygen level. And nobody really knows why.

It's called an oxygen oasis in time.

TLDR: Don't make your planet devoid of life, just make it uncomplex life, and the small population of animals the humans bring with will die off, leaving complex life to continue to evolve over billions of years, but certainly not present a thousand years after they leave, when the planet's artificially rizen oxygen levels plummet. Look into the The Lomagundi Event.

Life as we know it started in water. Since then it has adapted to all sorts of conditions, but in the beginning it was complete dependent on water to survive.

So, make a planet that is much drier than ours. No oceans, few lakes. Life as we know it will not develop. We don't know if there are other forms of life that might appear in these circumstances, but you can easily say that "Well, it didn't happen here anyway." without anybody saying you are wrong.

IMPORTANT: The rest of this answer is scientifically wrong. Leaving it here as a warning to others. See comments for details.

As others have been saying, a life-less planet wouldn't have free oxygen, so you need some terraforming. Fortunately this is easy. Just spread plant seeds around!

You will probably need to engineer a plant for the purpose, since the desert species we have don't grow very quickly. But hey, this is science fiction, your bio engineers will have know exactly how to sequence the perfect gene sequence for a plant that does the job.

Of course, when the plants have done their job, they will have converted all the CO$_2$ and they might starve and die. It is tempting at this point to assume that some of the plants would die and rot, releasing CO$_2$ back into the atmosphere, creating a cycle.

Unfortunately, things don't rot on their own. You need bacteria for that. So please remember to seed some of those too. Otherwise you might get fires, which will also release CO$_2$ but will generally be nasty in other ways.

Right, so you have oxygen. Time to add colonists! They are likely to establish their colony on the shore of one of the few lakes there is since humans are still pretty dependent on water.

Time for you to make a decision: Is the original terraforming plant edible? If yes, people have landed in paradise. If no, they have a tough job ahead of them. The same properties that made the plant work quickly as a terraformer also makes it a bad weed. The colonists will curse the short-sighted engineers who were only looking at the short term.

At this point somebody will suggest releasing a few herbivores, e.g. rabbits, to control the plants. Others will remember what rabbits did to Australia and say NO.

• Oxygenating a planet takes something on the order of hundreds of millions of years after the start of mass-producing oxygen; you won't have significant amounts of free oxygen in atmosphere until everything on the crust that can easily oxidate has done so, i.e. all the iron in the surface minerals has converted to iron oxide. On our planet, waiting for "when the plants have done their job" took around a billion years (en.wikipedia.org/wiki/Great_Oxygenation_Event) because of all the oxygen sinks. Even if smart terraforming magically does it thousand times faster, it takes million years. – Peteris Mar 8 '17 at 12:36
• @Peteris Agree, the only way around would be to find/manufacture a few quadrillion tons of oxygen in space (from a watery moon?) and then dump them on the planet. – kingledion Mar 8 '17 at 13:00
• Plant seeds cannot grow without oxygen unfortunately. They have the food in the seed, and they need oxygen to produce energy from that in order to grow. After that they can do photosynthesis and produce oxygen. And even then they will need enough oxygen during nighttime. They do not have a mechanism to store oxygen for later use since they do not need that on earth. – Nuri Tasdemir Mar 8 '17 at 14:56
• Ouch. Here you people come shooting down my nice theory with ugly facts. I have read too many SF novels were terraforming was easy. Taking a few years, certainly, but not millions. Oh well, nice theory while it lasted. – Stig Hemmer Mar 9 '17 at 8:20

What you're asking for is a puzzle box. You want a planet which has some planet-wide problem which makes it inhospitable that can be solved by intelligence, and must be continuously solved by intelligence.

The funny thing is, it's really hard to define what intelligence actually is. The harder you try to define it, the harder it is to create some naturally existing planet-wide issue which depends on it.

One approach would be to have it be an artificial problem. Make the planet a quarantine planet with a dead-man's switch. Every few years, a console on the planet displays some difficult problem like a discrete logarithm. If there is no intelligence to punch in the answer within a time period (perhaps a year), the device activates and bathes the entire planet in unholy amounts of ionizing radiation to sterilize it.

Such a planet might be highly useful for xenobiological engineering experients. Any violent species which seeks to leave the planet needs to reach a level of intelligence to solve such a math problem within a year, or the device wipes the experiment out.

Another approach would be to have a planet that naturally wants to be in an inhospitable orbit, but may be nudged out of that orbit on a regular basis by the intelligent species operating the planet. This would be simpler, but comes with the issue that an intelligent species couldn't live "easily" on the planet. They'd need to first expend the energy to move it into a hospitable place first.

High amounts of UV will do it. Single celled organisms would be sterilized, whilst those with a dead skin layer would be relatively fine, though likely subject to higher amounts of mutations and thus skin cancer. You can adjust the dose to keep the whole planet sterile, but perfectly livable for any organism that has developed sunglasses and clothes, and they can grow plants within a greenhouse (if you have dosed your planet with very large amounts of UV continuously). If you need a mechanism to explain the UV, go with high amounts of X-rays hitting an earth type atmosphere, resulting in down-conversion by the atmosphere to UV radiation.

If necessary, you can construct the history of your planet so that it develops complex life, that is then killed off by successive gamma ray bursts nearby. https://en.wikipedia.org/wiki/Gamma-ray_burst. You will need more than one, of course, because a single gamma ray burst will only effect one side of the planet.

Additionally, how sterile do you need it? Your story may be able to function with most complex life killed off, and the only life on the planet are single celled organisms around a deep thermal vent.

• Keep in mind that Earth already has a good bit of UV hitting it, and yet is not devoid of life. Ratcheting up the intensity would kill a lot of current organisms, but it wouldn't definitionally prevent other, better-adapted organisms from thriving. Deinococcus radiodurans is an example of an extremely UV-tolerant organism. (It's the honey badger of radiation.) – Sneftel Mar 8 '17 at 18:35
• Understood, it is just a matter of scale or dose. D. Radiodurans gets its resistance by keeping multiple copies of it's DNA and repairing the damaged DNA. You can still kill it with UV, It just takes a larger dose. ncbi.nlm.nih.gov/pmc/articles/PMC3685888 If one subscribes to the idea that life developed from single celled organisms that were either seeded or spontaneously developed, preventing simple single celled organisms from even developing, or killing them all off once they have developed, keeps the planet sterile but provides a habitable one to explorers. – Mauser Mar 8 '17 at 20:35
• Without the en.wikipedia.org/wiki/Ozone_layer the Earth would fit this scenario. Perhaps the colonists could keep it habitable by creating ozone or preventing it from being depleted? – JollyJoker Mar 9 '17 at 11:57

You may be overthinking this. We currently do not know how life forms. There are theories but none have been proven. Since we do not have an accepted theory of how life originates we certainly cannot make any informed claims about how likely it is for life to come into existence generally, nor specifically how likely the formation of life would be in a given environment.

Based on this there is no reason to believe that you are likely to find life on a planet just because it has the physical properties that we think are important to life (even it our assumptions about the conditions necessary for life turn out to be completely correct). Based on our current scientific understanding life can be extremely rare, extremely common, or anything in between. We still only know for sure that life originated once in our universe, and it may be incredibly unlikely regardless of the planetary environment.

So for your story you could develop it that as science progressed it was discovered that life is incredibly unlikely to form in any environment.

Or if you want the planet to be inhabited by things like plants but no sentient beings... then your problem is already solved. There is simply no scientific reason to suspect that intelligent life will evolve wherever life in general evolves. An astounding set of conditions must come in to play for intelligence to evolve (and we don't even know what all of them are yet), and there is no reason to assume that these conditions would be met on any or every planet with life.

• I was going to write this exact thing. I think you are bang on. In fact, the scientists of the other planet could be astounded to find this new planet with everything needed for life, yet life didn't evolve, and wondering why not. And the plot twist centering around how like a super-saturated fluid suddenly crystalises, maybe their arrival at the planet "seed" evolution of life there at an amazing rate and wierd stuff unfolds... – GreenAsJade Mar 10 '17 at 0:05

To produce life, you have to produce a lot of thermodynamical processes. It is the result of the second law of the thermodynamics. Life can't get energy from heat, it can get energy only from the heat difference.

If the planet has no daily and yearly cycles, then it will be a still, silent, maybe warm planet - without weather, without anything. It will dead, even if there is water on it.

Maybe the photons coming from the Sun could start some thermodynamical process, but the photosynthesis is a very complex process, plants won't develop on such a planet randomly. The first unicellular life on the Earth were chemosynthetical, it is the only method on which a life can start.

To avoid the planet to have daily and yearly cycles, it should be a tidal locked planet orbiting on a very regular, circle-like trajectory around its star.

It will result that the planets terminator can be habitable, although its Sun-directed hemisphere will be hot, and the opposite will be cold.

A little bit of weather will exist also there, because an little bit of atmosphere circulation will happen also there (although on different reasons as in our case). It can be the source of thermodynamical processes, if it is chaotic. Make it regular, ideally by a well-designed mountain system.

Such planets can exist nearing red dwarfs, they may be even more as our Earth-like ones.

It sounds too easy, but one of the options is to make your planet inhabitable. Remove at least one of the critical habitability factors and you'll end up with a planet that is unable to sustain life as we know it. If you don't go too far you'll end up with a planet where technological species can live with the help of life-supporting technology.

The simplest example that comes to mind is an ice planet

The planet that contains all the necessary elements and an abundance of water. Maybe even a breathable atmosphere. Intelligent life will just need to figure out how to heat it properly. They can live for an extended period under a heated dome, a cold planet still will get enough energy to support the limited population. But life will freeze as soon as they leave and/or their heating system shuts down.

As far as we know, life can't develop on a planet without liquid water.

The lifeforms produce toxic substances which would accumulate and make life unsustainable if not counteracted.
The only available means to neutralize the toxins involve transport mechanisms which are only performed by the more intelligent caretakers of the habitat.

Would that fit your parameters?

• Doesn't really work. What counts as a "poison" is relative once evolution is in play. Oxygen was poisonous to the first lifeforms; most hid from it under water or soil, but eventually some evolved to take advantage of it instead, and here we are. Pump the atmosphere with cyanide long enough, rather than the end of life, you'll just get cyanide-breathing animals. – Francesco Dondi Mar 8 '17 at 16:40
• @FrancescoDondi These toxins kill sooner than any adaptation or the like, perhaps? Et cetera. The question did not concern the viability of the planet beyond that boundary condition. – can-ned_food Mar 9 '17 at 20:49

An artificial "designer" planet (like Earth in Hitchhiker's Guide) could have been built by an advanced alien race, designed to support life but never populated for whatever reason. This could have been done as much as a billion years before your story begins, and life could still not have evolved naturally. After all, it took nearly a billion years for the first forms of life to appear on our planet, and even then it was primitive and microscopic for another 3 billion years. It might never evolve at all, especially if the planet was designed to lack all the conditions favorable for the spontaneous formation of primitive life.

A planet with a very short solar exposition caused by debris or giant planets. That would actually mean that you could still create life with artificial UV light. But with no intelligent life to maintain it all life would eventually die out. Of course life can also exist without light but there is also the possibility that life in this life never evolved in that way. There are a lot of explanations of why the planet now lacks sunlight you just had to pick one.

• Life developed even in places with no solar exposition at all. And usually UV light is dangerous to life. – L.Dutch Mar 9 '17 at 6:49
• @L.Dutch i did stated that. Still life in those places could not exist or because the way they evolved they would require other lifeforms that would require light or those life forms could never have been a thing to begin. "And usually UV light is dangerous to life." that is like saying oxygen is usually dangerous to life because in a ambient too concentrated with oxygen things would die out. UV is indeed dangerous in high doses but it is also necessary, some farmers use IR and UV light to grow crops so you know... – PeterKima Mar 9 '17 at 9:26
• I was referring to i.e. black and white smokers, which can sustain life not counting on light at all. Only the chemical energy provided by the hydrothermal activity en.wikipedia.org/wiki/… – L.Dutch Mar 9 '17 at 9:33
• @L.Dutch anyone who had biology knows what you are talking, that is why in the answer i wrote "Of course life can also exist without light". Still the UV thing is bluntly misleading as you stated it. – PeterKima Mar 9 '17 at 9:40

Maybe an scenario where a planet is constantly pelted by meteors having oxygen, nitrogen and other frozen gasses? the planet could have the ability to gain a hospitable environment over many millennia, and kill any life that may attempt to flourish.

• can you try to expand your answer? – L.Dutch Mar 9 '17 at 6:50

## You could have a planet with a weak magnetic field.

If it were just strong enough to stop the solar wind from stripping the atmosphere, but not strong enough to stop cosmic rays, then the constant bombardment would be devastating to life.

If you combine that with some periodic source of ionizing radiation (e.g. a astrophysical jet passing through part of the planet's orbit) then you would really be wiping the slate clean on a regular basis. While even this inhospitable environment would have some life (e.g. around deep-sea thermal vents), it could be very minimal and primitive (e.g. single-cellular). Colonizing aliens could boost the magnetic field in just a few years by building a super-conducting loop around the equator.

You could also add elements of geomagnetic reversal to make things more interesting. E.g. every few thousand years, a geomagnetic reversal happens while passing through the jet. All life is wiped out.

It seems to have happened on Mars according to this new scientist article. It did not get into details of how that happened. Oxygen would oxydze any pre-biotic molecule and life will not evolve. You still have to comment in your story how near is the atmospheric composition to that of their home world.

Mars, having a weak magnetic field, may have its water molecules dissociated and broken down into oxygen and hydrogen. Hydrogen is stripped by solar wind and oxygen is left behind or stripped more slowly. Oxygen reacts with soil minerals to create the oxydes giving Mars its red colors, but over time soil minerals will be saturated with oxyen and it will build-up in the atmosphere.

## protected by MołotMar 9 '17 at 6:22

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