Imagine a terrestrial planet extremely close to its star (assume it's identical to the Sun to make this simpler), maybe a bit like Kepler-78b but with an orbit of only about a week, for the purposes of a story. I'm thinking of an advanced alien species that has plopped a small base there so they observe the development of a nearby world (again, for sake of simplicity assume it's identical to earth in composition, size and orbit) and its species until their technological development allows them to be detected (it's a bit like the prime directive). The planet is generally like Mercury and the amount of inhabitants in the colony is minuscule, they can pull enough water, power and other stuff needed from the surrounding environment to keep themselves going for a long time.

What I want to know is what kind of challenges could make this idea unfeasible? First, how close to the star would a tidally locked world (assuming similar characteristics to mercury) have to be before the heat from the sun starts to bleed over onto the dark side and make it unfit for colonization? Could water be found on the dark side? Would there be particular effects from the star, ie tides, radiation, that could scupper this?

Related questions I was thinking about:

  • Would this world be detectable from earth prior to the invention of the telescope?
  • Would it be particularly difficult to study and image the surface using today's technology?
  • Can I assume that a lander and orbiter would be totally out of the question for many decades too? Would the sun's influence somehow interfere with communication?
  • 2
    $\begingroup$ "Would there be particular effects from the star, ie tides...", on a tidally locked planet? Well, no. No there would not. $\endgroup$ – candied_orange May 27 '15 at 7:41
  • 1
    $\begingroup$ If there is no atmosphere, the dark side of the planet would be quite cold until it was close enough to liquefy completely. Mercury is easily visible in the morning and evening skies with the naked eye (as long as you have a good horizon), so yes, it would be visible without a telescope. An interesting idea: A planet close enough to the primary that it was not visible even at sunrise/sunset due to the brightness of the star. However, if your primitive alien world has eclipses, it would show up then and only then, while the star was blocked. $\endgroup$ – Jason Patterson May 27 '15 at 16:26
  • $\begingroup$ CandiedOrange when I said tides I meant the whole planet being flexed by the sun's gravitational influence. $\endgroup$ – Khwarezm May 29 '15 at 2:25
  • $\begingroup$ @CandiedOrange Well, there would be libration and I guess it can produce some tides. $\endgroup$ – Radovan Garabík Feb 19 '16 at 17:44
  • $\begingroup$ @RadovanGarabík if we think of a non-tidally locked planet producing tides like a car driving over potholes and a tidally locked planet as a car stuck in a pothole then libration would be nothing more than the idle car rocking back and forth within the pothole. Sure there is motion but I don't see any transfer of energy to deform the planet. If you think this can produce tides please tell me how. $\endgroup$ – candied_orange Feb 20 '16 at 1:44

I'd be thinking something more like Venus. That planet is known as Earth's sister or twin. Of course we know it as pretty uninhabitable due to raining acid, thick atmosphere, and very hot surface temperatures. But it has an atmosphere, it has rocky landmasses, it had "water", etc. So in many respects it is very similar to Earth. Venus doesn't have any moons, but other planets out there could have close moons which could make them "tidally locked." Mercury is gravitationally locked with the sun in a unique way; "It rotates on its axis exactly three times for every two revolutions it makes around the Sun." Furthermore, "An observer on Mercury would therefore see only one day every two years."

If Venus had much less greenhouse gasses however, the temperature would drop, and the poles would eventually cool to a more reasonable level. So if there were a planet like Venus (but "tamer"), not exactly habitable by the society's consensus (and perhaps their particular physiology), but totally habitable by the aliens... then they could use that. Perhaps even work to terraform it by scrubbing out some of the greenhouse gasses to make it a little cooler. (Interesting result if society notices this, and starts looking at it more closely...)

Incidentally, the reason Humans likely haven't considered robotic terraforming of Venus is that it doesn't have a magnetosphere. Without that, solar radiation will reach the surface and strip off any water present (as well as slowly kill living things (as we know it) via radiation poisoning.

Keep in mind that days are determined by the planet's axial rotational velocity, and may vary greatly, as we've seen with Mercury. Years are it's orbital velocity around the star, and as @DJMethaneMan illustrated, do not have to follow the 365/1 Earth ratio at all. (But of course, planets with bigger orbits will generally have longer years, simply because the distance is further -- and likewise, smaller orbits = shorter years.) Here is an interesting comparison of the lengths of a "day" on our various planets.

Would this world be detectable from Earth prior to the invention of the telescope?

World, yes. Colony, not a chance.

Would it be particularly difficult to study and image the surface using today's technology?

Not really. We've already sent probes to peer at all of our planets. we have some great scans and photos. But they are orbital photos; without very high resolution, there is little hope of "spotting" a small settlement. "Spy satellites" are generally in a very low orbit, and use extremely high resolution.

Can I assume that a lander and orbiter would be totally out of the question for many decades too?

I can't answer this. If humans detected something on Mars 25 years ago, it would already have been investigated many times, likely with astronauts on their way there now for a little "meet and greet."

Would the sun's influence somehow interfere with communication?

Very likely. Solar radiation "flares", even protected by the Earth's magnetosphere, still manage to get through some and wreak havoc with many electronic devices occasionally. On a planet with no magnetosphere and closer to the sun, it might very well destroy electrical things outright, or "burn" skin, like a chemical or heat burn (which won't heal.)

  • $\begingroup$ Thanks for the answer, I just wanted to clarify a few things in my thinking. When I'm referring to an alien colony, I mean REALLY small, only a couple of hundred in number max. I assume that these aliens have extended their lifespans indefinitely if they've mastered interstellar travel so a critical mass for long term reproduction is not really a problem, they're simply there to observe and report. As a result they don't make many demands on the surrounding planet and live in a kind of enclosed habitat without having to worry about lack of atmosphere. $\endgroup$ – Khwarezm May 29 '15 at 2:07
  • $\begingroup$ As for the planet itself I rather stupidly didn't clarify that its year I put up was in Earth days rather than its own. Basically this world has no days so the dark side is nestled in endless darkness. Since that's the case I assume that the planet must be much closer to its star than anything in the solar system, though I might be misunderstanding how tidal locking works! The cold half never gets exposed to sunlight, similar to some craters on Mercury, so they could harbor some volatiles that would have been burned off Sunside billions of years ago, maybe delivered by some large comets. $\endgroup$ – Khwarezm May 29 '15 at 2:13

Even a planet very close to the parent star like Mercury should be visible with the naked eye (Mercury was one of the seven planets visible to ancient people's, and was placed in the pantheon of Greek gods as a result), but until the planet under observation undergoes a scientific and industrial revolution, they won't be able to make out any details about the sun locked planet, so the observers are safe enough there.

As for how they will live, as an advanced space going species, they will have the ability to dig and mine for resources not only on the base planet, but anywhere in the solar system, and as long as they are reasonably careful, without much risk of detection. Even solar sails wold not be visible in the night sky of the planet under observation unless there are a vast fleet and they are operating in or near the Earth-moon hill sphere (using our own solar system as the example).

If the planet where the base is located is tidally locked, then a base on the far side will be cool enough (on an airless planet it will actually be quite cold) to operate without much more than the sorts of systems needed for ay deep space craft. If the base planet rotates, then the best solution would be to tunnel in at the poles, where the amount of sunlight can be limited and the base will be insulated from most of the solar radiation without undergoing cyclic extremes.

The key issue of a base on a located close to the primary is the speed of rotation makes doing any sort of observation quite periodic and limited in scope. Mercury orbits the Sun every 88 days, and tidally locked planets around distant stars have been recorded to orbit even faster. If you want to keep something under observation, you want a clear line of sight and the ability to observe for long periods of time without interruption.

  • $\begingroup$ Just to check, I used Kepler-78b up there as an example since it is 40 times closer to its star than mercury (though the star is 27% smaller) and orbits in only 8.5 hours. This is all assuming the info in the wiki article is correct, but if it is wouldn't the glare from the star have a much larger effect there than for Mercury in terms of detection from an earth equivalent distance? $\endgroup$ – Khwarezm May 29 '15 at 2:23
  • 1
    $\begingroup$ There are a lot of factors that we don't know, so you may well be correct that the planet won't be visible to the naked eye the way Mercury is. OTOH, the planet may be visible because of the dark spot on the sun as it transits the star every 4 hours or so. How this works in your setting is up to you, of course. $\endgroup$ – Thucydides May 30 '15 at 2:15

You don't even need a tidally locked world. Get close enough to the poles and dig in and Mercury is comfortable. The temperate band is some distance from the poles (the poles themselves are way too cold!) so you could go even closer than Mercury and still have a suitable observation base.


While Mercury is not tidally locked in that it still rotates on its own axis, its days are almost as long as its years (its day being 58 Earth-days and its year being 88 Earth-days). This is the closest thing to hands on experience we have with something similar to what you want. On Mercury the average temperature on the dark side is -300 degrees fahrenheight or -184 degrees Celsius.

Since it is speculated that Kepler-78b has no atmosphere (due to the heat on the dayside) there will most likely be no water on you world. Any H20 would have to last for millions of years while constantly evaporating while exposed to the sun and re freezing on the dark side. I am not a scientist, but I think the radiation being emitted from its host star would strip the Hydrogen Atoms from the Oxygen atoms as well. Any ice would have to be buried underground to survive the sun, however the "lava line" on this planet is probably extremely close to the surface.

Midday going to be very hot. How hot? I don't know, but I would look to Mercury for an example. At its hottest Mercury is hot enough to melt tin and zinc. 427 degrees Celsius. I don't see how a base that is not mobile could survive those temperatures.

"Would there be tides, radiation etc that could scupper this?" If there was an ocean, yes. But there isn't, so no. I would see this being a very active world with many earthquakes and volcanoes, however.

The radiation would, as mentioned earlier, strip the atmosphere and separate Hydrogen molecules from Oxygen. If your colony was a glass dome, the glass would melt and your people fried when the planetary year passes (after all, the dark side is only dark for half a planetary year/solar orbit).

  • $\begingroup$ If my sentences are a little scatter brained, please excuse me. Semester exams are today and I am more than a little nervous :) $\endgroup$ – JDSweetBeat May 27 '15 at 15:46

Well for one thing, you're gonna have to consider the kind of atmosphere this planet has. The air has to be thick enough that the nearby star won't strip the planet off of it's atmosphere and it should be thin enough so that the air won't trap heat. The moon barely has any atmosphere due to it being locked into it's current form right now, not spinning and all.

Second, unless if the water is located deep underground where the star's heat won't evaporate it immediately then having water on the dark side is plausible. Even if it's on the permanently dark side of the planet, from the distance it has with the star surface water will not stay a feature.

Third, a colony living in this planet would have to either have access to metals that has insane melting points or be living underground to reach said water pools. When we tried studying Venus with a probe, the probe melted in under an hour. So whatever establishments the race you're gonna put in here must have better materials than we currently have available.

Overall, this seems to be a planet suitable for a temporary settlement for mining and/or gathering information. Living in this place is another matter altogether.


While the dark side of a planet is very cold that's actually easier to deal with than heat. Space is an excellent insulator so the only heat losses are due to radiation into vacuum which are easily manageable.

It's likely a base would be placed somewhere on the planet near the sunlit side though for two reasons. The first is that by picking the right location the external temperature can be appropriate for the base. The second is that solar power generation would be a tempting prospect and by being nearer to the sunlit surface it would be more practical to gather and use that solar energy.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.