In an intergalactic federation, space ships are extremely common. They run supply routes, defend planetary bases, and transport colonists to new worlds. To aid in these adventures, most ships would follow a defined path in space. It would make sense to only fly through charted territories, so random asteroids and pirates are not as much of a problem, right?

In this scenario, shady people, like pirates, would want to travel off the beaten path. Mostly to avoid detection and prey upon unlucky travelers.

Most of the galaxy is in a constant state of expansion. This means that people are always looking for new real estate, even amongst the most inhospitable environments. Even if the planet-in-question is far away from any current civilization, eventually it will be discovered. But currently, it is a decent distance from any operations.

Even so, those pirates or lost travelers could potentially find their way near the unmarked planet. With these points in mind, what is the probability of a ship finding the planet?


  • The planet is not labeled on any galactic maps
  • Most ships are equipped with planetary scanners (they can see planets from just outside the system)
  • The solar system where the planet is located is perfectly normal, nothing unusual
  • The planet/system does not contain anything of value
  • Most planets already discovered have resources and can support colonies/life

Bonus question: If the planet had life on it, would that affect the probability of accidental discovery?

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    $\begingroup$ What is the method of flight? Do they take tens or hundreds of years to travel between star systems or do they somehow jump from one to the next? Can the jump go wrong - see Battlestar Galactica or Vatta book series. $\endgroup$
    – Hukk2010
    Commented Apr 25, 2021 at 7:43
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    $\begingroup$ It's about the same chance as a specific tree on earth being touched by a human. There are many humans. The tree is not "hiding". Humans go everywhere in their millions. Yet there are many trees that grow, flourish and die without ever seeing a human. Because people tend to stick to familiar places, follow familiar routes, and most see no benefit in going into the woods and finding trees. $\endgroup$
    – PcMan
    Commented Apr 25, 2021 at 9:40
  • $\begingroup$ Given the infinite improbability drive... $\endgroup$
    – pipe
    Commented Apr 25, 2021 at 16:50
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    $\begingroup$ it doesn't make sense that pirates would keep off the trade routes. if they want bounty, they'll be near the densest routes. there's a reason piracy is a problem in the Red Sea. $\endgroup$
    – ths
    Commented Apr 26, 2021 at 10:05
  • $\begingroup$ Short answer: basically impossible. Space is 'vastly hugely mind-bogglingly big' and if you point a random direction into the night sky and then travel that way in a straight line for a couple hundred thousand light years, you're not going to run into anything except interstellar medium. $\endgroup$
    – Dragongeek
    Commented Apr 26, 2021 at 11:38

4 Answers 4


Frame Challenge

It's not a ship that would find a planet, it's an observatory.

Keep Watching the Skies

Right now, we've found 4 341 planets outside our solar system, and we've only barely sent one space ship outside of same. Even with many space ships, people wouldn't be sending ships to go find planets. Even in Star Trek, stellar cartography is mostly handled by enormous telescopes.

So that's what your planet would be hiding from. An enormous space telescope, probably built a long way from the star of its solar system, gradually cataloguing all the stars in the night sky. Watching for the dips in intensity that indicate a planet, and the spectral lines indicating what its atmosphere is made of. We can do that now with space and ground-based telescopes. Any civilization that has starships is going to have much, much better telescopes that are constantly on the lookout for where their next scouting team is going to be sent.

So then that's the challenge. The pirates happen to find a planet in an as-yet unmapped part of space... but the cartographers aren't sitting still. So the question is - how good are the telescopes, and how many of them are there? There are approximately 200 billion stars in the Milky Way. Assuming a Cartographic Observatory can process ten a day, it would take fifty million years to work its way through them all, and 25 million to find a particular planet at random. But if it can work through ten thousand a day, and if there are a thousand such installations... the pirates' secret planet's days are numbered.

Side Note: Space is Big

It would make sense to only fly through charted territories, so random asteroids and pirates are not as much of a problem, right?

Pirates may be a problem. Asteroids are not. Our solar system's asteroid belt is pretty dense as far as such things go in space. The average distance between any two objects in that belt is approximately 966 thousand kilometres.

There are not (indeed, cannot be) asteroid belts as dense as those pictured in Star Wars, because such a belt would aggregate into a planetoid or planet, unless it was a very, very recent phenomenon (Alderaan, for example).

So if you choose a direction at random in the sky, and fly your rocket in that direction for twenty lightyears, the odds that you hit anything of note once you leave the mess in Earth's orbit behind are astronomically (haha) low.

If you want a reason for people to take particular routes, it's best to have it associated with how you handle FTL - because asteroids, nebulae, and other space-borne objects are not a reasonable threat.

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    $\begingroup$ That being said, if the planet circles a sun such that the "pole" of the sun faces us, our scanners are going to have trouble detecting that $\endgroup$ Commented Apr 26, 2021 at 0:55
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    $\begingroup$ @MooingDuck - nah, in that case the sun's wobble can still be detected, giving length of orbit, mass of the planet, and proximity to the star. Plus, with a big enough lens and an occulter, it's possible to see the light reflected from the planet itself. $\endgroup$
    – jdunlop
    Commented Apr 26, 2021 at 7:22
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    $\begingroup$ @Hobbamok - perhaps I'm not getting the geometries quite right, but an orbit perpendicular to the telescope/star line would make the wobble in the star's motion even more pronounced. NASA claims 837 exoplanetary discoveries via this method, without a transit. $\endgroup$
    – jdunlop
    Commented Apr 26, 2021 at 16:32
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    $\begingroup$ @jdunlop: I think Hobbamok was envisioning a situation where we were viewing the orbit "from above", so that it would look like a circle on the sky if we traced out its path. In this case, the radial velocity of the star towards or away from us never changes; and the method you linked to (with 837 discoveries) requires a change in radial velocity (leading to a Doppler shift of the light.) Such planets can, however, be discovered via direct imaging (51 planets so discovered) or astrometry (1 planet so discovered, not including the spurious "discovery" I mentioned above.) $\endgroup$ Commented Apr 26, 2021 at 23:24
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    $\begingroup$ @jdunlop: That was my point. Habitable planets around stars aren't random. If they exist, they're found in a well-defined location (the star's "Goldilocks zone") that is very small compared to the rest of space. $\endgroup$
    – jamesqf
    Commented Apr 27, 2021 at 17:18

As mentioned by jdunlop, the question is not about detecting the planet. What follows from that is that a mapped but not yet visited planet is no better or worse for hiding than an unmapped planet. It fact, if there are a limited number of agencies doing most scouting (the Galactic Patrol or the Scout Service), a little insider info might yield a planet that has been detected, has been visited by an automated probe, and has been classified as less interesting than other, nearer worlds.

So, on to probabilities. Part of that is the well-known Fermi Paradox and the related Drake Equation -- so and so many planets in the galaxy, why haven't aliens visited yet?

Cribbing numbers from Wikipedia, there are 40,000,000,000 useful planets in the galaxy. So now you have to put numbers in your setting on the scouting ships and their speed. A fleet of 1,000,000 ships, visiting one planet per month, would give you one visit per planet every 3,333 years. A pirate would probably deem that safe enough.

A fleet of 10,000,000 scoutships cuts this to 333 years.

And so on.

  • $\begingroup$ Plus with that number of scout ships, loosing one will likely not raise any flags, so just shoot it down (or hack it if its automated to log your planet as "absolutely worthless" and youre good $\endgroup$
    – Hobbamok
    Commented Apr 26, 2021 at 10:42

As @jdunlop and @Hobbamok noted, current telescopes can detect extrasolar planets quite well. I disagree with Hobbamok that having the planet's orbital axis be pole-on toward the observer would be enough to hide the planet.

There's another way however.

Dark nebulae

Paraphrasing Wikipedia, a dark nebula is dense enough to absorb light from beyond it. So a sufficiently dense nebula could prevent detection of extrasolar planets (or even stars) beyond the cloud. How the ship in question winds up in the unsurveyed region beyond the cloud is part of the story.

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    $\begingroup$ This does, however, require @Hobbamok's other assumption, that there is only one location from which this observation is taking place. If Stellar Cartography arrays are built in several far-flung locations in galactic civilization, there are very few places where a nebula could hide a star. $\endgroup$
    – jdunlop
    Commented Apr 26, 2021 at 16:33
  • $\begingroup$ @jdunlop Space is big. Really big. To get a view "around" a dark nebula, the stellar cartography array would have to be very far flung. But as the question says "intergalactic" not merely "interstellar" it's just a matter of time and logistics until the array covers the "far side" of the nebula. $\endgroup$ Commented Apr 27, 2021 at 13:54
  • $\begingroup$ If I were building stellar cartography arrays, I'd be positioning them as far apart as possible for exactly this reason. $\endgroup$
    – jdunlop
    Commented Apr 27, 2021 at 18:12

EDIT: this only works against one way of telescope-discovery of planets. It'll help a bit but not make your planet hidden forever

As @jdunlop already stated, not ships, but telescopes will discover your planet. This means that our planet can be absolutely obvious to any ship already close to the system, that does not matter because it won't happen.

Right now (and probably forever because it works well), this is done by observing a star, and seeing if it changes luminosity over time (periodically). This happens when the planet gets in between the star and the telescope, or close enough for the reflection of the planet to cause a perceivable change in the star's look.

This is easily mitigated by having the planet's orbit at roughly a 90 degree angle from the telescope-star line. Since telescopes (of that size) are and will be expensive (because bigger), they will likely remain near the center of civilisation. A far-enough-out system could therefore have this "roughly 90 degree" angle for all telescopes mapping space for a while.

Your planet having a low reflectivity would help as well.

Remember: space is big and going there is expensive. Ships will NOT just fly around randomly looking at systems. They will only come if a good chance for resources etc has been observed. [Because telescopes are comparatively cheap since you save all that fuel and time compared to actually going somewhere]

  • $\begingroup$ As I commented on your comment, I don't see how this prevents detection of the planet by measuring the star's movement. Dips in luminosity are just one method of finding an exoplanet. $\endgroup$
    – jdunlop
    Commented Apr 26, 2021 at 16:34
  • $\begingroup$ @jdunlop, yeah I read up a bit more on how to discover planets and while this setup would hide it from the "cheapest" or easiest options of discovery, it wouldn't be a guarantee for other methods $\endgroup$
    – Hobbamok
    Commented Apr 27, 2021 at 8:29

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