Humanity detects a very suitable planet, lets call it Vaung, that is roughly 80 years away from us with their current technology. A ship is made and sent to colonize it, same old stuff.

The issue is the following. At some point in the journey, after 30-40 years did not decide yet, our astronauts suddenly discover a planet that is even more suitable for human habitat. The ship is fitted with powerful telescopes and whatnot so detecting the planet is not an issue.

The issue is how can we not detect this planet that is like 30-40 years closer to us? Note that humanity spent billions of dollars and over 4 years frantically searching for a suitable planet and deciding on the original chosen planet because of it's proximity.

So. I want a scientific or scientifically based phenomena that can lead the astronauts to be like: That makes sense as to why we did not discover the planet earlier. Guess we better consider setting up shop here because it's even closer.

This is an important point as there are other elements at play here and I want my astronauts to be really oblivious to anything unnatural happening on the planet.

I understand that deviating from the mission should not be accepted but this is not here or there. Nor what is on the planet since I do want to explain this in a normal natural way instead of hand waving it with magic or a super duper advanced alien civilization...etc

What explanation can be plausible?

Edit: Sorry if the distance is not clear. Lets just say planet B is half the distance of planet A. Planet A is the original destination that is discovered earlier, while B is the one we want to be discovered later.

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    $\begingroup$ Last I checked, year was a unit of time, not distance. Since distances (or at least their order of magnitude) matter, the question is ill-formed by missing crucial details. Did you mean light-years? $\endgroup$ Oct 15, 2021 at 6:40
  • $\begingroup$ @AdrianColomitchi edit makes it more clear? $\endgroup$
    – Seallussus
    Oct 15, 2021 at 6:52
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    $\begingroup$ @AdrianColomitchi has a point. You're asking for advice concerning astrophysics, but you're giving us "distance" in terms of travel time. If your ships only travel at 50% the speed of light, then we're working with an initial distance of only 40 light-years. However, Adrian, we might be straining at a gnat. Let's ignore the distance completely and assume that enough distance exists that something can cause the effect Seallussus is looking for: a detectable and habitable planet at distance A and a second at distance B where B < A, but the planet at B couldn't be detected from Earth. $\endgroup$
    – JBH
    Oct 15, 2021 at 7:28
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    $\begingroup$ @Seallussus: Are you (re-)writing en.wikipedia.org/wiki/Alien:_Covenant ? $\endgroup$ Oct 15, 2021 at 18:48
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    $\begingroup$ This isn't an answer to your question, but if you're using conventional physics to get to your planet, the halfway point is where you turn around and start decelerating. If you find your "hidden" planet "about halfway" to the intended destination, you're going to fly right by with no chance of slowing down in time. $\endgroup$
    – MikeH
    Oct 15, 2021 at 21:30

10 Answers 10


Short version: The planets of the system are small and the ecliptic is tilted vertically, and perpendicular as seen from Earth but the ship gets close enough to resolve an individual body or for its point of view to show transit events.

Long version: There are five ways to find an exoplanet from Earth, we need to eliminate them all. So if you have a star system with no gas giants in it, at least no big ones then Direct Imaging is out as it mainly detects massive young exoplanets in wide orbits. Radial Velocity will also fail as it is best for finding massive planets in tight orbits. All planetary orbits have an ecliptic, a flat plane along which the orbital track lies. For Transits to be visible the ecliptic of the exoplanet needed to cut across the star as seen from Earth, the same is true for Microlensing which is caused by the transit of a world too small to resolve in the glare of its parent star. Transit Timing Variations rely on being able to see transits in the first place. For the vast majority of the star systems we have observed it is the case that its orbital tracks are close enough to parallel to the Earth's own for transits to be visible. All these ecliptics lie close to the rotational plane of the galaxy as a whole but it is possible for a planetary system to have an ecliptic that is perpendicular to our own. If that were the case then we couldn't see any occlusion of the star by any of its planets. Given what we are continuing to learn from the Kepler data not being able to see any evidence of planets in orbit from Earth would be a curiosity worthy of scrutiny on the way past. Being much closer may allow the crew to directly observe the back scatter from a world with high albedo. Being at a different viewing angle may allow them to observe transit events that are invisible from Earth. Either of these may invite further investigation if slowing down is not too expensive.

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    $\begingroup$ Potentially six ways as of the last couple weeks! It was proposed a couple years ago that we may be able to detect exoplanets closely orbiting red dwarfs through radio-aurora activity in the dwarf induced by magnetic field interactions, similarly to what Io does to Jupiter, and within the last couple weeks there's been some early evidence of 19 more exoplanets via that method. $\endgroup$
    – Idran
    Oct 15, 2021 at 19:40
  • $\begingroup$ @JustinHilyard Awesome, I don't think that development changes my answer if we're talking about a Goldilocks Zone body but it might. $\endgroup$
    – Ash
    Oct 16, 2021 at 5:57
  • $\begingroup$ The nice part about the transit method is that you can finetune when exactly the planet is discovered, and also that it provides you the composition of the atmosphere as well as some estimate about the planet's radial size (to the extent that you can guess the star's own size from its luminosity and spectrum; with at least four light years of parallax, the distance should be obvious). $\endgroup$ Oct 16, 2021 at 9:43
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    $\begingroup$ "ecliptics lie close to rational plane of the galaxy as a whole" - is that really true? I would've thought that local gas rotation in star-forming region would be quite random... $\endgroup$ Oct 16, 2021 at 17:52
  • $\begingroup$ @NooneAtAll According to the data I've seen, which is not exhaustive it's not something I take pains to keep up with, within 10 degrees either side, it makes sense that the vast majority of matter in the galaxy is spinning along roughly the same plane, it all gained angular momentum along the same axes during the same events. $\endgroup$
    – Ash
    Oct 17, 2021 at 7:52

The habitable "planet" is actually a moon of a super Jupiter. Telescopes in the Sol system were unable to resolve the moon from the planet it orbited. However, as the colony ship passed much closer to it on its way to the planet that was detected, they got a better look and spotted the moon.


An lengthier answer in response to comments:

The question doesn't give any information about how fast the ship travels, just that journey time is 80 years. Lets assume a distance of about 20 light years. That makes the ship fast, but not completely unrealistic with near future tech. If they check every star system within that distance, that's a survey of 83 star systems. That feels like a lot of work to do in just four years.

The search was presumably for habitable bodies. Exactly how they orbited the star would be of less importance than having surface water, suitable atmospheric pressure/composition, comfortable gravity, protection from solar radiation and so on.

However, while detecting exoplanets is really hard (See Ash's excellent answer for a summary of the techniques used), detecting exomoons is even harder. As of October 2021 I don't believe that a single confirmed detection has been published, although there are several good candidates.

Given the difficulties and time pressure, it doesn't seem unreasonable that researchers might look at a system and say "There is a large Gas Giant that makes a Terrestrial planet in the habitable zone impossible... Lets move to the next system." Missing the tiny hidden signal that gives away the presence of the moon...

The searchers pick their (more distant) target and launch the ship. By good luck however, the star system with the better habitable body is close to the trajectory to the target star. Passing within a few thousand AU, the difficult to spot (at 10ly) moon sticks out like a sore thumb to the ships sensors.

  • $\begingroup$ 1) the question is about a planet. and 2) when humans decide to travel 80 years to colonize the system, you may assume the system was thoroughly studied and knowledge of astronomy is advanced enough to see the difference between a moon and a planet and 3) they are halfway.. why would they not have found this giant moon when studying the system at only twice the distance ? It suddenly "appears". $\endgroup$
    – Goodies
    Oct 15, 2021 at 8:35
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    $\begingroup$ @Goodies 1. The difference between a planet and a moon is simply what they're orbiting, Ganymede is the 9th largest discrete object in the solar system. 2. They may well have studied the original target system very thoroughly but you don't invest nearly as much looking at places you have decided you aren't going so while they know there is a gas giant in the star system close to halfway but not much about it. 3. The question is about a 2nd, better, target that is halfway to the 1st so at the 40 year mark they are, astronomically speaking, on top of the star with the new target. $\endgroup$
    – Ash
    Oct 15, 2021 at 9:33
  • $\begingroup$ @Ash 1) my remark was about the question: sealussus talks about a "planet". There is a clear distinction, a moon is a moon and a planet is a planet, the difference is not its detectability, the difference is a moon orbits a planet and a planet orbits a star 2) i understand it could have been overseen, but keep in mind the distance half-way is only half the research distance they used on Earth. Before planning the voyage and taking 80 years to travel there, humanity oversaw the elephant in the room ? $\endgroup$
    – Goodies
    Oct 15, 2021 at 9:54
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    $\begingroup$ @Ash The thing is the moon is much harder to detect and if the star/planet/moon system is in resonance it's completely undetectable from Earth other than by direct observation. If there were an Earthlike moon around a super-Jovian around Proxima Centauri I don't think we would have the slightest chance of detecting it with current tech even without a resonance. $\endgroup$ Oct 16, 2021 at 22:54
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    $\begingroup$ @Goodies It's seen because the ship is much closer to it than Earth was. Most gas giant moons are uninhabitable, but that's why we are saying super-Jovian--big enough to have a moon that's Earth size. And tidal locking to the planet doesn't make it uninhabitable, it just makes the day equal to it's orbital period--thus you need a low orbit. It will eclipse, but how is that a show-stopper? $\endgroup$ Oct 17, 2021 at 22:53

Have you ever walked in a fog so thick you don't see a pole until you slam your nose onto it?

This is what might have happened: if in the line of sight between Earth and the planet there was something increasing the noise, the signal of the planet presence would simply become too small to be detected.

It could be dust, it could be gas, it could be a more luminous star.


It wasn't visible from the original angle:

Since you're talking about distances in 10s of years, I have to assume the planets we're going to are extrasolar (that is it orbits a different sun than our own/their own).

If it has a very long orbit (Pluto take 247.78 years to orbit the sun), then perhaps it was just behind the sun the whole time! Once you got far enough around the sun, the slowly orbiting planet was revealed.

If they're inside the solar system, the timescales are a bit off I think, but you could do the same concept, where the orbits are synchronyzed with the planet always hidden behind our sun.

  • $\begingroup$ @MarcelKrüger A typo thank you. It's years not days. $\endgroup$
    – Ryan D.
    Oct 15, 2021 at 17:14
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    $\begingroup$ He's looking for a habitable world--nothing that far out will be habitable. $\endgroup$ Oct 16, 2021 at 22:55
  • $\begingroup$ @LorenPechtel It's orbiting a different star though. The habitable zone depends upon stellar mass as does orbital speed. For our sun the habitable zone extends out to mars, which is about a 2 year orbit around the sun. For a different star, you'd need to do some math to say for sure. $\endgroup$
    – Ryan D.
    Oct 20, 2021 at 20:31
  • $\begingroup$ To be habitable that far out the star in the center would have to be awfully big and hot. There's nothing like that at the specified range from Earth. $\endgroup$ Oct 21, 2021 at 23:57

The simplest answer: the star, and it’s planets were not visible from earth because a nebula was in the way.

Nebulas are clouds of gas and dust (mostly hydrogen) that obscure visible light. We don’t really know what may be on the other side of one and indeed something like half of the Milky Way is not visible to us because of obscuring nebula. So we can actually see more of the Andromeda galaxy than we can of our own.

Here’s a famous example, the Pillars of Creation enter image description here

In this image, the stars that look like they might be in the nebula are actually in front of it, we do not know what is behind it. However, a spaceship that had moved far enough away from the earth, could see what was behind it, including perhaps, previously unknown stars and their planets.

Here is an example of a true dark nebula, Barnard 68 enter image description here

Like all known dark nebula it is large and far away. However, this does not mean that all dark nebula are large and far away. In fact there is every reason to believe that smaller obscuring clouds of gas and dust (i.e., dark nebula) are very common, however, the problem is that like Barnard 68 above and the vast majority of dark nebula we cannot see them, we can only infer them because of what we cannot see. So we can "see" Barnard 68 only because of its size and the large bright star field behind it. (the Pillars of Creation nebula is one of the rare exceptions, we can truly see it because it is lit up by the stars just in front of it).

For smaller nebula that are not in our line of sight within the galactic plane, there are neither enough stars behind them, nor are they large enough to allow us to "see" them in this way. However, because stars are created from nebula, there's every reason to believe that nebula are quite common.

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    $\begingroup$ The scale is wrong. The planet in questions is like 30 to 40 light years away. We're not in the middle of a cloud hundreds of thousands of light years across. Note that solar systems hatch out of what appear as tiny balls in this image. $\endgroup$
    – JDługosz
    Oct 15, 2021 at 20:48
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    $\begingroup$ @JDługosz That nebula is ridiculously big. Dozens and dozens of our solar system would fit into one of the protruding fingers of the cloud.. But is it really hundreds of thousands of lights years across? The milky way is only 100,000 light years across. EDIT: The pillars of creation span a few dozen light years. $\endgroup$
    – DKNguyen
    Oct 16, 2021 at 1:38
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    $\begingroup$ Downvoter: care to explain? I stand by my initial statement, an intervening nebula is by far the simplest, most reasonable and most likely explanation. $\endgroup$ Oct 16, 2021 at 5:44
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    $\begingroup$ I like the update to the post: there may be small nearby dark clumps of dust that we have not charted today. $\endgroup$
    – JDługosz
    Oct 18, 2021 at 14:03
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    $\begingroup$ @RBarryYoung A lot of the answers have a downvote, including mine (with no comments). In case the person is reading this, see worldbuilding.stackexchange.com/help/privileges/vote-down for this S.E.'s standard of voting. It does not mean "I dislike this"or "not my favorite", but is supposed to indicate that the answer is incorrect or does not provide sufficient information to be useful. $\endgroup$
    – JDługosz
    Oct 18, 2021 at 14:14

I make no claim to sufficient expertise in astrophysics to assure you the following is credible... but I did sleep virtually (given a bit of literary license) in a Holiday Express last night, so you're in luck

OK, I admit it, I slept very comfortably in my own bed last night. Still...

  • Parallax: the effect whereby the position or direction of an object appears to differ when viewed from different positions, e.g. through the viewfinder and the lens of a camera.

  • Gravitational Lensing: A gravitational lens can occur when a huge amount of matter, like a cluster of galaxies, creates a gravitational field that distorts and magnifies the light from distant galaxies that are behind it but in the same line of sight. The effect is like looking through a giant magnifying glass.

  • The second planet orbits a white dwarf, a star that is hot, but dim.

  • The second planet orbits a star that dims oddly, meaning that there's some unknown effect causing the star to be dim at the time the first planet was discovered, so the second planet couldn't be easily detected. Examples of what theoretically could cause the dimming would be a massive dust cloud in orbit around the star or a massive planet in a closer orbit that orbits at one heckava inconvenient orbit.

  • To build on @L.Dutch's answer, a dust cloud or nebula that masks the view of the second planet from Earth, but the point of view changes as the ship travels, allowing it to see behind the dust cloud or nebula.

  • The planet orbits (very close orbit, here!) an unusually warm Y Dwarf star that's too cool and too dim to allow the planet to be seen from Earth, especially with that whomping close orbit....


(this answer assumes sub-lightspeed travel, say less than 30%)

They came too close, the cloaking does not work anymore

The newly discovered planet has an advanced civilization. Unfortunate events in the past were a good reason to develop a planetary cloak. This cloaking technology prevents the planet to be discovered by pirates roaming around the section at warp speeds. It is based on a trick involving satellites and light interference patterns.

This cloaking field has a certain distance threshold. When you come near, at some point you'll not see the interference patterns anymore, you see the planet. The colonists have come really close. Of course their approach was discovered by the aliens years go.. it was no reason to change the cloak, these primitive humans pose no threat..


Trojan planet

Trojan planets, which follow or precede the lighter of two stars at a 60-degree angle (L4/L5 points), can usually be detected by transit timing variations. This is important since the Earthlings would not have launched an interstellar-level project without some very good research, possibly including unmanned observatories flying several light years into space and measuring transits not visible from Earth. However, the paper cited also shows (Table 5 at the end) that the transit timing variations can be extremely small for a few cases. So there is a point at which astronomers could have written off the odds of a planet in a given system based on a limited period of instrumental data availability. Coming nearer the system, observing it with the best available instruments, the ship might detect more subtle variations, or brute force an image resolving the planet by eye, or use some cleverer astronomical deduction.


Consider the real-life case of trying to see planets orbiting our nearest neighbor: the binary stars are too close together so the glare spoils the effort.

The star in question could be a binary, or an "optical binary" where another star is in our line of sight behind it. Either way, having another star in the field where advanced telescopes carefully block out the image of the central star to try and see planets — and also obtain the atmosphere spectra of the planet — would prevent such an observation.

Perhaps we knew there were planets there. What we didn't know was the exact mass and radius and "life signs" primarily gained through visible direct observation of that planet.

Years later, such measurements became possible due to further advances in technology, or the movement of the stars out of the way, or observatories that are stationed far away from Earth.


A simple solution would be orbiting directly opposite the star


added from comments: A counter-Earth can't exist in our solar system specifically because of the gravitational effects of our exact solar system. Nothing in the question specifies this is the context. In another solar system, there's no reason a counter-earth-like planet can't exist.

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    $\begingroup$ As that article and any number of others will tell you, a "counter-Earth" can't exist. $\endgroup$
    – Mark
    Oct 15, 2021 at 20:55
  • $\begingroup$ A counter-Earth can't exist specifically because of the gravitational effects of our exact solar system. Nothing in the question specifies this is the context. In another solar system, there's no reason a counter-earth-like planet can't exist. $\endgroup$
    – TCooper
    Oct 15, 2021 at 20:58
  • $\begingroup$ Yes, there is. The specific circumstances of our solar system have nothing to do with it. Even in an idealized three-body system isolated in intergalactic space, the arrangement would be inherently unstable. $\endgroup$ Oct 16, 2021 at 5:44
  • $\begingroup$ @LoganR.Kearsley and what about in an idealized 114 body system? Also there are plenty of cases of 'unstable' orbits that are stable into the billions of years. Plenty of time for a plot line. $\endgroup$
    – TCooper
    Oct 18, 2021 at 16:29
  • $\begingroup$ @TCooper Then it would be even more unstable. Yes, there are technically-unstable systems which persist with minor perturbations for billions of years, but this is not one of them. $\endgroup$ Oct 18, 2021 at 18:00

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