4+ light years away, aliens living on Proxima Centauri B have space travel and incredible technology, but are not interested in traveling out of their system or in returning our radio-babble, rather going about their business.

They have built in their solar system a very large structure as a cultural monument.

With today's technology, would this necessarily have to be the size of a planet for us to 'notice' a new structure, or could it be smaller? How large does an artificial structure need to be for us to detect it — and could there be a known material/mineral/etc. to enhance a smaller object?

EDIT: we are only 4+ light years away; not 12.

  • $\begingroup$ Browse through the SETI weekly colloquium videos. Many are on instruments and details of detection. You can learn about real observation technology and put that in your story. $\endgroup$
    – JDługosz
    Apr 16, 2017 at 16:34
  • $\begingroup$ What we can detect with telescopes is radiation. If an object is arbitrarily large but does not radiate (and does not alter radiation of other objects) then we are not going to detect its presence. On the other hand, if an object radiates a lot then we can detect it, no matter how small its size is. $\endgroup$ Apr 16, 2017 at 19:10
  • 1
    $\begingroup$ Yes, we on Earth today, living on Earth today detecting the object. I have always known our nearest star to be a bit over four light years away, why had I written twelve?? Jeez I hope I don't mess with any answers by editing this. $\endgroup$
    – Mikey
    Apr 17, 2017 at 5:16

4 Answers 4


It depends on, what we understand on "detect it".

If it means: "we can detect it anyhow", then the answer is, we should use simple, ideally focused radio signals for communication. Any radio telescope (or sender) would be enough, and the answer is: some meters. (Unrelated, but important: if any civilization would exist in the outer space using a similar radio technology as we do, we would mutually detect eachother from around 100 light years. Sad truth is that there is radio silence.)

If it means: "we can see it in visible light", then it depends from its temperature. The maximal thermal radiation is around at 6000K in visible light (which is not surprising - our eye was evolved below the Sun). In this case, all depends on, how near is it to the Alpha Centauri. The limit of the current exoplanet detectors is in the order of Earth-sized bodies, although these aren't so hot. If something heats this body to 6000K, then it may be visible already if it is only Moon-sized. The object would be point-like in any current telescope.

If it means: "seeing it as a multi-pixel object", the answer is around Sun-size.


The easiest way of detecting a solid object in other star system is observing the weakening of the light of its star as it passes before it. The majority of exoplanets were discovered this way. So a thin dish having the diameter of some thousand kilometers would be ideal choice. But this works only if its orbit intersects the line between Proxima centaury and Earth. If the aliens don't want to send message to us, their construct might never passes before the star from our POV.

The other solution is to make the object emit very strong radiation on its own. This would mean a very high power energy source. Since they probably can not construct a new star, it would only radiate in a collimated beam, but will slowly sweep the full sky. This acts as a monument for the civilization, but is not a directed message for us.

  • $\begingroup$ B. Lorenz - you failed to notice that the "we" in the question must be aliens living 12 light years from Proximal Centauri and thus about 7.7 to 16.3 light years from Earth. $\endgroup$ Apr 16, 2017 at 15:46
  • $\begingroup$ Please examine my edit, for future reference. $\endgroup$
    – JDługosz
    Apr 16, 2017 at 16:31
  • $\begingroup$ M. A Golding Yes, the question is a bit confused about the distance of Proxima Centaury, but I think it is clear that the aliens are intended to live on Proxima Centaury b, and 'we' are Earth. But, anyway, the methods I described work for 16 ly too. Should I edit the question to 4.2 ly? $\endgroup$
    – b.Lorenz
    Apr 16, 2017 at 17:38
  • $\begingroup$ @M.A.Golding - this is entirely my fault, although I can't imagine why I stated 12 light years away, and I'm a little mortified. I have edited to to clarify WE and Proxima Centauri, 4 light years (and change) away. $\endgroup$
    – Mikey
    Apr 17, 2017 at 5:19

About the size and mass of Earth

Exoplanet discovery hasn't quite dropped below the "about earth sized" detection limit yet, and we've already detected one, possibly two or three, exoplanets in orbit around the Alpha Centauri system.

Proxima Centauri b (aka or Alpha Centauri Cb) is approximately 1.27 Earth masses with a radius between 0.8 and 1.5 Earth's. Although the data only gives a minimum value on the mass.

There are also two other possible detections (that unlike Alpha Centauri Bb haven't been ruled a data anomaly yet), one of which has a radius of about 0.92 Earth's. I can't find a whole lot of information about these.

Detecting anything smaller or less massive than that is beyond current technology and even current technology is based on very precise measurements that are prone to error (as evidenced by Alpha Centauri Bb). It's the equivalent of measuring the light intensity of a lighthouse beacon vs. the same beacon with a single poppy seed placed in front of it. Which is why there are such precise measurements on the radius of exoplanets but not their mass.

Now, if your alien civilization was building something like a Dyson Swarm we here on Earth would probably start picking that up as soon as the light level of the star dipped significantly. Most exoplanet research is done with observations over the period of weeks or months, so without one of those aimed at the Alpha Centauri system, you'd have to wait on the less focused observations to pick up the dimminig. But a dyson swarm could do that pretty easily with little mass involved, simply by congregating on one side of the star. But at the same time, such a swarm could easily avoid being detected simply by making sure it is sufficiently diffuse.

How quickly that would get noticed? No idea. Assuming that at 100% light absorption that any Joe on the street with his naked eye notices that the star is missing in a few seconds and that at 0% light absorption, even the exoplanet research teams see nothing wrong after 6 months of study, then there exists some value X between these two extremes such that someone, somewhere, eventually notices and the scientific community starts aiming more powerful telescopes at the system to see what's up (and even then they might only be able to conclude that "something is happening" because the actual image of the system is still only pixels in size, but 1% dimmer than a year ago!).

  • $\begingroup$ Did you see my answer, describing the detection of something smaller than our moon, and several times the distance? $\endgroup$
    – JDługosz
    Apr 17, 2017 at 20:07
  • $\begingroup$ @JDługosz I hadn't, but I am not sure that we're to that tech level yet. Cursory research shows that there are only proposed methods, none of which have been shown to work yet. I agree that the example case you mention would work, but I am not sure our precision is fine enough to be certain. $\endgroup$ Apr 17, 2017 at 20:12
  • $\begingroup$ No, that’s an actual observation. Google kepler data exomoon for example. $\endgroup$
    – JDługosz
    Apr 17, 2017 at 20:28
  • $\begingroup$ The Kepler spacecraft data may contain enough data to detect moons. I don't see results, I see speculation. $\endgroup$ Apr 17, 2017 at 20:31

The question doesn’t have a simple, single, answer.

If you browse through the videos for the SETI Weekly Colloqiums (best on YouTube), you’ll learn all about detection and instruments, including those hunting exoplanets.

You will see that the ability to detect can be a function not just of the size of the object (radius or mass, depending on the method) but on its orbital parameters and the characteristics of the star. You also have the albedo of the object to consider!

Planets can be detected much smaller than the ability to resolve as a disk with an optical instrument.

The case for a very small object that I can think of is an exomoon. The planet is large/close/fast enough to detect with transits. But jitter in the timing of those transits is explained by the planet wobbling slightly due to a moon.

Detecting this is dependant on the nature of its primary, such that the planet is detected — not just being big enough relative to the primary to make it wobble.


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