Inspired by: How much electromagnetic transmission noise does Earth have to emit to be noticeable?

Premise: Assume that the infamous Wolf 359 star system has one habitable planet and a second, habitable or not, upon which the Wolvians have developed a mining operation. The two planets (maximum separation: 3 AU) have receivers with a -200 db threshold. The transmissions are focused between the two planets, so they would not be continuously detectable by Earth (if detectable at all) as only the signal aligned with Earth when the two planets appropriately line up could be detected.

  • For the purpose of this question, assume that the ecliptic of both star systems are perfectly parallel on an arbitrary X-Y axis with one another.

  • The Z-axis difference between the two systems should be taken into account.

  • Assume space between the two planets is clear of any debris.

  • The planets are talking with each other, not with Earth. Our detection is incidental to their activities. Please assume the Wolvians are unaware of us.

  • Please ignore the fact that the actual Wolf 359 star (a red dwarf) is unlikely to promote life.

  • Do not assume the use of satellites to get around the line-of-sight problem when the planets are on opposite sides of their sun. The transmissions may originate from planet-based transmitters or from satellites in orbit around the planets, but may not be bounced off of satellites in orbit around the sun.

  • Assume the transmissions from each planet are continuous such that the signals could be represented by cones, emanating from each planet in the direction of the other.

  • Assume the technology on Earth that is currently available to SETI.

Question: Would the interplanetary transmissions in the Wolf 359 star system be detectable by Earth today?

Best Answer: The best answer will consider as many of the factors complicating this premise as possible. A defensible signal strength, nature of the focus, window of opportunity for detection, all three dimensions, etc. The best answer will produce, with reasonable extrapolation from current technology, a defensible solution for what signal strength would be used by the Wolvians and its detectability here on Earth.

Worldbuilding Justification: Building alien worlds that can be detected by Earth without Star Trek-ish exploration requires a better understanding of how they would justify the power needed for transmissions we can detect. Thus, from the perspective of building a civilization at Wolf 359, could that civilization have a justifiable reason (e.g., a mining operation on another planet in their own system) for pumping out the energy needed to be detected?

The linked question intrigues me because it assumes Earth would be pumping out on a general basis power levels that could be detected at very long distances. Assuming human logic (the Vogons would disapprove of that generalization), there isn't a reason to do that. There will likely never be a reason to do that. This suggests that we need an off-planet reason to pump out that kind of power — but off-planet solutions are directional, not global. Though simplified, this question suggests that the civilizations SETI's looking for are substantially more technologically advanced than we are and that the detection of their presence would strongly suggest specific applications of that technology.

  • $\begingroup$ Is earth's orbit for detection fine or does it have to be on earth? I am not certain, but i think the atmosphere is another hurdle that could weaken a signal. Just asking for clarification. $\endgroup$ Commented Sep 10, 2018 at 9:16
  • $\begingroup$ The amount of energy required would certainly be impressive, I have no doubt about that. Any signal given by an alien planet would require enough energy to be considerably more advanced than us today. $\endgroup$
    – Neil
    Commented Sep 10, 2018 at 9:27
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    $\begingroup$ Did i get that right, that we basically have to assume the signal is already just slightly above -200dBm, when it passes one planet? With that then a definitive no. When the signal is that freaking weak I highly doubt anything detectable would reach our solar system. $\endgroup$ Commented Sep 10, 2018 at 10:45
  • $\begingroup$ Or did you mean that at 3AU the signal loses less than 200dB in strength? I find that part to be unclear. $\endgroup$ Commented Sep 10, 2018 at 11:57
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    $\begingroup$ @Neil, that's literally why I found the question interesting. If SETI's simply listening for average broadcasts (the Wolfinian version of "I Love Lucy*), then why would they transmit at that power level? I'm testing a theory as to how to justify the transmission strength needed just for detection. $\endgroup$
    – JBH
    Commented Sep 10, 2018 at 15:58

2 Answers 2


No, because the Wolf-ians will use lasers

You waste a lot of energy broadcasting a signal to anywhere that isn't your intended target. In interplanetary communications, where the distances are great, you really want to narrow your beam down. This is a job for lasers.

NASA is working on developing such a system right now. If the Wolf-ians have gone through the trouble of establishing a colony on another planet in their solar system, then they will take the trouble to establish a transmission system that uses the least energy possible. Lasers are that solution (PS I know this because I just spent all weekend researching this question).

So now that we are using a directed energy, point to point beam to transmit information, the chances that this beam will then intersect Earth, some light years away, is effectively nil. The Wolf-ians desire for energy savings in their transmission equipment will cause them to use a transmission medium with such a narrow beam that Earth will never find it.

  • $\begingroup$ Precisely my thought, too. Why use barely directed methods of information transmission causing big losses, when you can have a narrow beam that will not send huge amounts of energy past the target. $\endgroup$ Commented Sep 10, 2018 at 12:39
  • $\begingroup$ Frankly, this is a perfectly valid response, and it would easily explain why SETI doesn't seem to detect anything... but light diffuses over distance, too, does it not? All electromagnetic transmission is photonic. Did you run the numbers to determine that it's impossible, given the conditions of my question, for the diffusion to include Earth? This answer suggests that a laser beam only to the moon would be 500 miles wide.... $\endgroup$
    – JBH
    Commented Sep 10, 2018 at 16:02

Since your question states that the two planets have an elliptic parallel to the Earth's heliocentric elliptic and there's a 0.95 light-year Z axis displacement between Sol and Wolf 359 (I have a data-table with the Sol relative XYZ co-ordinates for every star within 25 light-years of Sol, if you ever need it), any signal between them won't disperse enough to be picked up on Earth regardless of strength, the light-cones aren't wide enough (assuming Earth-sized planets).

If they were aiming at Sol we might get lucky; a signal from Wolf 359 is going to come in at an angle to our orbital plane of about 57.3 degrees (that assumes a matching elliptic for Earth and Wolf 359b and c), so a tight beam will miss us completely if aimed at anything but our planet directly. But you need a fairly large signal cone to even hit our solar system reliably since Sol and Wolf 359 have a total proper motion of about 4.7 seconds of arc per year as seen from Sol, so the signal, to hit Sol reasonably reliably the signal will be 0.2 light years (12650 A.U.) across at this end of the cone. That signal should hit Earth as well as Sol, most of the time.

But SETI probably can't pick the signal out of the background at that strength even if it was trained on Earth full time.

Do also note that unless the aliens are deliberately using a signal wavelength that doesn't come from their parent star in any significant quantity just about any conceivable signal is going to get lost in the glare from their star and put down to natural stellar variance if it's noticed at all.

  • $\begingroup$ I just finished pointing out to Kingledion that this answer suggests that pointing a laser at the moon results in 500 miles worth of diffusion. Are you sure the distance to Wolf 359 is insufficient to develop the cone? $\endgroup$
    – JBH
    Commented Sep 10, 2018 at 16:06
  • $\begingroup$ @JBH The two worlds take up 0.0008 degrees of arc in each others sky, if a signal was sent that was designed to blanket the facing hemisphere of each world it would be only 0.0001 light years (6.9 A.U.) across when it reached Sol, not enough to compensate for a 0.95 light year Z axis offset between parallel elliptics. That assumes you don't have the atmospheric scattering that leads to most of the 500 miles mentioned in that article. Lasers disperse very slowly in hard vacuum. $\endgroup$
    – Ash
    Commented Sep 10, 2018 at 17:02
  • $\begingroup$ This is why I'm having trouble convincing myself to continue with Seti@Home. I think their fundamental premise is wrong. Our local star systems could be swimming with life, and there'd be no common sense reason to know they were there unless they knew we were here and for an unfathomable reason decided to inentionally funnel planetary-level energy reserves into transmissions meant only for us. $\endgroup$
    – JBH
    Commented Sep 10, 2018 at 17:05
  • $\begingroup$ @JBH Then there's the problem of transmission wavelength, unless the signal is a deliberate mismatch with the star it's be transmitted from, either transmitted at a wavelength the star is very dim in, or better yet transmitted in one of the star's absorption lines, it will disappear in the glare of the starlight. $\endgroup$
    – Ash
    Commented Sep 10, 2018 at 17:18
  • $\begingroup$ Ooooh, that needs to go into your answer. I'm really starting to wonder what SETI hopes to accomplish. $\endgroup$
    – JBH
    Commented Sep 10, 2018 at 18:41

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