Our civilisation emits radio waves into space. But insofar as the Sun can be modelled as a black body, it is also emitting radio waves into space! For an alien civilisation too far away from us to be able to spatially separate the Earth and Sun sources, what would the relative strengths of these two radio signals actually be? Are our artificial radio waves visible at all over the Sun's radio noise? And could this be why no alien signals have been found yet?

  • $\begingroup$ One of the issues with artificial radio emissions is they are not particularly strong & because of the inverse distance squared law they lose power very quickly. Given the vastness of the cosmos it can be very difficult to notice an artificial weak radio signal from a long distance. $\endgroup$
    – Fred
    Commented Dec 28, 2021 at 17:12
  • $\begingroup$ To expand on @Fred point, capitalism will ensure that issue remains. Nobody is going to spend the money to create signals easily observable at galactic scales because that's just wasted money considering the target audience is necessarily on Earth and a super strong signal isn't necessary. The only few exceptions are signals specifically targeted for extraterrestrial life such as the Arecibo message. $\endgroup$
    – zephyr
    Commented Dec 30, 2021 at 15:54

1 Answer 1


The Sun emits incoherent radio signals over the full range of frequencies. The signals are not modulated in any meaningful way. Humanity emits signals at particular frequencies that are then modulated in various ways to contain information.

It is the careful analysis of signals in narrow frequency bands that enables you to sort out the signal from the noise. As a particular example, if you take a TV signal carrier wave leaking from the Earth - it would be a signal at a particular frequency that does not correspond to any natural frequency due to some molecular or atomic transition. As a result, the contribution from the Sun over that narrow band could be very small. A careful analysis of the spectrum would reveal the modulation in the form of closely adjacent frequencies and sidebands. Further, if you were to study this signal over the course of time, you would observe the Doppler shift in the frequency as the Earth orbits the Sun, clearly identifying it as coming from the planet rather than the star.

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    $\begingroup$ This answer doesn't really address the question. It is a good explanation on how the two signals would be interpreted if found, but doesn't address anything related to the relative strengths or potential for the Sun to mask Earth's signals. The essence of jamming communications is to inject noise into the signal, which the Sun does in spades. $\endgroup$
    – David S
    Commented Dec 28, 2021 at 19:18
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    $\begingroup$ @DavidS For example, on ~10cm broadband, Sun emits about 3 kW/Hz. So 1 MW transmitter with signal spectral width about 300 Hz would have same power as Sun even in anisotropic mode. With antenna, we can get 10^3 and even 10^6 improvement in some direction. $\endgroup$
    – Vashu
    Commented Dec 29, 2021 at 4:18

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