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The Kepler telescope looks at dark sides of exoplanets. If the exoplanet was earth-like would it not have emitted light? Would those lights be detectable, or what technological advances would be required to see them?

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    $\begingroup$ The Earth is dark on the dark side, except where there is human activity. We’ve never seen a planet with any life on it at all, much less life that’s in the business of creating powerful illumination. $\endgroup$ – Jacob Krall Aug 20 '19 at 16:23
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    $\begingroup$ Do you mean "if it was like Earth" in that it was inhabited, or "Earth-like"? We can see light pollution from space, but that will also depend on how much light is being emitted $\endgroup$ – Punintended Aug 20 '19 at 16:23
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    $\begingroup$ Are you asking about artificial lights on an extrasolar planet? $\endgroup$ – Organic Marble Aug 20 '19 at 16:28
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    $\begingroup$ We have difficulty enough seeing exoplanets to begin with. Asking for the dark side where artificial lighting would be far dimmer than what gets reflected in the daytime seems a bit much for me. And if Earth were any indication, radio detection would be an excellent alternative for detecting an advanced civilization. $\endgroup$ – Oscar Lanzi Aug 20 '19 at 16:57
  • $\begingroup$ The Earth, being a black body of sorts, emits radiation at night. Just not much in the visible spectrum. $\endgroup$ – Mad Physicist Aug 21 '19 at 18:54
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Keep in mind that earth like exoplanets are mostly found as they cross in front of their sun.

If you hold a flashlight between you and the sun, you would not be able to tell if it was on or off, because the sun is so bright. The same would apply to city lights on an exoplanet.

Even if the explanet is partly around the sun, it is so far away from us, that it's sun would make city lights on the exoplanet not visible to us.

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City lights are not detectable today, but it is possible in the future. It would require:

future generations of telescopes on the ground and in space that would have the capacity to detect phase modulation due to very strong artificial illumination on the nightside of planets as they orbit their parent stars.

-- "Detection Technique for Artificially Illuminated Objects in the Outer Solar System and Beyond" - A. Loeb, E. Turner

See also: My answer to "How would we detect an Earth doppelganger planet?" at Astronomy Stack Exchange

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Exoplanets are found in a variety of ways. Two rather common ways are star transits and radial velocity changes.

Star transits are found by continuously measuring the brightness of the Star, and looking for dips that resemble a planet blocking out a portion of the solid angle.

Radial Velocity Changes detection works by finding small redshift/blueshifts in the star’s color. These are from planet(s) gravity pulling the star as they orbit, which changes its radial velocity relative to earth in a predictable manner.

From the precise magnitudes and timings of these events, many parameters like planet radius, orbital period, and (inferred) mass are known. If both transits and radial velocity changes are detected for the same star system, there may be enough data to tell if there is a significant light emission coming from the “dark side” of the planet. This would likely need to be much brighter than Earth city lights to be detectable. Think Death Star lazer in terms of necessary power.

Resources:

Exoplanet Archive

Detection Types

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    $\begingroup$ I think that you seriously underestimate power of Death Star laser. According to Boom table (projectrho.com/public_html/rocket/usefultables.php), its output is comparable to 10000 years of total Sun output ;) I think we can find out way smaller energy sources $\endgroup$ – Artur Biesiadowski Aug 21 '19 at 12:01
  • $\begingroup$ Alright, maybe just the power of a small star, instead. $\endgroup$ – CourageousPotato Aug 21 '19 at 12:03
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No, The Kepler telescope lacks the sensitivity to see earth originating lights at inter-stellar distances. Its capability is around the sensitivity required to detect the blocking in the parent star, so unless the planet was emitting within-orders-of-magnitude as much light as a star, it would not be sensitive enough to detect it.

It also is really only sensitive to the change in light coming off the planet as it lack the angular resolution to detect where in the target star-system the light is coming, so unless the light source changed over time it would be assumed part of the parent star light, even if enough photons could be collected to determine the presence in principle.

All is not lost though. Albeit with many orders of magnitude more sensitive instruments, by looking at the spectrum of the emitted light and how this was shifted, It may be possible to isolate light originating-from-or-reflected off a planet.

With even more high fidelity measurements separating this into reflected light and local sources may be possible but this is a long way off our current capability.

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