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My thought is, if an intelligent race were to stem from such a planet, locating it would most likely be a lot different than finding a regular planet orbiting a star.

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  • $\begingroup$ What's your definition of deep space? Are you talking interstellar space? I think a lot of the answer would have to do with the life present on the planet. How is this intelligent life surviving without a sun to power them? Are they emitting RF at strange frequencies? $\endgroup$ – Cort Ammon Aug 14 '16 at 2:58
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This depends on who is trying to locate the rogue planet. if it's the rogue planet aliens, then they will need to use the methodology of interstellar navigation to first work out what is its galactic orbit. Then a spaceship from the rogue planet will be able to find their way back to their planet by navigation alone.

The most likely form of interstellar navigation uses pulsars as beacons. For information about this just navigate your way to here

For sapient lifeforms that don't come from the rogue planet, they will find locating the rogue planet will be a bit more difficult. if the rogue planet is passing either through or near the Oort clouds of a planetary system its presence will show up because of the gravitational perturbation of objects in the Oort cloud.

Another way would involve the detection of electromagnetic emissions from the technology on the rogue planet and then homing in on it. It is possible that infrared radiation from the rogue planet will be detected too. Also, if a civilization on the rogue planet possesses space travel, then their space technology activity might also be detected.

There is also radar astronomy. While this isn't used for ordinary radio astronomy because the energy requirements to generate radar pulses will be exorbitant. Radar astronomy might be used to map the positions of objects in the Kuiper Belt and in the Oort clouds. If a rogue planet was sufficiently close, it would show up in radar astronomy surveys. Radar pulses, especially if they were powerful enough, could continue on beyond the Oort clouds and reflect off any rogue planet passing by. This makes it possible that rogue planets could be found.

Of course, if astronomers wanted to locate rogue planets they would use many of these methods in concert a survey of rogue planets. However, astronomers have been able to discover some rogue planets using convenient astronomy. See here and here for more information.

Considering whether a rogue planet is habitable, David Stevenson has suggested rogue planet might be better environments than expected.

Interstellar planets generate little heat nor are they heated by a star. In 1998, David J. Stevenson theorized that some planet-sized objects adrift in the vast expanses of cold interstellar space could possibly sustain a thick atmosphere that would not freeze out. He proposes that atmospheres are preserved by the pressure-induced far-infrared radiation opacity of a thick hydrogen-containing atmosphere.

If intelligent life evolved on rogue planet under a hydrogen atmosphere its biochemistry would be radically different from that on Earth. This doesn't rule out the possibility. Admittedly this now gives you the problem of working out how a hydrogen breathing biosphere could function. However, that can be the subject of another question.

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  • $\begingroup$ It should be noted that perturbations of Oort cloud objects will take centuries to become very noticeable (by some of the objects falling into the inner system and becoming visible as comets). Infra-red seems the likeliest way for a warm rogue planet to be detected: the James Webb Space Telescope would plausibly be capable of this. $\endgroup$ – John Dallman Aug 14 '16 at 17:33
  • $\begingroup$ @JohnDallman Good point. Thanks for adding it. $\endgroup$ – a4android Aug 15 '16 at 4:50
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One way to discover one would by by accident, as it passed in front of another star. This would cause gravitational lensing effects to distort the star or galaxy being observed.

Imagine a survey looking through anamalies—bad exposures normally thrown away—for telltale signs that this was the cause in some case.

Check out this video from the SETI weekly seminars, particularly some of his slides.

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