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Let's imagine a civilization that has a way of colonizing rogue planets (ones not orbiting a star). It would want to expand, and therefore search for new worlds. What physical principle and rough design of a sensor would they likely use? In their world, being efficient is the key as energy and materials are scarce.

Would they use a swarm of tiny ships to search for other rogue planets? I'm guessing that they could use some technology to detect the gravity of rogue planets. Assuming a planet of interest has at least 1% mass of earth, from what distance could such planet be sensed? How would the sensing device look like? Would gravity of the galaxy be an obstacle, meaning the search for rogue planets could be more efficient in inter-galactic space?

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You talk about detecting planets like it is searching for boats in the sea at night, looking for their lights.

One can search for the optical signature of a planet, but that requires a star to shine some light on the planet itself, and you are looking for a gravitational signature.

Gravity affects masses, and to detect gravity we therefore need other masses. The main problem with gravity is that it's a very weak force when compared to other forces. For our daily life it becomes important because of the huge masses involved with Earth and Sun. Moreover gravity fades with the square of the distance.

Detecting a rogue planet gravity with a mass placed here is therefore a no go: we are talking about an effect way way way way smaller than what we can practically detect, without taking into account the noise caused by all the other massive bodies which are closer to us than the rogue planet.

A solution to the above would be to put another mass closer to the planet and observe it with a bright enough light that we can see from very far. This is what exoplanet search projects do: in some implementations they try to observe the alteration on the light emitted by the central start caused by planets orbiting it (either occultations or star wobbles).

But as you see we don't detect gravity, but the side effects of gravity on the emitted light. And a rogue planet, by definition, has no paired star.

The only way seems then to be to wait for a rogue planet to pass close enough to a star and perturb its light curve.

Based on my gut feeling observing such an event is very unlikely.

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I hope you know that rogue Planets are about as close to stealth as you can get in space. Furthermore, it is kind of Questionable why anyone would look for these Planets in the first place but ok.

In terms of Detection, your best bet is to just look at the sky and wait for Stars light to be blocked by something. And since there are a lot of Stars, you can get a pretty good idea of the Planets dimensions just by looking how much it blocks.

This could be done by using a lot of very big Telescopes build in space looking at everything at once. How long your Distance is depends on your Budget. Nobody is saying that you cannot build an Array of Telescopes the sizes of the Solar System to see every last grain of Dust in a Radius of a few Billion Lightyears. But that is not cheap.

Besides that, sending out drones is a waste of everything. Again, those mf´s are stealth so you will know that you are near one, when you are REALLY close. Which will take a long time.

Using Gravity is also a waste of time. First, we can hardly detect Black Holes, how are we suppose to find something with 1% the mass of Earth ? It might as well not be there.

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Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.

What you are proposing is the equivalent of searching the sun for individual atoms of iron, only more so. You would need immensely sensitive detectors far past any we have now. In particular, we would need a way to directly detect gravity, on the scale of detecting, here on Earth, the gravitational attraction of a tennis ball out in the Oort Cloud.

Then you would also need technology far past ours to colonize rogue planets on account of their being cold as the Oort cloud or colder.

Citing gravity would be a start, but after that I think you may have to invent your technobabble.

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The question seems to show inadequate research.

Astrophysicist Takahiro Sumi of Osaka University in Japan and colleagues, who form the Microlensing Observations in Astrophysics and the Optical Gravitational Lensing Experiment collaborations, published their study of microlensing in 2011. They observed 50 million stars in the Milky Way by using the 1.8-metre (5 ft 11 in) MOA-II telescope at New Zealand's Mount John Observatory and the 1.3-metre (4 ft 3 in) University of Warsaw telescope at Chile's Las Campanas Observatory. They found 474 incidents of microlensing, ten of which were brief enough to be planets of around Jupiter's size with no associated star in the immediate vicinity. The researchers estimated from their observations that there are nearly two Jupiter-mass rogue planets for every star in the Milky Way.[13][14][15] One study suggested a much larger number, up to 100,000 times more rogue planets than stars in the Milky Way, though this study encompassed hypothetical objects much smaller than Jupiter.[16] A 2017 study by Przemek Mróz of Warsaw University Observatory and colleagues, with six times larger statistics than the 2011 study, indicates an upper limit on Jupiter-mass free-floating or wide-orbit planets of 0.25 planets per main-sequence star in the Milky Way.[17]

https://en.wikipedia.org/wiki/Rogue_planet#Observation[1]

Astrophysicist Takahiro Sumi of Osaka University in Japan and colleagues, who form the Microlensing Observations in Astrophysics and the Optical Gravitational Lensing Experiment collaborations, published their study of microlensing in 2011. They observed 50 million stars in the Milky Way by using the 1.8-metre (5 ft 11 in) MOA-II telescope at New Zealand's Mount John Observatory and the 1.3-metre (4 ft 3 in) University of Warsaw telescope at Chile's Las Campanas Observatory. They found 474 incidents of microlensing, ten of which were brief enough to be planets of around Jupiter's size with no associated star in the immediate vicinity. The researchers estimated from their observations that there are nearly two Jupiter-mass rogue planets for every star in the Milky Way.[13][14][15] One study suggested a much larger number, up to 100,000 times more rogue planets than stars in the Milky Way, though this study encompassed hypothetical objects much smaller than Jupiter.[16] A 2017 study by Przemek Mróz of Warsaw University Observatory and colleagues, with six times larger statistics than the 2011 study, indicates an upper limit on Jupiter-mass free-floating or wide-orbit planets of 0.25 planets per main-sequence star in the Milky Way.[17]

https://en.wikipedia.org/wiki/Rogue_planet[2]

Apparently only one Earth mass rogue planet has been detected yet, but Jupiter -sized rogue planets might have Earth-sized moons orbiting them.

So if there is a civilization that seeks out rogue planets to settle, they might detect them the same way Earth astronomers do, by occulations and gravitational lensing of background stars that rogue palnets pass in front of.

They might establish space observatories in many widely scattered regions of space. Each observatory would have many telescopes, sufficient to observe a full 360 degrees sphere of space. Those observatories would record occultations and gravity lensing events which might be the result of rogue planets passing in front of stars. So over decades, centuries, and millennia they would accumulate many records of those events and begin to narrow down the distances of those rogue planets.

And maybe those observatories would also send out very intense radar beams that would be aimed at each tiny segment of the sky in turn. And also send radar beams in the directions where rogue planets have been detected, of course. Eventually, after years of travel at the spead of light, very faint radar echos would be received from any beams which had struck a rogue planet. And the time taken by the beam to reach the rogue planet and return would show how far away the rogue planet was.

So eventually the civilization would find the locations of a vast number of rogue planets.

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