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Is there a way to make a planet invisible naturaly? I dont want the entire thing to be clear, but have a natural "field" or something that prevents the planet from being seen (or makes it much harder) from the outside. It should still allow someone to get to the planet.

It only needs to be invisible to the naked eye and some levels of lower tech (like early radar and thermal vision), but it can be visable to the higher level technologies. I would prefer it to stay invisible from all angles until the "person" passes through the "field". The "field" would be either in the atmosphere or the stratosphere (but, if there is a way for to happen (but the field had to be somewhere else), then the placement does not matter).

The planet would probably be in or near a nebula.

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  • $\begingroup$ A singularity shell, a planetary dyson sphere made from black holes to bend light around the planet. Ignoring all details, it's easy. $\endgroup$ – Samuel Feb 13 '15 at 20:22
  • $\begingroup$ Ignoring all details, is it possible to be natural, and could someone get to the planet? $\endgroup$ – Shadow Z. Feb 13 '15 at 20:24
  • $\begingroup$ Hmmm, no, on both counts. It was tongue-in-cheek comment. Do you have a distance or direction that it would be invisible from? Or does it need to be invisible until someone gets sucked into the gravity well? $\endgroup$ – Samuel Feb 13 '15 at 20:34
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    $\begingroup$ What sort of distances are we talking here? What range would you like people to approach before they see it? $\endgroup$ – Tim B Feb 13 '15 at 21:58
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    $\begingroup$ Does the gravity of the planet need to be 'invisible' as well? If you want to hide the planet from someone in the same solar system, remember that Neptune was discovered from its gravitational effect on Uranus in the mid-1800s. $\endgroup$ – 2012rcampion Feb 13 '15 at 23:51
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How to Cloak a Planet

The basic idea behind cloaking is very easy; bend the light rays around the planet in such a way that you restore the light to a path it would have traveled if the planet were not there. This sounds much easier than it actually is. There are only a few ways you can do this.

Gravity?

Gravity bends light, and so it can theoretically do the trick. In practicality, gravity as we know it cannot perform this. This is because the amount of gravity you need involve black-hole sized masses, and we would no longer call that a planet, but a black hole.

So gravity won't work.

Metamaterials

Most "cloaking" devices nowadays involve metamaterials (see this academic article! and this list from wikipedia!), materials which are composed of other materials but whose structure is arranged so that you get specific properties. Yes, you can get metamaterials to redirect light, absorb light, or simply prevent a lot of scattering from your object. Essentially, you can use them to cloak things.

Specifically, you're looking for materials with a negative reflective index. Your atmosphere will need a very specific arrangement of its layers, one that one likely be destroyed by any amount of weather. Also, it looks like this layer of atmosphere will be made up of metals.

Not that you can't have an atmosphere with metallic elements in it, it's just very unlikely.

Birefringence or "The Wonders of Calcite"

Some researches have been able to use calcite to cloak macroscopic objects. This is due to a special property they have called birefringence. In theory, your atmosphere can have some special arrangement of layers to cause this birefringence and other conditions needed to hide it. Alternatively, you can be in a nebula which produces such a layer around the planet.

Surface Cooling

You could, in theory, cool the surface of your planet such that it does not give off much light. This may not jive with your concept of how this cloaking should work, but I'm mentioning it here. Essentially, you cool an object's emissions from blackbody radiation enough that you can't distinguish it from the surroundings. This would be very, very cold; the planet would be near the temperature (on the surface) as space.

Some Issues with Cloaking

  1. If you're bending all the light waves around a planet, the planet itself doesn't get any light on the surface, because that light is being bent around it. This makes living life on the inside of your cloak hard.
  2. Sir John Pendry, a renown physicist who is considered a father of metamaterials, thinks that this is impossible. He doubts you can get enough of a material to hide anything more than a few centimeters across. Hiding a person, much less a planet, is a fantasy to him.
  3. The atmospheres of exoplanets which we know of do not contain massive amounts of metals, which many metamaterials need to work. Of course, the sci-fi genre has never been totally stopped by this before. Just realize that this boots your planet out of the hard sci-fi genre.
  4. The fact that humans see a spectrum of colors, not just one, prevents many cloaks from ever taking off. You may be able to hide from one wavelength, but not the others. Calcite seems to avoid this. I have focused on visible light, although invisibility cloaks for IR and microwave light exist. In fact, some tanks supposedly can do this in IR.
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You can cover the entire planet in an artificial structure that has very low reflectivity. For example you could cover the top in solar panels covered with material that absorbs the frequencies the solar panels do not use. Then use the solar power to cool the air trapped above the structure with heat pumps. If the heat was pumped into large gas bags, they'd function as hot air balloons and give aerostatic lift.

It would probably be easier to just have optics that guide sunlight on top to inside the planetary structure. Such lens/fiber optics already exist, so probably could be mass produced efficiently by anyone trying to "hide a planet". A combination system where some frequencies go to photoelectric systems and parts to optics that guide them to the surface might be possible.

It might also be possible use solar thermal to drive the heat pumps directly. Systems where a small "hot area", easily achievable by focusing sunlight, is used to drive a cooling cycle have been used for refrigerators in areas without electricity. Photoelectrics might be simpler though.

Height of the structure is problematic as you would want it be above as much of the atmosphere as possible and certainly above the mountains, but it is easier to build even a floating structure closer to the ground. Maybe that should be left as an implementation detail?

The system as described would essentially turn the planets albedo to be very close to zero,trapping all the light inside. This would make the planet very difficult to detect and probably be sufficient to answer the question. Two issues remain though.

A minor issue is that this would leave a shadow. There would be a perceptible dip in the luminosity of the star when the planet transited it. The planet would also still have a gravitional effect on the star, causing it to move slightly. This is also detectable from interstellar distances.

A MAJOR ISSUE is that with the planets albedo dropped to near zero, the planets temperature would rise rapidly and by a large amount.

First solution is to have a wide enough orbit. Planet on a wide orbit has lower gravitational effect on its star and also has habitable temperature even with much lower albedo. A previously habitable planet would need to be moved to a far orbit, a challenging engineering problem of its own. More practically otherwise inhabitable ice ball with naturally correct orbit could be terraformed to habitable temperature. This would probably be accompanying with use of artificial lights inside the cloaking structure to maintain visibility with reduced natural light.

Second solution is to radiate the heat selectively in a specific direction and with specific spectrum. Generate hydrogen plasma with temperature close to that on the surface of the star and guide the resulting radiation from a small opening in the structure to direction opposite from the star. From another star system this would hide the tell tale dip in the luminosity of the star and give a convenient place to dump lots of excess energy. Doing this would require a way to take excess heat from the planet and convert it into heat of the hydrogen plasma. Probably possible but it would have to be ridiculously expensive.

Finally, the best way to hide is to be where nobody is searching. Have your home far from those annoyingly bright stars and hide in the darkness. An orbit in the Kuiper belt might be enough unless you want to hide from somebody with permanent presence in the system.

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It really depends what range you want it invisible from. "From the outside" is quite a large range of values. I'm assuming "orbit around the star" (quite hard) rather than "orbit around a nearby star" (much easier) or "orbit around the planet itself (impossible)".

If you want a natural effect, probably the best you can do is to hide it the same way that islands were hidden in tales of yore - in a cloud.

A planet is defined as an object which has cleared its orbit, so technically, just by cluttering the orbit, you make the planet literally disappear - there is no longer a planet in that orbit, by definition.

But there's still one hell of a big rock. From a large enough distance, it'd be enough to fill the planet's orbit with enough high-albedo "chaff" that the planet would not be comparatively noticeable to instruments either when crossing the sun ("eclipse"), or when reflecting it ("full moon").

The closer you need the non-detection, the more dust you need. A light mist will conceal an island from ten miles out, but you need a pea-souper to conceal it from ten yards!

Now, downside is, the dust will make the star look really weird. "A star that age, with so much dust, such thickly-populated rings? How strange."

The dust will also affect the planet, reducing incoming sunlight, and falling constantly into the atmosphere in some brilliant and beautiful eternal aurorae, perhaps laced with shooting stars of the larger particles. The planet would be better protected against radiation, and could possibly be heated by the aurorae at least to some extent, but I'm not sure how effective that would be.

There'd also need to be something to produce the dust, which might in turn be detectable.

[Edit: If you only need to protect from viewing directly in line with the planet's orbital plane, then you don't need dust in the planet's path - you can have a larger ring outside the planet, like our asteroid belt but much thicker, which would obscure it sufficiently, from that one angle.]

[Edit2: Note that ring thickness is unstable, but rings can be cabled, and if we're assuming that the rings can be produced, they can be renewed anyway. A solid ring (Dyson ring) resolves the stability problem but becomes a world more complex to create, and creates its own stability issues.

And all this is moot in the longer term anyway, since as @2012rcampion commented on another answer, with sufficiently sensitive instrumentation, and long enough study (at least one year, poss many more), merely observing the motion of any visible planets and the sun should be enough to find a planet in the mist even from another star.]

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