# Interstellar “reefs”, plausibility and detection [closed]

Most of the interstellar medium is, as far as we can tell with current technology, pretty empty and most of what is there is in the form of single atoms or tiny particles only a few molecules across. Is it plausible then to have denser regions of matter in deep interstellar space, possibly the remnants of long dead star systems? And if these areas did exist how close would we need to get with current technology to detect them?

For the purposes of giving a coherent answer assume that the areas in question are similar in volume, number of objects, spatial density, and overall mass to Jupiter's L4 Trojan Asteroids and have similarly low albedos (0.04 to 0.12). Note that for the purposes of detection these regions would be outside the Heliosphere of all the nearest stars so the amount of radiation available for them to reflect or occlude will be correspondingly low.

Note carefully that I'm not asking if we can detect something like the Trojans since obviously we can we're in the process of counting them; I'm asking how close we have to be to detect a scatter of such small dark objects when they are far from the nearest star.

## closed as off-topic by Monty Wild♦Oct 30 '18 at 6:55

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• I feel like this is more appropriate for a different Stack Exchange. space.stackexchange.com – Sora Tamashii Oct 29 '18 at 16:58
• @SoraTamashii Maybe I'll give it 24hrs here and see what happens with it though since it's already posted. – Ash Oct 29 '18 at 17:20
• It will be hard. Best bet is to wait for your cloud to obstruct a star. Also, why wouldn't a bunch of asteroids collapse into a small planet, given enough time? Or why wouldn't they fly away from each other? Keeping them in a single cloud requires some careful engineering :) – Bald Bear Oct 29 '18 at 19:23
• @SoraTamashii, Monty Wild: This is not on topic at Space Exploration. As it is about observation of interstellar space, it could have been considered for Astronomy, not Space Exploration. Space Exploration is focused on what we can send spacecraft to. – called2voyage Oct 31 '18 at 20:11
• Ah, my apologies. I typed in the one that came to mind first. Sorry! – Sora Tamashii Nov 8 '18 at 22:47

Answering the second part of your question on how to detect such objects: Since there is not much light for those objects to reflect, the best available tool would be radar astronomy, where you effectively provide your own light-source in the form of radar waves.

The rest then just is numbers. I'm not sure what the current state of the art is, but I can try to give a rough estimation. Wikipedia mentions the Galilean moons as objects that we have observed. So say that we can observe a 3000km diameter object at 5AU. The largest trojans are about 100km in size which leaves us with about $$1/30^2=1/900$$th of the surface. I'm not sure about the difference in albedo, since we are not using visible light, so I'll just assume it to be roughly the same. Radar loses signal strength with the fourth power of the distance (squared on the way out, then again squared on the way back in), so we'll get about $$\sqrt[4]{1/900}=\sqrt{1/30}\approx1/5$$th of the distance, in other words about 1 AU of range.

Of course you specify today's technology while talking about interstellar travel. If you are willing to lift a sufficient amount of mass into space, today's technology could build some quite impressive radar dishes when not having to deal with pesky gravity or securing sufficient funding for research without direct application. So 10 or even 100 AU might be feasible as well.

• Astronomy by radar can only observe a known object, it cannot detect objects. According to wiki, they need precise location of the object (ephemeris). B/c they can only look at one point in the sky, and they need to wait minutes (or hours) for return signal, and they have only a few receiver dishes: en.wikipedia.org/wiki/Radar_astronomy#Disadvantages – Bald Bear Oct 29 '18 at 19:20
• Yes, but this is a financial, not a technological problem. Its perfectly possible to find objects using this technique by just pinging a signal somewhere into the sky and waiting a few hours for any result and then repeat at a slightly different angle ad nauseam. It is just rightly considered a terrible waste of perfectly good radio-telescope time. – mlk Oct 29 '18 at 20:47

Yes, many rogue planets have been discovered, and they are believed to be very common, with hundreds of millions or billions in our galaxy. To answer the rest of your question, they can be detected with our current technology, and they have been.

If you didn’t mean to include rogue planets in your scope, you will have to rephrase the question, since a planet is certainly a “dense region of matter”.

• An earthlike planet is both far too large and far too small to fit the parameters of the question as originally written. The Trojans cover a vast area of the solar system but with a mass less than 1% that of Earth's moon. Furthermore the 16 known rogue planets are all super jovians massing thousands of times more than the Earth. – Ash Oct 29 '18 at 14:21
• @Ash The 16 known rogue planets are super-jovians because they're the easy ones to detect & observe. There could be lots of interstellar rubble out there. Also, what you describe sounds like what might be found in the Oort clouds. – a4android Nov 9 '18 at 1:18