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Thread: The number of rogue planets in the universe

  1. #1
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    The number of rogue planets in the universe

    Is there any reason to assume there aren't billions of rogue planets all around us that we can't detect? From what we have observed the are objects of all sizes out there. There are the supergiant stars, and everything smaller. From what we have observed, the most common type of star is a red dwarf. It would make sense, that the smaller the object, the more common it would be in the universe. We have trouble detecting even small stars with our current technology. I would then assume that there are many trillions of objects out there that are just too small to detect. Is there any reason there couldn't be billions of jupiter sized planets or planets of any size for that matter out in space that we can't detect that essentially are the center of their own solar system? I'm not well versed in this area, so I have no idea, if there's a reason that this isn't likely what is that reason?

  2. #2
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    There is a limit to how many there are which is imposed by gravitational microlensing studies. See the Wikipedia article for a readable introduction. Small planets and stars where a candidate for dark matter which is why they were searched for quite a lot. The conclusion was that they made up at most 20% or so of it.

    The other reason would be that there is no easy formation mechanism for these objects. Disks large enough to collapse gravitationally in reasonable time tend to form star sized objects and then planets form in the denser disk around them. So the number of small planets is likely to be limited by the number of stars. And then there is the metal problem - small, rocky planets would have taken some time to start appearing as early on there was just about only Hydrogen and Helium around.

  3. #3
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    Personally I think it's reasonable to assume there are more unbound planetary mass objects (PMOs) in the galaxy than there are proper stars. This is based on the following reasoning:

    Sumi et al in their microlensing study found about 1.8 Jupiter+ mass PMOs per visible star. Jupiter-mass was their detection limit, smaller PMOs wouldn't have been detected if present.

    Observations of star-forming regions show that fewer PMOs are formed by the stellar formation mechanism than stars themselves.

    This probably means that the majority of those 1.8 Jupiters per visible star were formed as planets, but have been ejected from their home systems early on in development.

    Planetary ejection is found to happen quite commonly in system formation models. These models are primarily about single-star systems, the feeling being that planetary ejections would be much more common for double or multiple star systems (which make up a large proportion of the stellar population).

    Since smaller mass planets are ejected preferentially over larger mass planets, that 1.8 Jupiters per star figure is the tip of the iceberg. There should be lots more smaller PMOs yet to be detected.

  4. #4
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    The numbers are huge unless you go to powers of ten. Our galaxy is more than 10^5 light years wide and about 10^4 thick so it has a volume over 10^14 cubic light years, so the number of rogue planets bigger than Mercury could be about 10^14 = 100,000,000,000,000 = one per cubic light year. Possibly none are presently within 0.1 light years, but one or two have passed that close in the last 1000 years and several are heading this way and will pass closer than 0.01 light years in the next 5000 years. Some of the detection methods have a slight chance of detecting an Earth size rogue planet 0.1 light years away, but most of the detection methods require a near-by star. Possibly the next full sky survey will find a rogue planet approaching and 0.01 light years away, because it occulted a more distant star briefly, while we were looking in that direction. Is panning data searched for occults = partial eclipse on some large telescopes? Panning data might also find faster than light stars, if there are any. On a photographic plate, panning produces parallel curved lines of uniform thickness. A thin spot in a line would indicate an occult. A non parallel line would be produced by meteors, aircraft and faster than light stars. Likely computerized data can also be searched easily.
    Several rogue planets per cubic light year is likely rare due to lack of metals in the early universe, but I suppose gas midgets (not giants) are possible until it passes within a few million kilometers of a star = interstellar comets. Neil
    Last edited by neilzero; 2013-Jun-19 at 07:19 PM. Reason: I believe that should have been over 10^9 cubic light years

  5. #5
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    neilzero wrote:

    ...faster than light stars...

    Never heard of this one before ! Please elaborate

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    Bump: I agree, there are likely no "faster than light stars" closer than about 13 billion light years, but we will likely miss them is we don't make a dedicated search, especially if they are much faster than light, which is even less probable. Neil

  7. #7
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    Quote Originally Posted by neilzero View Post
    Bump: I agree, there are likely no "faster than light stars" closer than about 13 billion light years, but we will likely miss them is we don't make a dedicated search, especially if they are much faster than light, which is even less probable. Neil
    I assume you mean faster-than-light stars, meaning stars that go faster than light, rather than things that go faster than "light stars," whatever those are. In which case, where did you ever get the idea that there even might be stars going faster than light? Don't you know that it's illegal to exceed c?
    As above, so below

  8. #8
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    Assume a rogue planet approaches within .01 ly (600 au) every few thousand years, wouldn't that mean a lot more disturbance in the Oort cloud and Kuiper Belt than we seem to actually see?

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