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Rogue Planets

  1. Oct 28, 2009 #1
    What is the chance of planets forming from gas clouds with out a star? has anyone calculated this? are they more likely to form than stars? could there be billions of rogue planets floating arround our galaxy?
     
  2. jcsd
  3. Oct 28, 2009 #2
    Hi there,

    By definition, a planet is form around a star. What you are referring to is called a "brown dwarf". A brown dwarf is formed when the initial mass of the cloud is not enough to start thermonuclear reaction in its center. Therefore, you have an object that never shines.

    For the rest, you are right, there might be millions of them just in our galaxy.

    Cheers
     
  4. Oct 28, 2009 #3
    Yea, I'm familiar with brown dwarfs, I was wondering about objects even smaller than brown dwarfs though. Say instead of condensing into a star system a cloud condensed into objects like earth, mars, merc and venus as well as jupiter, saturn and those. Basically a solar system without the solar part. Was just wondering what an "earth" would be like if it evolved without a stelar companion, not even a brown dwarf. Not to mention the possibility of planets that did evolve with a star but when the star "turned on" so to say and started shining, what if that initial explosion caused the planets forming with it to be ejected from that system, causing yet more "rogue planets"
     
  5. Oct 28, 2009 #4

    Wallace

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    The lower end of the stellar mass function has a lot of uncertainty. We really don't know how many condensed lumps of material too small to ignite fusion at the core there are out there. We have some limits from microlensing experiments, which have shown more signal than we expected, but they do at least tell us that the galaxy is not completely full of these things (in say the quantities that would explain dark matter).

    The question of whether you call something that is the same size and composition as Jupiter, but sitting isolated on its own is a semantic one. Such an object is technically referred to as a planet, and the term star is reserved for something which does burn at the core. The dividing line between star and planet is about mass, not orbital neighbours. Again though, it is semantics since there is no exact line that seperates the two and really it is a continuum from low mass stars to high mass planets.
     
  6. Oct 28, 2009 #5
    In the distant future, the Sun will become a red giant. It will swell to a hundred times its current size and the outer layers of the Sun will only barely be held in place by gravity. These outer layers will "fluff" away. About half of the Sun’s mass will be lost in this way and it will eventually become a white dwarf. This shift in mass may cause the outer planets to become rogue. It is reasonable to assume that other white dwarf stars may have lost exoplanets in a similar way.
     
  7. Oct 28, 2009 #6
    Not really an "initial explosion" that expels rogue planets, but your question now makes more sense. Oligarchic planet formation theories "predict" that a large number of smaller bodies - between Moon and Earth size - are mutually perturbing each other in the outer solar system. The migration of the gas giants may well eject quite a number of such "planetary embryoes" into interstellar space. There could be billions in the Galaxy.

    Alternatively a heavy gas/dust disk forms beyond 40 AU around a solar mass star. If the disk is heavy enough then it will gravitationally fragment into gas giant-brown dwarf mass bodies and these will disperse into the surrounding nebula. Smaller objects may well form with such bodies or be formed by the disruption of an object that hasn't had time to fully condense into a gravitationally bound object.

    So lots of options exist to make rogue planets and many billions probably float around the Galaxy.
     
  8. Oct 29, 2009 #7

    Chalnoth

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    I don't think you'd get rocky objects forming on their own in interstellar space. The reason why you get rocky objects in our own solar system is that the heat from the Sun has blown away most of the lighter materials, leaving behind what would otherwise be very rare rocky material (metals, silicates, and such).

    I imagine that below a certain size, gaseous bound objects just fail to form. Though that size is probably quite a bit below the smallest of the gas giants in our own solar system.
     
  9. Oct 29, 2009 #8

    Chronos

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    Gas clouds small enough to form brown dwarfs appear to lack the mechanics necessary to form planets of any appreciable size - so far as I know. You need repulsive forces, like fusion ignition and solar winds, to shock remnant materials into forming planets.
     
  10. Oct 29, 2009 #9

    Chalnoth

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    Naively I would expect the copious numbers of moons around our own gas giants would seem to argue otherwise. However, I will grant that perhaps the formation of those planets was aided and abetted by the Sun.

    Edit: Let me just add that when first formed, brown dwarves are quite hot indeed.
     
    Last edited: Oct 29, 2009
  11. Oct 29, 2009 #10

    Wallace

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    Brown dwarfs (dwarves?) with planetary companions have been found, but only a handful (look up 2M1207b and MOA-2007-BLG-192Lb on wiki or elsewhere. Catchy names those!).

    Also note that planets form via gravitational accrection in the dusty disk of protstars. You don't need shocks or solar winds to facilitate this process, just gravity.
     
  12. Oct 29, 2009 #11

    Chronos

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    I think that is arguable, Wallace. Gravity does the heavy work, but, without shock waves, how do you suggest accretion rings manage to clump and form planets? I believe I'm in good company suggesting shock waves account for most structure formation in the universe.
     
    Last edited: Oct 29, 2009
  13. Oct 29, 2009 #12
    When a star forms, how much of the proto planet mass gets ejected, instead of forming into planets circling the star? Could there be more earth sized planets in deep space then orbiting stars?

    This leads to another question: What are the chances of an earth sized or larger object entering the inner solar system? Is "When worlds collide" plausible within the life of the solar system? For all we know, some of the jovian moons could have formed on the other side of the galaxy. (Triton, anyone?)
     
  14. Oct 29, 2009 #13

    Wallace

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    As has been mentioned, the protoplanetary disks around Jupiter and Saturn (for instace) managed to form discreet orbital bodies without shocks or a solar wind. As is also the case in brown dwarf systems with planets. What else is the protoplanetary disk going to do? It is inherently unstable due to gravitational growth of perturbations, so either it will fragment and form into collapsed bodies, or simply accreate onto the star itself.

    Graviational heating during the collapse of a proto-star is sufficient to prevent runaway accreation onto the star, hence fusion reactions in the core of a system is not a requirement for the formation of satellites.
     
    Last edited: Oct 29, 2009
  15. Oct 29, 2009 #14

    Wallace

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    Certainly some mass would get ejected, though I don't have a good idea what proportion, it would likely depend on the proto-star mass. Note that the material ejected would be hot gas and dust, not formed planets since these form much more slowly. Three body gravitational interactions could eject some planet sized objects later on during the formation of a solar system I guess, but I'm not sure how common that would be?

    Very very low. The distance between stellar systems is many orders of magnitude greater than the orbital seperation of the parts of a system. Even if a planet was somehow ejected from a nearby system, its velocity would be low, and the target (us) so small that the odds are miniscule.

    The entry of such a body into a stable configuration like the solar system would cause enormous distruption, which would not be conducive the life. If life has evolved on a planet then it would suit the conditions on that planet and could be killed off by some change to its orbit inducude by the newcomer, that is supposing it finds a new stable orbit.

    No, the distances are too great and the velocites of any ejected planets far too small for a moon to cross the galaxy, even given the entire age of the Universe (let alone the age of the galaxy). I think it would be hard enough to get from star to star within the age of the Universe, let alone across the galaxy.

    It is also highly unlikely that a body entering the solar system anew would be captured into a stable orbit around say Jupiter. If you look at what happens to comets etc on plunging entries into the solar system from the Oort cloud, they don't end up orbiting the planets.
     
  16. Oct 29, 2009 #15

    Chronos

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    I doubt anyone can make a reasonable guess at the proportion of ejecta during stellar formation - too many variables. I would guess this is a large proportion, given Oort cloud observations. I also believe planetary formation is a reasonable consequence of mass exiting the solar system through the accretion ring during this phase of stellar formation. This is the kind of shock needed to clump matter into planets, imo.
     
    Last edited: Oct 29, 2009
  17. Oct 29, 2009 #16

    Wallace

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    Sorry, but that is a mash up of conflicting concepts and terms.

    The Oort cloud is a cloud of material graviationally bound (if only loosely) to the solar system, i.e. it is part of the solar system, not material ejected from it.

    What has the formation of planets within the solar system got to do with the ejection of material out of it? If anything ejection of material can potentially solve the angular momentum problem more efficiently that the formation of planets, so the greater the angular momentum shed via ejection the more the remaining material can directly accreate onto the star rather than forming planets.

    I think most of the material ejected during the pre-main sequence phase of a star is in the form of axial jets, which doesn't have much to do with what goes on in the circumstellar disk, which is where planets form.
     
  18. Oct 29, 2009 #17
    Just 15 km/s is enough to travel 50,000 ly in a billion years. What counts is the relative velocity between systems, not the usually low dispersal velocity of stars from their birth nebula. Getting between stars takes mere hundreds of kiloyears at 15 km/s.

    Some break-up, some collide, some end up in the Sun. Some escape. Oort cloud objects can be traded between stars because the Cloud is such a huge target relative to the inner system and so weakly bound. But you're right that it would be rare - but not impossible - for an interstellar interloper to be captured by a planet. Would require a binary pair to carry off excess momentum by one member escaping. Tidal dissipation can capture moons and planetoids too, but probably not at a high hyperbolic excess.
     
  19. Oct 29, 2009 #18

    Chronos

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    Perhaps, although I find it likely the Oort cloud originated from something suspiciously resembling an ejection or gravitational stripping event.
     
    Last edited: Oct 29, 2009
  20. Oct 29, 2009 #19

    Chronos

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    The sun may have originated as a member of a binary system that was gravitationally ravaged billions of years ago. The possibilities are unbounded.
     
    Last edited: Oct 29, 2009
  21. Oct 29, 2009 #20
    Droll, very droll. There's always Edward Drobyshevski's theory that Jupiter is the Sun's binary companion, but it failed to condense fully, instead throwing off mass that became the planets. It's quite a good theory. He predicts thousands of Moon-mass objects in the Oort Cloud too.
     
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