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Thread: Exoplanets always so large?

  1. #1 Exoplanets always so large? 
    Forum Bachelors Degree x(x-y)'s Avatar
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    I've been wondering recently why pretty much all (if not all) of the exoplanets discovered- including the large amount of recently discovered exoplanets by the Kepler telescope- are larger than the Earth and much larger than smaller planets in our solar system such as Mercury and Mars. Do the laws of physics favour the formation of larger planets from the disc of matter that originally surrounds a star? Or have we just not found smaller planets yet for the very reason that they're small and don't exert a large gravitational wobble upon their parent star?

    I'm just interested to find out the reason to why only large exoplanets (especially those such as Wasp-12B [I think that's what it's called]) have been found.

    Thanks...


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    This issue is caused by observational bias. We are not sensitive enough to detect smaller (= less massive) planets. Their gravitational pull on the star they are revolving around is too small to produce observable effects.


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    Quote Originally Posted by Dishmaster
    This issue is caused by observational bias. We are not sensitive enough to detect smaller (= less massive) planets. Their gravitational pull on the star they are revolving around is too small to produce observable effects.
    Ah, I thought this might be the reason. So, how will we go about detecting these smaller planets then? Simply (well, I say 'simply') making our equipment more sensitive? Or using an alternate method?
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    Quote Originally Posted by x(x-y)
    Quote Originally Posted by Dishmaster
    This issue is caused by observational bias. We are not sensitive enough to detect smaller (= less massive) planets. Their gravitational pull on the star they are revolving around is too small to produce observable effects.
    Ah, I thought this might be the reason. So, how will we go about detecting these smaller planets then? Simply (well, I say 'simply') making our equipment more sensitive? Or using an alternate method?
    yes
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    In fact, both alternatives are possible and are actually tried. A good site for looking for detailed answers is:
    http://exoplanet.eu/
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    In fact we are discovering more smaller planets now as more observing time, and more sensitive methods are employed. It's still very hard though.
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    Quote Originally Posted by Dishmaster
    In fact, both alternatives are possible and are actually tried. A good site for looking for detailed answers is:
    http://exoplanet.eu/
    Thank you, that site is very useful actually!
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    There have been reports lately (on BBC etc) that free floating planets the size of jupiter maybe more common than stars.
    That is 10 already found within 10 au.
    These at the moment are all big found by gravitational lensing, but should we expect smaller ones out there being even more common?
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    Ah yes, I read about that (and heard it on the radio). The "free floating" planets found (10 of them) were found by gravitational microlensing, however we cannot be sure that they definitely don't have a parent star at this point apparently. Still very interesting though, these regular fascinating findings are a part of what make me love physics!
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    Another link to another rogue exoplanet story; http://physicsworld.com/cws/article/news/46022
    How freaking cool are rogue exoplanets!
    I wonder if this will cause a recalculation of dark matter estimates?
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    Quote Originally Posted by GiantEvil
    Another link to another rogue exoplanet story; http://physicsworld.com/cws/article/news/46022
    How freaking cool are rogue exoplanets!
    I wonder if this will cause a recalculation of dark matter estimates?
    The gravitation lensing technique they used is biased towards find larger objects, so many more smaller planets could be out there.
    However, they only saw object that past near a background star, so bigger ones could be out there too.
    Possibly it could add a significant amount of matter to the hard to see category, and maybe knock a few percent of the dark matter estimates, as a guess.
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    Quote Originally Posted by PetTastic
    Quote Originally Posted by GiantEvil
    Another link to another rogue exoplanet story; http://physicsworld.com/cws/article/news/46022
    How freaking cool are rogue exoplanets!
    I wonder if this will cause a recalculation of dark matter estimates?
    The gravitation lensing technique they used is biased towards find larger objects, so many more smaller planets could be out there.
    However, they only saw object that past near a background star, so bigger ones could be out there too.
    Possibly it could add a significant amount of matter to the hard to see category, and maybe knock a few percent of the dark matter estimates, as a guess.
    Apparently this guess has been made by others. It has the problem of not being able to account for a significant portion of dark matter thought to exist without causing big problems for the currently accepted Lamda-CDM big bang cosmological model:

    Theoretical work simultaneously also showed that ancient MACHOs are not likely to account for the large amounts of dark matter now thought to be present in the universe.[5] The Big Bang as it is currently understood simply couldn't produce enough baryons without causing major problems in the observed elemental abundances,[6] including the abundance of deuterium.[7] Furthermore, separate observations of baryon acoustic oscillations, both in the cosmic microwave background and large-scale structure of galaxies, set limits on the total baryon-to-total matter ratio. These observations show that a large fraction of non-baryonic matter is necessary regardless of the presence or absence of MACHOs.
    (ref. http://en.wikipedia.org/wiki/Massive...considerations )

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    I did only guess a few percent :wink:

    One of the interesting things being discussed about this is the assumption that these were ejected from planetary systems and not part of the brown dwarf or failed star population .

    To eject a Jupiter size object you need a gravitation boost from a very big object or you need to pass very close to an object of say 5 Jupiter masses otherwise you will not get escape velocity.
    Jupiter only has a surface gravity of 2.4g so the 'guess' is it would be ripped apart by a close very close transit behind an object of 5 x its mass.
    The probability of multiple boost from a few planets all positive seems unlikely to explain such a large population of free planets of such size.

    Even bigger guesses
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  15. #14  
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    Quote Originally Posted by PetTastic
    To eject a Jupiter size object you need a gravitation boost from a very big object or you need to pass very close to an object of say 5 Jupiter masses otherwise you will not get escape velocity.
    Evidence? Reference?

    We are not talking about fully evolved planetary systems here. The idea is that they were ejected during the formation of planets out of a dusty, gaseous and viscous disk. Tidal and gravitational interactions in such an environment is much more difficult to simulate or describe than a simple two body problem. So, I am not convinced that you really need single massive objects for an ejection.
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    So the thought is that these rogue planets were ejected from their systems through orbital resonance or by some other mean?
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  17. #16  
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    Quote Originally Posted by x(x-y)
    So the thought is that these rogue planets were ejected from their systems through orbital resonance or by some other mean?
    Interacting with other planets can cause it. For example: a "gravitational slingshot" approach was used by NASA to propel the Voyager satellite into the outer areas of the solar system by bringing it close to some of the inner planets in a way that would increase its velocity.

    http://en.wikipedia.org/wiki/Gravity_assist
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  18. #17  
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    http://arxiv.org/abs/1103.0556v1
    http://arxiv.org/abs/astro-ph/0701485v2
    Some freely downloadable material on protoplanetary disks and system formation.
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  19. #18  
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    Quote Originally Posted by Dishmaster

    Evidence? Reference?

    We are not talking about fully evolved planetary systems here. The idea is that they were ejected during the formation of planets out of a dusty, gaseous and viscous disk. Tidal and gravitational interactions in such an environment is much more difficult to simulate or describe than a simple two body problem. So, I am not convinced that you really need single massive objects for an ejection.
    I thinks you have forgotten the arguments we had when I first joined, about the massive particles systems(for 25 years ago anyway) that I helped program.

    It is easier to eject a planet from a fully formed system, because escape velocity is only 1.41 (root 2) times orbital velocity. For a partially formed system with not all mass concentrated at the centre this number increases rapidly, because the planet is orbiting inside some of the mass hence has a much lower orbital velocity, but escape velocity remains the same, as the mass is the same just spread out over more space. Also in a disk environment tidal force act will limit the the availability of objects in an eccentric orbit, that could give it the kick.

    It is hard to see how tidal forces from the disk can assist as the disk is travelling just below orbital velocity and slowly falling in. (Slowed by tidal forces from matter joining the disk)
    I can only see tidal forces slowing something that is trying to traverse the disk at escape velocity. Plus a juptier size object would probably acrete some slower moving mass on its way out.

    As for the estimates they were based on near best case numbers, that is traversing behind an planet on an eccentric orbit as it approaches the star, and that planet staying between the star and the planet being kicked.

    Remember the total forces on the planet are much greater than those needed just to increase its speed by 41%.
    That 41% is only the difference between the speed gained as the planets approached each other and the speed lost as they move apart. Speed at closest approach could easily be ten times that or more.
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