Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How big bodies can exist in outer Solar System?

  1. Jan 9, 2013 #1
    Well, how big things can exist unseen?

    We did see Pluto - but very importantly managed to not see Eris till very recently. What else may we not see?

    Eris at 95 a. u. was found recently. The total luminosity of a reflective object decreases with fourth power of distance, so that we could miss an Earth-sized object at about 200 a. u., Neptune sized object at 450 a. u. and Jupiter sized object at 800 a. u.

    There is stuff out there at 900 a. u., because that is where Sedna´s aphelion is. Can we observationally rule out a brown dwarf star at Sedna´s orbit? Nothing is bigger than Jupiter except red dwarf and bigger stars, something which we do have observational evidence against (a star the size of Proxima Centauri and as far from Sun as Proxima is from Toliman would be a naked eye object, though an inconspicuous one).

    We know that Neptune has some inner heat. But then, we know where to look. Would a body as big and hot as Neptune (50 000 km and 50 K) located far enough to be missed in reflected visible light (say 500 a. u.) be seen in infrared?
  2. jcsd
  3. Jan 9, 2013 #2


    User Avatar
    Science Advisor

    There is no fundamental size limit. The limits we have are technological - how powerful are present day telescopes.
  4. Jan 9, 2013 #3


    User Avatar

    Staff: Mentor

    How can something that isn't even theorized to exist already have a name? :rolleyes:
  5. Jan 9, 2013 #4


    User Avatar
    Science Advisor
    Gold Member

    There are constraints on how much mass can exist in the kuiper belt/oort cloud without noticeably affecting planetary orbits. It is not more than a few earth masses.
  6. Jan 18, 2013 #5
    Found a claim that WISE would have seen Neptune at 700 AU and Jupiter at 63 000 AU.
  7. Jan 18, 2013 #6
    *Disclaimer - not a physicist but thought I'd try and chip in.*

    Firstly, when you say big, don't underestimate how 'big' space is. The needle in a haystack analogy does no justice to the relative size of dwarf planets and the solar system.

    Where did you hear/read that jupiter was the biggest thing appart from red dwafs and bigger stars? Theoretically, in terms of gaseous planets, they can exist to be up to 15 Jupiter masses or so before there is enough pressure in the core to ignite deuterium fusion, hence becoming “brown dwarfs” rather than planets. The biggest planet in the Universe (at the time of this writing)* is TrES-4, which is located 1,400 light years away in the constellation Hercules. The planet has been measured to be 1.4 times the size of Jupiter, although it is only 0.84 times Jupiter’s mass.

    * http://www.universetoday.com/13757/how-big-do-planets-get/#ixzz2ILYfkycT

    ... or did you mean it's the biggest thing you can see with the naked eye that isn't a star?

    What makes you think that something 500AU away would not be visible? where did you get that information?

    Is your question referring to the size of an object and the limits of the lenses we use, i.e. resolution of an object due to the focal length etc. of the lens you are using not allowing you to see objects beyond a certain distance?

    As for the distance of our observations, "According to calculations, the comoving distance (current proper distance) to particles from the CMBR, which represent the radius of the visible universe, is about 14.0 billion parsecs (about 45.7 billion light years), while the comoving distance to the edge of the observable universe is about 14.3 billion parsecs (about 46.6 billion light years)"**

    UDFj-39546284 is the designation given to a stellar structure reported January 27, 2011, as light from the oldest object detected through infrared, this is 13.75 light years away***

    ** http://en.wikipedia.org/wiki/Observable_universe

    *** http://en.wikipedia.org/wiki/UDFj-39546284

    1 light year is roughly 63,241.077 AU so we can at least observe things well in excess of 500 AU away. But as I mentioned the way in which you observe them i.e. the lenses you use etc will effect the resolution of what you can observe...

    Again I should note that I'm not a physicist so my apologies if none of this is useful in answering your question. And apologies to any physicists if I've made any blinding errors!
  8. Jan 19, 2013 #7
    Yes, but it is a hot Jupiter bloated by nearby star - and its size was a theoretical surprise even so.

    Jupiter is 3 times the mass of Saturn, but mere 15% bigger, because of compressibility. It is generally expected that objests more massive than Jupiter but less massive than red dwarfs are compressed by gravity so that they are not bigger than Jupiter.
    Read the second and third paragraphs in my original post.

    Eris, 2300 km across and 97 AU away, was spotted only recently - missed until then.

    A planet 5 times further away than Eris but 25 times larger in radius would have 625 times the area of Eris. This area would receive and reflect back 25 times the sunlight Eris does, but the amount of reflected light reaching Earth would equal the amount received from Eris. Since we missed Eris till recently, we could have missed a planet bigger than Neptune and inside 500 AU in the same way.
  9. Jan 19, 2013 #8


    User Avatar
    2017 Award

    Staff: Mentor

    The Kuiper belt does not have enough mass for really big (solid) objects -> see here for some numbers.
    The Oort cloud, on the other hand, could contain bigger objects, even as big as Jupiter.
  10. Jan 19, 2013 #9


    User Avatar
    Science Advisor
    Gold Member

    Weissman estimated the Oort cloud contains around 2 earth masses - http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1983A%26A...118...90W&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf [Broken]
    Last edited by a moderator: May 6, 2017
  11. Jan 20, 2013 #10


    User Avatar
    2017 Award

    Staff: Mentor

    That is just the mass of comets ("The total mass of comets in the Oort cloud is calculated"). Anything orbiting in a circular orbit (or at least with a perihelion in the Oort cloud) is not included, I think.
  12. Jan 29, 2013 #11
    orbits would be affected. the bigger the body the bigger the effect.
    the effects could lead orbital migration, differing orbital resonences
    so what can dictate the size of a body in the outer solar system?
    well if it was made within the solar system, the amount of gas and or rock the body can gravitationally attract before it either runs out of material ( due to the materials scarsity and/or the protosun undergoing steller wind/bi polar outflow blowing the material away). and then plantery migration can cause this bosy to migrate to the outer system.
    another senario is that a massive body formed else where somehow got captured (unlikely).

    and calculations have shown that the orbits of the current system tally with the mass detected.
  13. Jan 29, 2013 #12


    User Avatar
    Science Advisor
    Gold Member

    Eris is the 4th brightest TNO (trans-neptunian object), but it was about the 500th one discovered. Brightness isn't everything. Eris went undetected because of how slow it moves against the background stars. The discovery of Sedna, also slow-moving, made Mike Brown lower his velocity threshold in the searches to discover more slow-moving objects. But the lower the threshold, the more false-positives.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook