franka@mmintl.UUCP (Frank Adams) (01/03/86)
I have for some time suspected that there are a fair number of sub-stellar objects in the galaxy, which are not associated with any star system. I have seen estimates that a mass of .07 times that of the sun is the minimum required to ignite fusion. Proxima Centauri seems to be near this lower limit. Although there may be a lower limit to the size of the objects produced by the processes which produce stars, it seems unlikely that this point coincides with the ignition point. How many such objects are there likely to be? For stars, there is a clear relationship between size and frequency: the smaller the size, the more stars there are at that scale. There is no reason to believe this does not hold down to near the lower limit for the process. Thus if objects appreciably smaller than .07 solar mass can be created in this way, one expects them to considerably outnumber the stars. There is reason to think that objects as small as .0001 solar mass are possible, since this is the approximate size of the outer giant planets (Uranus and Neptune), which seem to have been formed in a similar fashion (although as part of the formation of the sun, not as independent events). The question is, could such objects be responsible for the observed perturbations of the orbits of the outer planets? A quick calculation reveals that an object of .05 solar mass would have to be at a distance of about 300 AU to produce an effect comparable to a giant planet at Pluto's orbit. This is still only 1/2000 of a light year; it doesn't seem likely that there could be enough objects of this size that one would expect an object this close, but we could be dealing with an unusually close approach. The other possibility is that objects on the order of .0001 solar mass are freqent enough that one would expect the nearest to be no more than 100 AU or so away. I don't have access to a size/frequency comparison for stars, so I don't know how reasonable such a frequency is. This would still, I think, be low enough that one would not expect an observably close encounter of such an object with the sun in the time in which we have been making observations. The uniformity of the orbits of most of the planets puts an upper limit on the frequency of such encounters; however I suspect that the strangeness of Pluto's orbit can be explained as the result of such an encounter. (Perhaps also Uranus's rotational tilt? I don't know how likely it would be for a close encounter to change the rotational axis while leaving the orbit roughly circular.) Disclaimer: I have no professional training in Astronomy, and have not attempted to really rigorously work out the consequences of this hypothesis. Frank Adams ihpn4!philabs!pwa-b!mmintl!franka Multimate International 52 Oakland Ave North E. Hartford, CT 06108
friesen@psivax.UUCP (Stanley Friesen) (01/06/86)
In article <971@mmintl.UUCP> franka@mmintl.UUCP (Frank Adams) writes: > >I have for some time suspected that there are a fair number of sub-stellar >objects in the galaxy, which are not associated with any star system. >.... Although there may be a lower limit to the size of the objects >produced by the processes which produce stars, it seems unlikely that this >point coincides with the ignition point. > >How many such objects are there likely to be? For stars, there is a clear >relationship between size and frequency: the smaller the size, the more >stars there are at that scale. There is no reason to believe this does not >hold down to near the lower limit for the process. Thus if objects >appreciably smaller than .07 solar mass can be created in this way, one >expects them to considerably outnumber the stars. There is reason to think >that objects as small as .0001 solar mass are possible, since this is the >approximate size of the outer giant planets (Uranus and Neptune), which >seem to have been formed in a similar fashion (although as part of the >formation of the sun, not as independent events). > As far as I know the mode of formation of the outer planets is *very* close to that for stars. In fact a number of simulations suggest that a very small change in initial conditions would have left a small (type M or K) star where the Jovian planets are now. Given the large number of binary(and larger) star systems, this seems quite reasonable. Thus I see no reason at all why Uranus sized object could not form independently also. Actually, I would expect the distribution of sizes to peak at some point and taper off, thus the smallest objects would likely be quite rare. But this would still leave quite a few Jupiter/Saturn sized objects running around. -- Sarima (Stanley Friesen) UUCP: {ttidca|ihnp4|sdcrdcf|quad1|nrcvax|bellcore|logico}!psivax!friesen ARPA: ttidca!psivax!friesen@rand-unix.arpa