Comets, Planet X and the orbit of Neptune

Abstract

The recent discovery that Pluto's mass is negligible compared with that of the Earth¹ has raised again the question of the cause of the important discrepancies between the observed and computed positions of the outer planets^2,3. The unmodelled force appears to require a mass of ≈1M_⊕ at a distance of order 50 AU from the Sun, but the non-discovery^4,5 of any single body of this size puts the ‘Planet X’ hypothesis in severe difficulty. Recent work on cometary theory⁶ has shown, however, that in order for the Oort cloud to survive the strong perturbations induced by close encounters with giant molecular clouds^7,8 it is necessary to postulate a much more centrally condensed comet cloud than Oort⁹ originally described. Thus a measurement of the cometary space density just beyond the planetary system could provide a crucial test of interstellar and primordial theories of comet origins. One way by which this could be done was suggested previously¹⁰. Here I have modified an earlier suggestion by Whipple^11,12and propose that a dense inner core for the Oort cloud could provide the unmodelled force which perturbs the planets