On the number of planets in the outer solar system: Evidence of a substantial population of 1000-km bodies

Abstract

Where have all the Tritons gone, long time ago? A natural consequence of the accumulation stage of planetary formation is the scattering of planetesimals during the accretion epoch. In the outer Solar System, such scattering contributed to the formation of the Oort cloud (with an orbital semimajor axis distribution 10 3 < a < 5 < 10 4 AU) and the Kuiper disk (30 < a < 500 AU). We suggest here that in addition to comets, a potentially large number of planetary bodies in the size class near 1000 km may have also been scattered into the Kuiper disk and Oort cloud. This hypothesis implies that the present population of planets in the outer Solar System is much larger than previously recognized. To support this claim, we present calculations showing that the large tilts of Uranus and Neptune, the capture of Triton, and the formation of the Pluto-Charon binary each argue for the past presence of numerous 1000-km bodies in 20–50 AU region. We then summarize arguments that scattering dominated other loss processes for these objects, leading to the conclusion that many of these bodies should presently be located beyond the “regular” planetary system. Finally, we examine the detection prospects for such bodies, and compare the existing observational constraints to our model predictions of the number of such bodies in the region between 30 and 500 AU from the Sun.