Oort Cloud Bombardment by Dark Matter

TL;DR

This work investigates whether lunar-mass dark matter objects, such as primordial black holes or similar macroscopic DM, can dynamical perturb the Oort cloud to deliver comets into the inner Solar System. It introduces a toy dynamical framework with a spherical Oort cloud, a power-law size distribution $N \propto D^{-\alpha}$, and a Dehnen-like density profile $n \propto \frac{a}{r^2(r+a)^2}$ (with $a=10^5$ AU), then numerically evolves $2.5\times10^5$ protocomet test particles under DM impulses to obtain the delivery rate $rate_2$ from the DM arrival rate $rate_1$, including corrections for impact parameter limitations. The results indicate that, for lunar-mass PBHs in the range $m_{DM} \in [10^{-9},10^{-5}]\,M_\odot$ and plausible $b_{max}$, $rate_2$ can exceed $10\,\mathrm{yr}^{-1}$, while reducing the PBH fraction to $f_{PBH}\approx 0.1$ lowers the flux toward unity; the study also compares the available inner-Solar-System comet reservoir (roughly $8.2\times10^{4}$ objects inside 300 AU) with observed HC/NPC rates to assess compatibility. The authors discuss observational tests via microlensing with Rubin/Roman to constrain lunar-mass DM, emphasize the need to include planetary perturbations in future work, and highlight how such macroscopic DM hypotheses could inform our understanding of DM particle mass scales and solar-system formation processes.

Abstract

The realization that primordial black holes (PBHs) might be some fraction of the dark matter begged the question, how often do PBHs enter the solar system? For a Neptune radius solar system the answer is, rarely. For an Oort cloud sized system the answer is different. Simulations of bombardment of the Oort cloud by dark matter suggest that dislodgement of protocomets and their entry into the inner solar system can match the observed frequency of comets, if that PBH fraction is high enough. Comets were traditionally considered as messengers, usually omens. After 50 years of puzzlement regarding dark matter, we need a hint from the dark universe about the size and nature of dark matter particles.

Figures (6)

• Figure 1: The Dehnen (1993) distribution in the solid line. Other authors suggest a radial power law with index -3.5, and this is is shown by the dashed line. Brasser et al. (2006) finds that the median distance falls off as $\surd$n. Two variants of this density law are shown as the red and green dashed lines respectively. The distribution adopted by Kaib & Quinn (2008) is represented by the solid symbols, based on the cumulative distribution they provide.
• Figure 2: Distribution of particles in the xz plane 0.05 Myrs into run 25.
• Figure 3: Distribution of velocities 0.05 Myrs into run 25.
• Figure 4: (a) Numbered comet semi-major axes from the JPL small bodies database (above). Orbital eccentricities (below).
• Figure 5: Simulated comet arrival rate in the inner solar system as a function of DM mass and maximum impact parameter. On the right is the colour scale which is logarithmic and ranges from 0.2 to 20. Deeper blue $<$ 0.1.