Dark Messengers from the Edge of the Solar System: Can Dark Matter Trigger Comets?
Comets have fascinated humans for millennia, but in Oort Cloud Bombardment by Dark Matter, Jeremy Mould asks whether these icy visitors might also be telling us something fundamental about dark matter. The paper explores a bold idea: if some fraction of dark matter is made of relatively massive objects, such as primordial black holes (PBHs) or other lunar-mass bodies, then their gravitational flybys through the distant Oort cloud could knock comets toward the inner solar system. This would link the observed rate of long-period comets to the properties of dark matter itself.
Background: Comets, the Oort Cloud, and Dark Matter
The paper begins by reviewing what is already known about how comets are produced. The Oort cloud is a vast, roughly spherical reservoir of icy bodies far beyond Neptune. Traditionally, gravitational tides from the Milky Way and passing stars are thought to be the main forces that disturb Oort cloud objects and send them inward. Mould points out that if dark matter is not made of tiny particles like WIMPs or axions, but instead includes macroscopic objects with masses around the mass of the Moon, then these objects could act as additional perturbers. Because dark matter is much more numerous than stars in the solar neighborhood, the rate at which such objects pass near the Oort cloud could be extremely high.
The Toy Model of Oort Cloud Bombardment
To test this idea, the author introduces a simplified “toy model” of the Oort cloud and its interaction with dark matter. The model assumes a realistic size distribution for protocomets and adopts commonly used density profiles for how those objects are spread with distance from the Sun. Instead of simulating every comet individually (which would be computationally impossible), Mould follows the motion of 250,000 representative particles. These particles experience gravitational kicks from passing lunar-mass dark matter objects, whose velocities reflect both the Sun’s motion through the Galaxy and the random motions of the dark matter halo.
Tracking Comets into the Inner Solar System
The key physical process in the model is momentum transfer during a close flyby. When a dark matter object passes within a certain impact parameter, it can significantly change a comet’s velocity. The simulations track which protocomets are perturbed strongly enough to fall inside a radius of 300 AU, marking their entry into the inner solar system. While this radius is larger than where comets are usually observed, it allows the author to estimate comet production rates without excessive computational cost.
Results: Matching the Observed Comet Rate
The results show that the simulated rate of comets entering the inner solar system can match or exceed the observed rate, if a substantial fraction of dark matter is made of such objects. If all dark matter were lunar-mass PBHs, the predicted rate would be far too high. However, if only about 10% of the dark matter is in this form, the comet arrival rate becomes comparable to what is inferred from observations. The exact rate also depends on assumptions about the structure of the Oort cloud, with shallower distributions producing fewer comets.
Comet Supply Versus Observed Demand
The paper then compares this “supply” of comets to the observed “demand.” Using data from the JPL Small Bodies Database and recent surveys, Mould estimates how many comets currently reside within 300 AU and how quickly they are lost through hyperbolic or near-parabolic orbits. The simulated dark-matter-driven supply can replenish this population on timescales much shorter than a million years, but only if the fraction of massive dark matter objects is not too small. This comparison places indirect constraints on how much of the dark matter could exist in lunar-mass form.
Discussion and Conclusions
In the discussion and conclusions, Mould emphasizes that this work is an exploratory first step. The model neglects important effects such as planetary perturbations and uses a simplified treatment of encounter distances. Still, the results suggest that macroscopic dark matter could play a partial role in shaping the comet population. Future microlensing surveys with telescopes like Rubin and Roman could directly test whether such dark matter objects exist. If confirmed, dark matter might not only help explain how galaxies formed, but could also have influenced the delivery of water, and possibly the conditions for life, on Earth.
Source: Mould
