Solar System Dark Matter

I review constraints on solar system-bound dark matter, and discuss the possibility that dark matter could be gravitationally bound to the earth and other planets. I briefly survey various empirical constraints on such planet-bound dark matter, and discuss effects it could produce if present, including anomalous planetary heating and flyby velocity changes.

💡 Research Summary

The paper provides a comprehensive review of the possibility that dark matter (DM) may be gravitationally bound not only to the Galaxy but also to the Solar System as a whole and to individual planets. It begins by noting that most DM constraints come from galactic‑scale observations—rotation curves, cluster dynamics, cosmic‑microwave‑background anisotropies—and that these do not directly limit the presence of DM on sub‑AU scales. The author therefore turns to Solar‑System dynamics to set quantitative bounds.

Using long‑term ephemerides of the outer planets, laser ranging to the Moon and to artificial satellites, and radio‑science tracking of spacecraft, the paper derives an upper limit on the average DM density within the Solar System of roughly 10⁻¹⁹ g cm⁻³. This is several orders of magnitude higher than the canonical Galactic halo density (≈10⁻²⁴ g cm⁻³), meaning that a locally enhanced “planet‑bound” component could, in principle, exist without violating current observations.

The analysis then focuses on individual planets. For Earth, high‑precision tracking of LAGEOS, GRACE, and satellite laser ranging data constrain any additional mass at the planet’s centre to be less than about 10⁻⁹–10⁻⁸ of Earth’s total mass. Similar limits are obtained for Mars, while for the massive gas giants the constraints are slightly weaker (≈10⁻⁸–10⁻⁷ Mₚ) because of the larger gravitational sphere of influence and the presence of the Galilean moons, whose orbital residuals provide independent checks.

Having established the allowed mass budget, the author explores the physical consequences if such bound DM were present. The most direct effect would be a contribution to planetary heat flow. As DM particles sink toward the planetary core, they would be compressed by the planet’s gravity, releasing a modest amount of gravitational potential energy as heat. For Earth, the total geothermal flux is about 44 TW; the DM‑induced component would be at most a few hundred megawatts—far below current detection thresholds. For Mars, where the internal heat budget is much smaller, a DM contribution could be a non‑negligible fraction of the observed heat flow, potentially influencing localized volcanic or geothermal activity.

The paper also addresses the “flyby anomaly,” a set of unexplained velocity changes (typically a few cm s⁻¹) observed during some Earth‑gravity‑assist maneuvers. The author proposes that a non‑uniform DM halo surrounding Earth could generate tiny, direction‑dependent gravitational perturbations that manifest as the observed anomalies. By modeling a spherically symmetric DM cloud with a density profile consistent with the mass limits above, the calculated perturbations can reach the order of magnitude of the reported anomalies. However, the statistical significance of the flyby data remains low, and alternative explanations (atmospheric drag, thermal radiation pressure, Earth’s tides) have not been definitively ruled out.

Finally, the paper discusses the compatibility of planet‑bound DM with direct‑detection experiments. Existing underground detectors (LUX, XENON1T, PandaX) place stringent limits on DM–nucleon cross sections, implying that any Solar‑System DM component must be essentially non‑interacting except through gravity. Consequently, observable signatures are limited to gravitational effects on orbital dynamics, internal heat budgets, and possibly subtle timing variations in high‑precision clocks aboard spacecraft.

In conclusion, while current observations do not exclude a small population of DM gravitationally bound to the Sun, Earth, or other planets, the permissible mass is extremely limited—typically less than one part in a hundred million of a planet’s total mass. Detecting such a component will require next‑generation ranging and timing experiments, improved models of planetary interiors, and a larger statistical sample of spacecraft flybys. The paper highlights these avenues as the most promising paths toward either confirming or definitively ruling out planet‑bound dark matter.