How Dark Matter Could Be Measured in the Solar System

In a new study, Edward Belbruno and colleagues show that dark matter contributes a measurable galactic gravitational force within the solar system, roughly balanced by ordinary matter (about 45% dark matter and 55% baryonic matter) and increasing in influence with distance from the Sun. They identify a transition where galactic gravity surpasses solar gravity at around 30,000 astronomical units, inside the Oort Cloud. The authors propose a direct measurement approach using a radioisotope-powered spacecraft with a reflective ball and a laser to compare trajectories while subtracting thermal forces. They also discuss implications for spacecraft such as Pioneer 10/11, Voyager 1/2, and New Horizons, and for distant objects like Oumuamua and potentially Planet X.

Overview

In a study published in the Monthly Notices of the Royal Astronomical Society, co-authors Edward Belbruno and Jim Green quantify how dark matter from the Milky Way subtly alters gravity within the solar system. They find that the total galactic force acting on objects in the solar system is made up roughly of 45 percent dark matter and 55 percent baryonic matter, implying a near half-and-half mix of dark and normal matter in our local cosmic neighborhood. The halo of dark matter surrounding the galaxy concentrates the dark component toward the outer regions, explaining why the solar system experiences a relatively modest dark-matter contribution overall.

<>"I was a bit surprised by the relatively small contribution of the galactic force due to dark matter felt in our solar system as compared to the force due to the normal matter," - Edward Belbruno, Princeton University and Yeshiva University

Dark Matter in the Solar System

The researchers describe a planetary-scale gravitational field, termed the galactic force, produced by all galactic matter, with dark matter forming the dominant component in the halo. Because most dark matter resides in the Milky Way’s outer regions, its local influence within the inner solar system remains small but nonzero. A few key distances matter: Pluto lies far inside the regime where solar gravity dominates, while the Oort Cloud extends to about 100,000 astronomical units (AU), bringing solar-system dynamics closer to the realm where dark matter’s gravity grows more significant.

Potential Measurements and Mission Concepts

The authors propose that measuring dark matter’s gravity does not require venturing to extreme distances. At around 100 AU, a spacecraft with a carefully designed experiment could directly observe the galactic force. Their concept envisions a radioisotope-powered spacecraft carrying a reflective ball. The ball would feel only galactic forces, while the spacecraft experiences both galactic gravity and a thermal recoil force from its power source. By dropping the ball and using a laser to monitor the two objects flying in parallel, researchers could subtract the thermal contribution and isolate the galactic force’s effect on trajectories. The Interstellar Probe mission concept, which aims to reach ~500 AU, offers a natural platform for such an experiment.

"If spacecraft move through the dark matter long enough, their trajectories are changed, and this is important to take into consideration for mission planning for certain future missions," said Belbruno. The study also notes that dark matter’s gravity could have influenced the paths of historic probes like Pioneer 10/11, Voyager 1/2, and New Horizons, albeit by only a few meters over billions of miles, making direct measurement challenging with past data.

Broader Implications for Outer Solar System Bodies

Beyond mission design, the researchers suggest dark matter’s gravity could have lingering effects on outer solar-system objects, including objects in the Kuiper Belt or even a hypothetical Planet X. They speculate that dark matter might have contributed to the speed of interstellar visitors like Oumuamua or caused some comets in the Oort Cloud to escape solar gravity. The work thus connects galactic-scale physics with the dynamical evolution of the solar system’s distant frontier, offering a tangible way to probe dark matter in a local setting.

Source: Monthly Notices of the Royal Astronomical Society