Why do we not see effects of dark matter in our solar system and other nearby star systems?
Curran Rode
Ammannsville, Texas
Dark matter refers to material that does not absorb, reflect, or emit any electromagnetic radiation. Astronomers have ascertained the existence of dark matter through the gravitational influence it exerts over visible matter. In fact, we estimate that approximately 90 percent of the Milky Way consists of dark matter. That figure is based principally on observations of stellar motions. The individual velocities of a galaxy’s stars are proportional to the amount of material contained within it — the greater the mass, the faster the speed. Stars within the Milky Way are moving quite quickly. Even at Earth’s location, about two-thirds the distance from the galactic center to the outer boundary of our galaxy, the Sun is traveling at some 138 miles (220 kilometers) per second. Visible matter alone cannot account for such rapid velocities. So, there must be much more galactic matter than meets the eye.
This is how the existence of dark matter was first inferred over large (galactic) scales. But could dark matter similarly affect motions within our own solar system, and could we somehow observe its effects through its interactions with normal matter?
Let’s approach this question by examining the motions of objects within the solar system. Containing 99.8 percent of the solar system’s matter, the Sun exerts the predominant gravitational force over everything moving around it. In other words, the solar system consists of the Sun and a comparative smattering of extraneous material (planets, asteroids, etc.) scattered about it. Thus, its matter is highly concentrated in a specific object and highly diffuse in a vast region around it. Conversely, dark matter is likely to be extremely rarefied and uniformly dense. It should permeate the solar system just as it does the galaxy, without any alteration in density. Consequently, the inner planets should experience less gravitational force than the outer planets, since the latter’s orbits encompass more of this dark matter than the former. To be more specific, astronomers estimate that Earth’s orbital motion should be affected by the 1013 kilograms of dark matter it encircles, while Uranus’ orbit contains 1016 kg of dark matter. Although those amounts might seem considerable to us, they are negligible when compared to the masses of the Sun (2 x 1030 kg), Earth (6 x 1024 kg), and Uranus (8.7 x 1025 kg). Observing such minuscule differences between planetary orbits with and without dark matter is lamentably beyond our current detection capabilities.
However, according to a paper by Edward Belbruno and James Green published in the March 2022 issue of Monthly Notices of the Royal Astronomical Society, “When Leaving the Solar System: Dark Matter Makes a Difference,” distant spacecraft such as Pioneers 10 and 11 or Voyagers 1 and 2 could be affected in a detectable manner by the dark matter throughout the galaxy. Unfortunately, dark matter’s effect on these vessels’ motions would only become detectable once they attained a distance of at least 30,000 astronomical units from the Sun. (One astronomical unit, or AU, is the average Earth-Sun distance of 93 million miles or 150 million km. For reference, Voyager 1, the most distant of all our spacecraft, was a mere 163.8 AU from the Sun in June 2024 and will require about 8,525 years to reach the 30,000 AU mark.)
All the same, even 30,000 AU is well within the outer solar system bound of 100,000 AU as defined by the Oort Cloud. In addition, Belbruno and Green suggest that a future spacecraft at a distance of “merely” 100 AU could release a reflective ball that would establish its own trajectory independent of the spacecraft. By measuring how galactic forces, or the combined forces of normal and dark matter, affect the motions of both spacecraft and ball, scientists could then measure how dark matter can cause deviations in their respective trajectories.
So, dark matter does indeed most likely pervade the solar system like an invisible fog. We just lack the means by which to measure its effects on solar system bodies … for now.
Edward Herrick-Gleason
Staff Member, Southworth Planetarium, University of Southern Maine, Portland, Maine
