Detecting dark matter by observing changes in Mars’ orbit could be an innovative approach, as suggested by researchers.
A recent study by MIT physicists proposes that if most of the universe’s dark matter is composed of tiny primordial black holes—an idea originating in the 1970s—these minuscule gravitational entities should pass through our solar system roughly once every decade. According to the researchers, such an event would create a detectable wobble in Mars’ orbit using current technology.
The successful detection of this wobble could reinforce the hypothesis that primordial black holes are a significant source of dark matter in the cosmos.
“Over the years, scientists have measured the distance between Earth and Mars with around 10 centimeters accuracy,” explains study author David Kaiser, a professor of physics and the Germeshausen Professor of the History of Science at MIT. “We’re leveraging this precisely measured area of space to search for minute effects. Discovering such an effect would provide a compelling reason to further investigate the intriguing concept that dark matter is composed of black holes that originated shortly after the Big Bang and have been traveling through the universe for 14 billion years.”
Kaiser and his team share their findings today in the journal Physical Review D. The co-authors include lead author Tung Tran ’24, now a graduate student at Stanford University; Sarah Geller ’12, SM ’17, PhD ’23, currently a postdoc at the University of California at Santa Cruz; and MIT Pappalardo Fellow Benjamin Lehmann.
Understanding Beyond Particles
Visible matter—such as stars, planets, and everyday objects—constitutes less than 20 percent of all matter in the universe. The remainder is made up of dark matter, a theoretical form of matter that, although invisible to all forms of light, is believed to exist throughout the universe and has enough gravitational influence to affect the motion of stars and galaxies.
Scientists have constructed detectors on Earth to attempt to identify dark matter and understand its characteristics. Most experiments to date assume dark matter consists of exotic particles that may scatter and transform into detectable particles during their passage through detection equipment. However, these particle-based investigations have largely been unsuccessful.
Recently, another theory, revisited since the 1970s, has gained attention: dark matter might not be a particle but instead consist of microscopic primordial black holes formed shortly after the Big Bang. Unlike the black holes that arise from collapsing stars, primordial black holes are theorized to have been created from dense gas regions in the early universe, scattering throughout space as the universe expanded and cooled.
These primordial black holes could contain a vast amount of mass in a very small volume. They could range in size from that of an atom to the mass of large asteroids. This raises the possibility that these tiny but massive entities could contribute to the gravitational effects associated with dark matter. Noticing this potential led the MIT team to ponder a somewhat whimsical question.
“Someone once asked me what would happen if a primordial black hole passed through a human body,” recalls Tung. After a quick analysis, he found that if such a black hole came within a meter of an individual, it could propel them about 6 meters—or around 20 feet—away in just one second. However, he also noted that it would be extremely unlikely for a primordial black hole to come close to any person on Earth.
Intrigued, the researchers expanded upon Tung’s calculations to consider how a black hole might influence larger celestial bodies like the Earth and the moon.
“We extrapolated to estimate the effects of a black hole’s passage through Earth and any wobbles it might induce in the moon,” explains Tung. “However, the results were complex, with many other dynamics in our solar system that could counterbalance any expected wobble.”
Exploring Close Encounters
To gain clarity, the research team created a simplified simulation of the solar system, factoring in the orbits and gravitational interactions of all planets and some of the largest moons.
<p”As the most advanced solar system simulations track over a million objects, each with a minuscule influence, we were able to discern a tangible effect in our more focused model,” says Lehmann. “Working with a limited number of objects still allowed us to investigate the effects in greater detail.”
They calculated how often a primordial black hole could traverse the solar system, based on existing estimates of dark matter density in specific regions and the mass of hypothetical black holes, which they posited to be comparable to the largest asteroids in our solar system.
“Primordial black holes aren’t stationary in the solar system; instead, they travel through space independently,” remarks co-author Sarah Geller. “It’s estimated they pass through the inner solar system at some angle approximately once every decade.”
Using this rate, the researchers simulated the passage of various asteroid-mass black holes through the solar system, at speeds around 150 miles per second, and assessed the effects of close encounters. They discovered that while Earth and the moon showed uncertain responses, Mars provided a more promising outcome.
They concluded that if a primordial black hole were to pass within several hundred million miles of Mars, it would induce a detectable “wobble” or minor deviation in Mars’ orbit. Within a few years of such an encounter, the shift in Mars’ orbit could be around one meter—an incredibly minute change given the planet is more than 140 million miles from Earth; yet, this could be measurable with the precise instruments currently monitoring Mars.
If such a wobble is observed within the next few decades, the researchers emphasize that more rigorous work would be essential to determine if the effect came from a passing black hole or a typical asteroid.
“We need utmost clarity regarding typical backgrounds, including the usual speeds and trajectories of regular space debris versus the distinct pathways and speeds of primordial black holes,” notes Kaiser. “Fortunately, astronomers have tracked ordinary space rocks for years, allowing us to compare their characteristics with those of primordial black holes.”
To facilitate this goal, the team is considering collaboration with experts who have a wealth of experience in simulating a broader variety of celestial objects.
“We are currently working on modeling a vast number of entities, from planets to asteroids, observing their movements over long periods,” Geller explains. “We aim to introduce scenarios of close encounters and analyze their impacts with greater accuracy.”
This research received funding from the U.S. Department of Energy and the U.S. National Science Foundation, including support for a Mathematical and Physical Sciences postdoctoral fellowship.
