Pictures of the planet Mars taken with the recently refurbished Hubble Space Telescope (HST) will provide the most detailed global view of the red planet ever obtained from Earth. Credit: David Crisp and the WFPC2 Science Team (Jet Propulsion Laboratory/California Institute of Technology), Public domain, via Wikimedia Commons.

MIT study finds watching for changes in the Red Planet’s orbit over time could be new way to detect passing dark matter.

In a new study, MIT physicists propose that if most of the dark matter in the universe is made up of microscopic primordial black holes — an idea first proposed in the 1970s — then these gravitational dwarfs should zoom through our solar system at least once per decade.

A flyby like this, the researchers predict, would introduce a wobble into Mars’ orbit, to a degree that today’s technology could actually detect. 

Such a detection could lend support to the idea that primordial black holes are a primary source of dark matter throughout the universe. 

Study author David Kaiser, professor of physics and the Germeshausen Professor of the History of Science at MIT, said: “Given decades of precision telemetry, scientists know the distance between Earth and Mars to an accuracy of about ten centimetres.

“We’re taking advantage of this highly instrumented region of space to try and look for a small effect.

“If we see it, that would count as a real reason to keep pursuing this delightful idea that all of dark matter consists of black holes that were spawned in less than a second after the Big Bang and have been streaming around the universe for 14 billion years.”

Dark matter

Less than 20% of all physical matter is made from visible stuff, from stars and planets, to the kitchen sink.

The rest is composed of dark matter, a hypothetical form of matter that is invisible across the entire electromagnetic spectrum yet is thought to pervade the universe and exert a gravitational force large enough to affect the motion of stars and galaxies. 

Physicists have erected detectors on Earth to try and spot dark matter and pin down its properties.

For the most part, these experiments assume that dark matter exists as a form of exotic particle that might scatter and decay into observable particles as it passes through a given experiment. But so far, such particle-based searches have come up empty. 

In recent years, another possibility, first introduced in the 1970s, has regained traction: rather than taking on a particle form, dark matter could exist as microscopic, primordial black holes that formed in the first moments following the Big Bang.

Unlike the astrophysical black holes that form from the collapse of old stars, primordial black holes would have formed from the collapse of dense pockets of gas in the very early universe and would have scattered across the cosmos as the universe expanded and cooled.

These primordial black holes would have collapsed an enormous amount of mass into a tiny space.

The majority of these primordial black holes could be as small as a single atom and as heavy as the largest asteroids.

It would be conceivable, then, that such tiny giants could exert a gravitational force that could explain at least a portion of dark matter.

Simulation

To get a clearer picture the team, including study lead author Tung Tran ’24, who is now a graduate student at Stanford University and co-authors Sarah Geller ’12, SM ’17, PhD ’23, who is now a postdoc at the University of California at Santa Cruz, and MIT Pappalardo Fellow Benjamin Lehmann,

generated a relatively simple simulation of the solar system that incorporates the orbits and gravitational interactions between all the planets, and some of the largest moons.

Lehmann said: “State-of-the-art simulations of the solar system include more than a million objects, each of which has a tiny residual effect, but even modelling two dozen objects in a careful simulation, we could see there was a real effect that we could dig into.”

The team worked out the rate at which a primordial black hole should pass through the solar system, based on the amount of dark matter that is estimated to reside in a given region of space and the mass of a passing black hole, which in this case, they assumed to be as massive as the largest asteroids in the solar system, consistent with other astrophysical constraints.

Geller said: “Primordial black holes do not live in the solar system. Rather, they’re streaming through the universe, doing their own thing and the probability is, they’re going through the inner solar system at some angle once every ten years or so.”

Given this rate, the researchers simulated various asteroid-mass black holes flying through the solar system, from various angles, and at velocities of about 150 miles per second, where the directions and speeds come from other studies of the distribution of dark matter throughout our galaxy.

They zeroed in on those flybys that appeared to be ‘close encounters’, or instances that caused some sort of effect in surrounding objects.

They quickly found that any effect in the Earth or the moon was too uncertain to pin to a particular black hole. But Mars seemed to offer a clearer picture.

Wobble

The researchers found that if a primordial black hole were to pass within a few hundred million miles of Mars, the encounter would set off a ‘wobble’, or a slight deviation in Mars’ orbit.

Within a few years of such an encounter, Mars’ orbit should shift by about a metre — an incredibly small wobble, given the planet is more than 140 million miles from Earth. And yet, this wobble could be detected by the various high-precision instruments that are monitoring Mars today. 

If such a wobble were detected in the next couple of decades, the researchers acknowledge there would still be much work needed to confirm that the push came from a passing black hole rather than a run-of-the-mill asteroid. 

Kaiser said: “We need as much clarity as we can of the expected backgrounds, such as the typical speeds and distributions of boring space rocks, versus these primordial black holes.

“Luckily for us, astronomers have been tracking ordinary space rocks for decades as they have flown through our solar system, so we could calculate typical properties of their trajectories and begin to compare them with the very different types of paths and speeds that primordial black holes should follow.”

The study is published in Physical Review D.