Black holes are formed either by the collapse of a massive star or by the merger of heavy objects. However, scientists suspect that smaller “primordial” black holes, including some with masses similar to the Earth, may have formed in the chaotic early moments of the universe. When we think of black holes, we tend to picture enormous cosmic monsters, such as stellar-mass black holes with masses tens or hundreds of times that of the Sun. We can even imagine supermassive black holes, with masses millions (or even billions) of times that of the Sun, sitting at the hearts of galaxies and dominating their surroundings. A team of scientists has predicted that NASA’s Nancy Grace Roman Space Telescope could detect a class of “light” black holes that have eluded detection until now.
However, scientists speculate that the universe may also be populated by much less massive, relatively light black holes with masses close to that of Earth. These black holes could potentially have a mass equal to that of a large asteroid. Scientists also speculate that such black holes may have existed since the beginning of time, some 13.8 billion years ago. Aptly called “primordial black holes,” these black holes have remained purely theoretical, but Roman, scheduled to launch in late 2026, could change that.
“Finding a population of Earth-mass primordial black holes would be an incredible step for both astronomy and particle physics, because these objects cannot be produced by any known physical process,” said William DeRocco, a postdoctoral researcher at the University of California, Santa Cruz, who led the study of how Roman might detect them. A paper describing the results was published in the journal Physical Review D. “If we find them, it will shake up the field of theoretical physics.”
Stephen Hawking proposed that black holes might slowly shrink as they leak radiation. The slow leakage of what is now known as Hawking radiation would eventually cause the black hole to simply evaporate. This infographic shows the estimated lifetime and event horizon — the point at which infalling objects cannot escape the black hole’s gravitational grip — diameters of black holes of various low masses. NASA Goddard Space Flight Center
The smallest black holes ever confirmed to exist are stellar-mass black holes, which are created when massive stars run out of the fuel needed to fuse nuclear energy in their cores. Once that fusion stops, these stars collapse under their own gravity. Typically, the minimum mass a star needs to leave behind a stellar-mass black hole is eight times the mass of the Sun — any lighter and the star will end up as a neutron star or a smoldering white dwarf.
However, conditions in the early universe were very different from those of the modern era. When the cosmos was hot, dense, and turbulent, it could have allowed much smaller clumps of matter to collapse and form black holes.
But a minimum mass is required: at least eight times the mass of our Sun. Lighter stars will become either white dwarfs or neutron stars.
This artist’s concept is a whimsical take on how small primordial black holes look. In reality, such tiny black holes would have a hard time forming the accretion disks that make them visible here. NASA’s Goddard Space Flight Center
But conditions in the very early universe could have allowed much lighter black holes to form. One of them, weighing about the mass of Earth, would have had an event horizon – a point of no return for falling objects – about the width of a U.S. dime.
Scientists believe that when the universe was just beginning, it went through a brief but intense phase known as inflation, when space expanded faster than the speed of light. Under these special conditions, regions that were denser than their surroundings could collapse to form low-mass primordial black holes.
While theory predicts that the smallest of them should evaporate before the Universe reaches its current age, those with masses close to that of Earth could survive.
The discovery of these tiny objects will have a huge impact on physics and astronomy.
“This would impact everything from the formation of galaxies to the content of dark matter in the universe to cosmic history,” said Kailash Sahu, an astronomer at the Space Telescope Science Institute in Baltimore, who was not involved in the study. “Confirming their identity will be hard work, and astronomers will need a lot of compelling evidence, but it will be worth it.”
This diagram shows a primordial black hole causing gravitational lensing, revealing its existence to the Roman Space Telescope. Robert Lee
General relativity predicts that all objects with mass cause a warp in the fabric of space and time itself, which are combined into a single four-dimensional entity called spacetime. When light from a background source passes through the warp, its path is curved. The closer the light gets to the lensing object, the more its path is curved. This means that light from the same object can arrive at a telescope at different times. This is called gravitational lensing.
When the lensing object is incredibly massive, like a galaxy, the background source may shift in apparent position or even appear in multiple locations in the same image. If the lensing object is smaller in mass, like a primordial black hole, the lensing effect is smaller, but it can cause background sources to appear brighter than they can be detected. This is an effect called microlensing.
Microlensing is now being used to great effect to detect rogue planets, or worlds that are floating around the Milky Way without a parent star. This has revealed a large population of rogues of roughly Earth mass — more than theoretical models predict. Scientists predict that with this model, Roman will increase the detection of Earth-mass rogues by a factor of ten.
The abundance of these objects has led to speculation that some of these Earth-mass objects may actually be primordial black holes. “It’s impossible to distinguish Earth-mass black holes from rogue planets on a case-by-case basis,” DeRocco said. “The novel will be an extremely powerful way to distinguish between the two objects statistically.”
This illustration shows the Nancy Grace Roman Space Telescope surrounded by primordial black holes. Robert Lee/NASA
Microlensing is an observational effect that occurs because the presence of mass warps the fabric of spacetime, like the imprint left by a bowling ball on a trampoline. Whenever an intervening object appears to drift near a background star from our perspective, the star’s light must traverse the warped spacetime around the object. If the alignment is particularly close, the object can act as a natural lens, focusing and amplifying the light from the background star.
Separate teams of astronomers using data from MOA (Microlensing Observations in Astrophysics) – a collaboration that conducts microlensing observations using the Mount John University Observatory in New Zealand – and OGLE (Optical Gravitational Lensing Experiment) have discovered an unexpectedly large population of isolated objects with masses equal to those of Earth.
Theories of planet formation and evolution predict specific masses and abundances of rogue planets—worlds that wander the galaxy, unattached to a star. The MOA and OGLE observations suggest that there are more light objects floating around the galaxy than the models predict.
“It’s impossible to distinguish between Earth-mass black holes and rogue planets on a case-by-case basis,” DeRocco said. But scientists expect Roman to find 10 times more objects in that mass range than ground-based telescopes. “Roman will be extremely powerful at distinguishing between the two statistically.”
DeRocco led the effort to determine how many rogue planets should be in that mass range and how many primordial black holes Roman could discern among them.
Finding primordial black holes would provide new information about the very early universe and provide strong evidence that early inflation did occur. It could also explain a small percentage of the mysterious dark matter that scientists say makes up most of the mass in our universe but have so far been unable to identify.
“This is an exciting example of what scientists could do with the data Roman already has as it searches for planets,” Sahu said. “And the results are exciting whether or not scientists find evidence of Earth-mass black holes. Either way, it will strengthen our understanding of the universe.”
The Nancy Grace Roman Space Telescope is managed at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, with participation from NASA’s Jet Propulsion Laboratory and Caltech/IPAC in Southern California, the Space Telescope Science Institute in Baltimore, and a science team comprised of scientists from various research institutions. Major industrial partners include BAE Systems, Inc. in Boulder, Colorado; L3Harris Technologies in Rochester, New York; and Teledyne Scientific & Imaging in Thousand Oaks, California.