Astronomers may soon be able to confirm the existence of a primordial black hole –gravity wells formed just moments after the Big Bang–in our Solar System with gravitational-wave experiments that will look back to a time before the formation of the first stars. “Ancient black holes would give us access to physics we would never otherwise be able to do,” wrote Dan Hooper, head of the theoretical astrophysics group at FermiLab, in an email to The Daily Galaxy. “If primordial black holes are real,’ he wrote, “they’d have potential to solve a whole host of the biggest problems in cosmology, not the least being the mystery of dark matter, considered to be the backbone to the structure of the universe.”
On March 2, 2019 The Daily Galaxy reported that astronomers from the University of California concluded that there are probably tens of millions of enigmatic stellar-remnant black holes lurking in the Milky Way. Now, physicists are proposing that the long-sought for Planet Nine, an elusive dark body in the outer reaches of our solar system, may actually be a primordial black hole (PBH), a theoretical remnant of the Big Bang, which could account for the odd orbits observed in the distant solar system.
Theoretical remnant of the Big Bang
This intriguing conjecture is presented by physicists Jakub Scholtz with Institute for Particle Physics Phenomenology at Durham University and James Unwin, theoretical particle physicist at the University of Illinois at Chicago, in a paper published on the arXiv preprint server this week. They are building upon the work of Caltech’s Mike Brown and Konstantin Batygin, who predicted the existence of a ninth planet in the outer solar system in 2016.
Reminiscent of a Super-Earth
“At five Earth masses, Planet Nine is likely to be very reminiscent of a typical extrasolar super-Earth,” says Batygin, an assistant professor of planetary science and Van Nuys Page Scholar at Caltech. Super-Earths are planets with a mass greater than Earth’s, but substantially less than that of a gas giant. “It is the solar system’s missing link of planet formation. Planet Nine is going to be the closest thing we will find to a window into the properties of a typical planet of our galaxy.”
“A solution with an ordinary planet and a solution with an exotic compact object like a primordial black hole are very similar,” said Unwin. “However, the search strategy you need to identify a black hole is drastically different,” he added. “We’re not currently using all the tools in our toolbox to search for this thing.”
Primordial Black Holes (PBHs)–hypothetical objects
“Primordial black holes formed just fractions of a second after the Big Bang and remain hypothetical objects for the moment,” says Alvise Raccanelli of CERN. “Initially proposed by Stephen Hawking in 1971, they have come back to the fore in recent years as possible candidates for explaining dark matter.”
“For normal black holes, you need to have at least a solar mass because it is created out of a star,” said Scholtz, who is a junior research fellow at the Institute for Particle Physics Phenomenology at Durham University. “These primordial black holes can be much lighter; for example, an Earth mass, or in fact, even lighter.”
Primordial Black Holes or Free-ranging Planets?
Scholtz and Unwin, reports Becky Ferreira at Motherboard, started developing the paper after recognizing the connection between the Planet Nine hypothesis and potential PBH observations captured by the Optical Gravitational Lensing Experiment (OGLE) project. OGLE scans the skies for microlensing events, which occur when distant objects—typically thousands of light years beyond our own solar system—pass in front of even more remote stars. The gravitational fields of these small objects, which could be primordial black holes or free-ranging planets, bend light from background stellar sources, producing a lensing signature that OGLE can detect.
The lensing events enable scientists to estimate that the enigmatic objects are about one half to 20 times as massive as Earth. This is about the same mass scale expected for Planet Nine, a coincidence that Scholtz and Unwin call “remarkable” in the paper.
Brown and Batygin provided indirect evidence that the sun is orbited by an as-yet-unseen planet, which is about 10 times the mass of Earth and with an orbit that is about 20 times farther from the sun on average than Neptune. Planet Nine, based on their calculations, appears to orbit at about 30 degrees off from the other planets’ orbital plane. The hypothetical Planet Nine influences the orbits of several Kuiper Belt objects in the outer solar system, which is how Brown and Batygin came to suspect a planet existed there in the first place.
“It continues to amaze us; every time we look carefully we continue to find that Planet Nine explains something about the solar system that had long been a mystery,” says Batygin.
A Free Floating Exotic World –Planet or Black Hole?
Proponents of the Planet Nine hypothesis have noted that the expected location of this world, which is about 20 times as far from the Sun as Neptune on average, reports Motherboard, “is a strange place to find a planet native to the solar system. One explanation is that Planet Nine was once a free-floating world, meaning a planet with no host star, that was ensnared by the Sun’s gravity.”
Scholtz and Unwin calculated that either a rogue planet or primordial black hole have the same probability of getting captured by the solar system are roughly the same. “It could equally be some sort of exotic object that got captured by the solar system at some point,” Unwin said.
Theoretically, a primordial black hole would produce “annihilation signals” says Unwin and Scholtz. These annihilation signals occur when the black hole’s dark matter particles are destroyed upon contact with their anti-particle counterparts, generating light radiation that could potentially be picked up by instruments like the Fermi Gamma-ray Space Telescope or the Chandra X-ray Observatory.
“We actually expect [annihilation signals] to happen at quite a significant rate, so these things have the potential to just be glowing sources in the sky,” Unwin said. To that end, Scholtz and Unwin are planning to analyze public data from the Fermi telescope to look for signals that might correspond to a local black hole.
Fermi-LAT Data Harbors the Clues
“The first thing we could do is send something out there and do a lot of tests of general relativity,” Unwin said. “The hypothetical orbit of Planet Nine is far away, but it’s not an inaccessible distance.”
“We are only going to get better information,” Unwin said. “If you exhaust your conventional searches in visible light, this explanation that it could be a more exotic object really starts to gain traction.”
In an email to The Daily Galaxy, Unwin wrote: “A PBH in our solar system would likely evade traditional searches. While my collaborator, Jakub Scholtz, and I are currently working on the necessary methodology to search the Fermi-LAT data for such an object, the pandemic has somewhat delayed our progress. Needless to say, a positive result if confirmed as a nearby PBH would be a spectacular discovery.”
Image credit: Jeremy Schnittman/NASA/ Goddard Space Flight Center