In 1971, Stephen Hawking suggested that a mini black hole from the early universe could be lurking at the center of the sun. His proposal was extended in 1975 by Don Clayton and collaborators who suggested that the power generated by the infall of matter onto such a black hole could explain the observed deficit in neutrinos of the electron flavor from the Sun. This deficit was known at that time as the solar neutrino problem, formulated by calculations of my early mentor, John Bahcall. Having a second power source in addition to nuclear fusion would have naturally reduced the production of solar neutrinos through nuclear reactions and accounted for the neutrino deficit. By now, better quantitative data from the Sudbury Neutrino Observatory in Canada, for which the Physics Nobel Prize was awarded to Art McDonald in 2015, implies a different solution to the solar neutrino problem, in terms of the transformation of neutrino flavors inside the Sun.
Nevertheless, could the Sun still host in its belly a primordial black hole that does not contribute much to its luminosity? After all, we know that 85% of the matter in the Universe is invisible. Primordial black holes with a mass similar to that of asteroids in the size range of 1–100 kilometers, could account for dark matter. If this is the nature of dark matter, is it possible that some stars captured primordial black hole in their belly? If so, what would be their fate?
It is easier to address the second question. A black hole captured by a star could change its evolution and internal structure. The interiors of stars can be diagnosed through their oscillations, just like the use of seismic signals to probe the inner structure of Earth. The unusual evolution and interior structure of stellar hosts of mini black holes could be searched for in the future.
Given the high speed of dark matter in the Milky-Way, the likelihood that the Sun captured a primordial black hole is one in ten million. Nonetheless, given the hundreds of billions of stars of the Milky-Way galaxy, there could still be tens of thousands of Milky-Way stars which have captured a mini black hole. Because of the smaller characteristic speed of dark matter in dwarf galaxies, most of the stars embedded in ultra-faint dwarf galaxies, like Tucana III and Triangulum II, could have captured a mini black hole.
After consuming their nuclear fuel, the core of Sun-like stars contracts to make a white dwarf, a metallic sphere roughly the size of the Earth. Since the radius of the Earth is a hundred times smaller than the radius of the Sun, the average mass density of white dwarfs is about a million times larger than that of the Sun. The accretion rate of matter onto an embedded mini-black would therefore increase a million times, potentially igniting the white dwarf and triggering a supernova explosion. Rare explosive transients of a new kind could be searched for in the data pipeline of the Rubin Observatory, which will start operations next year.
The effect of a mini black hole would be even more dramatic if it is trapped in the core of a massive star with more than 8 times the mass of the Sun. Such a core collapses to make a neutron star after consuming its nuclear fuel. The density of the neutron star resembles that of an atomic nucleus, a hundred trillion times higher than the mean density of the Sun. In that case, the rapid accretion of matter could turn the neutron star into a black hole, as I pointed out in a 2014 paper with my former postdoc, Paolo Pani, who is currently a professor in Italy.
Under these circumstances, the primordial black hole may be regarded as a seed that grows to consume its host star and transforms it into a stellar-mass black hole. This channel could lead to black holes with the mass of a neutron star, an outcome which is not expected under normal astrophysical evolution. Currently, the LIGO-Virgo-KAGRA observatory identifies compact objects as neutron stars or black holes by their mass, identified through their gravitational wave signal. The existence of a neutron-star channel to a black hole would cause confusion in this identification scheme and result in events where compact objects of neutron-star mass are detected in gravitational waves but release no electromagnetic radiation, due to the absence of matter.
As I pointed out in a recent paper, just accepted for publication in The Astrophysical Journal Letters, the growth of mini black holes of asteroid mass is suppressed by quantum mechanics. This is because the size of their event horizon is smaller than the size of atoms.
If primordial black holes make up dark matter, then the nearest black hole would be inside the solar system. Having a black hole close to home opens an opportunity to study quantum gravity experimentally. A black hole with an event horizon the size of a proton would radiate spontaneously, according to Hawking’s 1974 paper, a power of 1 gigawatt, mostly in gamma-ray photons with an energy of a hundred times the electron rest mass.
If we ever witness an asteroid-mass black hole in the solar system, it can be used as a testbed for quantum-gravitational experiments on a subatomic scale. Understanding it would guide us in developing a predictive theory that unifies quantum mechanics and gravity. Having such a theory would in turn inform us of what might have led to the Big-Bang. And knowing that would bring us closer to appreciating our cosmic roots.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
