Is Jupiter secretly planning to vanish? Recent research implies that given the proper cosmic conditions, a gas giant would not simply remain parked in orbit it could implode, from the inside out, into a black hole.
The process starts with a rare type of particle: superheavy, non-annihilating dark matter. Unlike the lighter contenders like axions, such particles would not annihilate one another when they meet. Instead, over billions of years, they might get gravitationally captured by a massive exoplanet and slowly leak out of kinetic energy as they ricochet through the dense body. The particles would migrate inward toward the core, building an ever-larger clump. If that clump becomes larger than a critical mass, gravity would dominate all the others, and collapse into a microscopic black hole would ensue.
From there, things become catastrophic for the host planet. The black hole would accrete material around it, even engulfing the whole world. “If the dark matter particles are heavy enough and don’t annihilate, they may eventually collapse into a tiny black hole,” said Mehrdad Phoroutan-Mehr of the University of California, Riverside. “This black hole could then grow and consume the entire planet, turning it into a black hole with the same mass as the original planet.”
Such an object would be unique among black holes but proved. The lightest existing black holes currently are stellar remains with masses above about 3.8 solar masses, a boundary defined by stellar evolution and the Tolman–Oppenheimer–Volkoff limit, which is the upper mass limit of a neutron star at the point where it will collapse. Planetary-mass black holes, on the other hand, would be much smaller Jupiter’s mass is only 0.001 solar masses but still share the same intense density and gravitational pull.
Such objects could be detected, although with difficulty. A planet that has collapsed into a black hole would still have the same gravitational pull on its star, maintaining radial-velocity and astrometric signals. Yet its physical size would be reduced to a size too small to obscure starlight during a transit, eliminating the characteristic dip in brightness. This disconnect stellar wobble without accompanying transit might indicate a candidate worthy of further inspection. Microlensing surveys, especially in dark matter–dense areas like the Milky Way’s galactic core, provide another tool. The future Nancy Grace Roman Space Telescope will observe tens of thousands of microlensing events and could unveil a population of compact, planet-mass objects.
Thermal signatures can also be indicative of dark matter in exoplanets. As particles accumulate or tiny black holes evaporate through Hawking radiation, they could deposit heat or high-energy particles into the planet’s atmosphere. In principle, this would elevate infrared emission above what is predicted by cooling models. Current telescopes are not sensitive enough to see such small deviations, but future observatories might make the difference.
The failure to find planet-mass black holes would be just as illuminating. If Jupiter-mass worlds survive in settings in which the model says they must collapse, that would set limits on the mass and interaction cross-section of dark matter particles. “If astronomers were to discover a population of planet-sized black holes, it could offer strong evidence in favor of the superheavy non-annihilating dark matter model,” Phoroutan-Mehr said. Failing to find them would serve to eliminate some of the parameter space.
It also fills the gap in current astrophysical probes. Neutron stars, for instance, might trap some dark matter candidates, which would then annihilate and warm the star. The observation of an ancient cold neutron star would thus disfavor such models. Exoplanets add to the arsenal, providing a new range of environments in which to probe dark matter theories.
With close to 6,000 discovered exoplanets and many more in the pipe, the statistical power of planetary surveys increases. Areas close to the galactic center, where dark matter density is speculated to be greater, could be the best hunting grounds. If even a single planet-mass black hole is discovered in such a region, it could be the long-awaited smoking gun for a type of dark matter that has up to now managed to stay clear of every terrestrial detector.
