What if some of the alien worlds we've discovered are not actually planets at all?
Astronomers have spent years cataloging thousands of worlds orbiting distant stars, assuming that if something has the mass of a planet and exerts a gravitational pull on its parent star, it must be a planet.
But there may be a ghostly alternative lurking in the early universe. In a recent paper that was uploaded to the arXiv preprint server but has not been peer-reviewed, researchers suggest that some "exoplanets" we've detected might actually be something far more exotic — primordial black holes.
These are not your garden-variety black holes, born from dying stars. Instead, they are hypothetical leftovers from the Big Bang itself, formed when the newborn universe was a chaotic, high-pressure soup of energy. These "mini" black holes could have the mass of Earth or Jupiter but be the size of a grapefruit.
Our current methods for finding planets are exceptionally good at measuring mass but less so at determining the physical size of a planet. For example, we often use the radial velocity method — a technique that involves watching a star "wobble" because the gravity of an orbiting object is yanking on it. If the wobble is big, the object is heavy. If the wobble is small, the object is light.
But here's the catch: A planet with the mass of Neptune and a black hole with the mass of Neptune produce the exact same wobble.
In an attempt to separate the two, the authors of the new study looked at exoplanets that have been detected via these wobbles but have never been seen crossing the face of their star — a process called a transit. When a planet transits, it blocks some light, telling us its physical size. If an object pulls on a star but never blocks any light, it might be because it is too small to see, or it might be because it is a black hole.
The researchers identified several intriguing suspects, including Kepler-21 Ac, HD 219134 f and Wolf 1061 d. These objects are heavy enough to make their stars wobble, yet they remain invisible to our telescopes. The team pointed to microlensing events — brief flashes of light caused when a massive object passes in front of a distant star and acts like a magnifying glass — as potential hiding spots for these ancient nomads.
The authors admitted that these candidates are merely representative possibilities, rather than a definitive gallery of tiny black holes. Most will likely turn out to be ordinary planets that just happen to have tilted orbits that prevent them from transiting.
The next decade of data from missions like the Nancy Grace Roman Space Telescope — a NASA telescope that will take a broad survey of exoplanets, due to launch as soon as this fall — will be crucial for learning more about these objects. We might catch one evaporating via Hawking radiation, a theoretical process whereby black holes slowly leak energy until they vanish. If so, we might discover that the universe is a lot more crowded with ancient black holes than we ever imagined.
