Invisible Black Holes: Planets or Something Else? express
A star dancing without a visible partner — that’s how astronomers perceive its wobble. Doppler shift usually reveals a planet, but sometimes the companion remains invisible. Then it could be a primordial black hole — a planetary-mass object born in the early Universe. Unlike ordinary planets, such a hole neither reflects light nor blocks the star (no transit).
Sifting through data on thousands of exoplanet candidates, scientists identified six objects that reveal themselves only through gravity. Among them are companions of the stars Kepler-21, Gliese 686, and four others. Two cases of brief light bending from distant stars (microlensing) are also suspicious.
The gravitational dance gives away both a planet and a black hole equally. Telling them apart may rely on Hawking radiation: tiny holes evaporate faster and could emit a faint signal. If confirmed, these invisible partners would become the key to dark matter — the mysterious substance that perhaps makes up most of the Universe.
Original abstract
as written by the authors arXiv:2512.03118
We have sifted through astrophysical data collected by various radial velocity and gravitational microlensing searches for exoplanets with the goal of identifying potential signs of the presence of primordial black holes (PBH). Our motivation is that those hypothesized remnants of inhomogeneous energetic fluctuations in the early universe, though too small for direct detection, are thought to have a mass range similar to that of planets. Thereby, if captured by stars, they could conceivably make their presence known through stellar wobbles picked up by means of Doppler spectroscopy in the radial velocity method, or alternatively through microlensing. In our analysis of such data, we have identified potential PBH contenders by ruling out any exoplanet candidates that have been detected through direct imaging or transit methods, as they would have sizes consistent with planets rather than PBHs. In particular we focus on the objects Kepler-21 Ac, HD 219134 f, Gliese 686 b, HR 5183 b, HD 20794 e, and Wolf 1061 d, each of which has been found using the radial velocity method but never imaged (either directly or through transit). We also examine the microlensing events MOA 2009-BLG-387L and OGLE-2016-BLG-1540, which offer promise as candidate PBHs. We present these as a representative, but not exclusive list, of potential PBH contenders. Furthermore, future imaging, especially focused on signals of planetary dimensions versus evaporation signatures, might clarify which of these are indeed exoplanets.
A primordial black hole with Earth’s mass would fit inside a grape — its diameter is just 9 millimeters.