You may have heard about Planet Nine—a hypothetical planet thought to exist in the outer reaches of the solar system. One possibility is that it’s not a planet at all but a tiny black hole. New research outlines a potential strategy for detecting this supposed black hole, in a search that could begin as early as next year.
Harvard astronomers Avi Loeb and Amir Siraj have proposed a new strategy for detecting a grapefruit-sized black hole in the outer solar system, in a paper that has been accepted for publication in The Astrophysical Journal Letters. Using the Vera C. Rubin Observatory, still under construction in Chile, astronomers could indirectly detect this object by observing it do what black holes do best: gobble up stuff.
The reason for thinking a black hole might be lurking out there has to do with an unexplained set of astronomical observations. Something– we don’t know what– appears to be affecting a group of objects beyond the orbit of Neptune. A possible explanation is an undetected planet, dubbed Planet Nine, with a mass between 5 and 10 Earth masses and in an elongated orbit between 400 and 800 AU from the sun, in which 1 AU is the average distance from the Earth to the Sun. Recently, scientists proposed another explanation: a primordial black hole of a similar mass.