Astronomers Have Discovered Closest Black Hole to Earth Yet

Artists illustration of Gaia BH1

The closest black hole to earth has been found at a mere 1600 light years away, being approximately three times closer than the previous candidate, an X-ray binary in the Monoceros constellation. The black hole neighbour, dubbed Gaia BH1 is a dormant black hole weighing 10 solar masses located in the constellation of Ophiuchus, making this the first distinct detection of a dormant stellar mass black hole in our galaxy.

Black holes are regions of spacetime where gravity is so strong that not even light can escape. These extreme astrophysical objects form from the remnants of massive stars which end their lives in supernovae explosions. Black holes can be active or dormant, depending on if they are in the feeding stage: active black holes engulf material from nearby stellar objects, which becomes superheated resulting in the generation of jets along with powerful x-rays. Stellar-mass black holes are a common type, weighing approximately 5 to 10 solar masses. It is estimated that there are 100 million of these black holes in the milky way alone, however, only a few have been confirmed since their small size (for a black hole) makes them hard to detect using telescopes. The handful that have been discovered were so due to their interactions with a companion star, which is a similar case to Gaia BH1, however, this black hole was not feeding off of its companion star but instead causing changes to its motion.

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Initially, the team led by Kareem El-Badry, an astrophysicist at the Centre for Astrophysics, Harvard & Smithsonian and the Max Planck Institute for Astronomy analysed data from the European Space Agency’s Gaia spacecraft which pointed towards a system that potentially hosts a black hole. This manifested as tiny irregularities in the stellar companion’s motion, suggesting some unseen massive object. Follow-up observations were carried out to look at the system in more detail, using the Gemini multi-object spectrograph instrument on Gemini north to measure the velocity hence the precise orbital period of the star as it orbited the black hole.

The preceding observations proved to be vital to constraining the orbital motion and therefore the masses of the objects in the system, leading to the team identifying the central body as a 10 solar mass black hole orbiting the system's centre of mass once every 186 days. El-Badry states “We could find no plausible astrophysical scenario that can explain the observed orbit of the system that doesn't involve at least one black hole." If the mystery binary object was a star, it would be much brighter than its companion and a lot easier to see, which was not insinuated whatsoever in the observations.

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However, this discovery has uncovered many new questions about our understanding of black holes. New models are currently being pursued to explain how the Gaia BH1 system could’ve formed, with the main issue being the progenitor star which later evolved to be a black hole. This star would’ve had to have been at least 20 solar masses, thus living only a few million years. Additionally, if both stars had formed at the same time, the more massive star would’ve engulfed its companion when it became a supergiant. This can’t be the case, since the solar-like star seemingly survived as indicated by the observations. All theoretical models that allow for a surviving companion star predict a much tighter orbit than observed.

These issues highlight a gap in knowledge of the formation and evolution of black holes in binary systems, additionally implying the existence of an unexplored group of dormant black holes in binaries. NSF Gemini Program Officer Martin Still explains, “While this potentially augurs future discoveries of the predicted dormant black hole population in our Galaxy, the observations also leave a mystery to be solved—despite a shared history with its exotic neighbor, why is the companion star in this binary system so normal?"

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Cover image: International Gemini Observatory/NOIRLab/NSF/AURA/J. da Silva/Spaceengine/M. Zamani

Journal source: K. El-Badry et al., A Sun-like star orbiting a black hole, MNRAS (2022). https://doi.org/10.1093/mnras/stac3140