Astronomers discover closest black hole to Earth, and it is 10 times heavier than the Sun

Only a handful of black holes, the most extreme objects in the Universe, have been confirmed to date. And nearly all of them are active - they "shine brightly in X-rays as they consume material from a nearby stellar companion", and are rather far away.

All records were broken on Friday when the closest black hole to the Earth was discovered. Called Gaia BH1, the dormant black hole is about ten times more massive than the Sun and is located about 1,600 light-years away in the constellation Ophiuchus. This makes it three times closer to Earth than the previous record holder, an X-ray binary in the constellation of the Monoceros.

All credit goes to the astronomers utilizing the Gemini North telescope in Hawai‘i, one of the twin telescopes of the International Gemini Observatory, operated by NSF’s NOIRLab. A companion Sun-like star that orbits the black hole at about the same distance as the Earth orbits the Sun, helped discover Gaia BH1.

"Take the Solar System, put a black hole where the Sun is, and the Sun where the Earth is, and you get this system," explained Kareem El-Badry, an astrophysicist at the Center for Astrophysics | Harvard & Smithsonian and the Max Planck Institute for Astronomy, and the lead author of the paper, in a statement.

"While there have been many claimed detections of systems like this, almost all these discoveries have subsequently been refuted. This is the first unambiguous detection of a Sun-like star in a wide orbit around a stellar-mass black hole in our Galaxy."

The article has been published in the Monthly Notices of the Royal Astronomical Society.

A companion star led to the black hole

The European Space Agency's Gaia had captured irregularities in the star's motion, which seemed to be caused by the gravity of an undiscovered massive object. El-Badry and his team prodded further and turned to the Gemini Multi-Object Spectrograph instrument on Gemini North, which "measured the velocity of the companion star as it orbited the black hole and provided a precise measurement of its orbital period", according to the release. 

A wobble was noticed in the star's position. The heavier the companion, the larger the wobbling seen. And this time, the wobble was caused by a heavy dormant black hole, with a weight of about 10 Solar masses.

"Our Gemini follow-up observations confirmed beyond reasonable doubt that the binary contains a normal star and at least one dormant black hole," elaborated El-Badry. "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."

Current systems do not explain the configuration of Gaia BH1

Currently, astronomers' models of the evolution of binary systems are unable to explain the "peculiar" configuration of the Gaia BH1 system.

To elaborate, the star that died and turned into a black hole would have been at least 20 times as massive as the Sun. This likely means that it must have lived only for a few million years.

But, if both stars formed at the same time, the gigantic one would have puffed up and swallowed the other star before it could become a "proper, hydrogen-burning, main-sequence star like our Sun". Or, if the star survived, it should have been on a much tighter orbit.

"It is interesting that this system is not easily accommodated by standard binary evolution models," concluded El-Badry. "It poses many questions about how this binary system was formed, as well as how many of these dormant black holes there are out there."

Study Abstract:

We report discovery of a bright, nearby (⁠G=13.8;d=480pcG=13.8;d=480pc⁠) Sun-like star orbiting a dark object. We identified the system as a black hole candidate via its astrometric orbital solution from the Gaia mission. Radial velocities validated and refined the Gaia solution, and spectroscopy ruled out significant light contributions from another star. Joint modeling of radial velocities and astrometry constrains the companion mass to M2 = 9.62 ± 0.18 M⊙. The spectroscopic orbit alone sets a minimum companion mass of M2 > 5 M⊙; if the companion were a 5 M⊙ star, it would be 500 times more luminous than the entire system. These constraints are insensitive to the mass of the luminous star, which appears as a slowly-rotating G dwarf (⁠Teff=5850KTeff=5850K⁠, log g = 4.5, M = 0.93 M⊙), with near-solar metallicity (⁠[Fe/H]=−0.2[Fe/H]=−0.2⁠) and an unremarkable abundance pattern. We find no plausible astrophysical scenario that can explain the orbit and does not involve a black hole. The orbital period, Porb = 185.6 days, is longer than that of any known stellar-mass black hole binary. The system’s modest eccentricity (e = 0.45), high metallicity, and thin-disk Galactic orbit suggest that it was born in the Milky Way disk with at most a weak natal kick. How the system formed is uncertain. Common envelope evolution can only produce the system’s wide orbit under extreme and likely unphysical assumptions. Formation models involving triples or dynamical assembly in an open cluster may be more promising. This is the nearest known black hole by a factor of 3, and its discovery suggests the existence of a sizable population of dormant black holes in binaries. Future Gaia releases will likely facilitate the discovery of dozens more.