An elusive type of black hole has been discovered in a neighboring galaxy for the first time, according to a new study based on observations from the European Southern Observatory's (ESO's) Very Large Telescope.

Dormant stellar-mass black holes, which form when massive stars reach the end of their lives, are particularly hard to spot since they do not interact much with their surroundings. This is because, unlike most black holes, dormant ones don't emit high levels of X-ray radiation.

While thought to be a fairly common cosmic phenomenon, this type of black hole previously had not been "unambiguously detected outside our galaxy," according to the team of US and European researchers involved in the study.

The newly detected black hole, called VFTS 243, is at least nine times the mass of our sun, and it orbits a hot, blue star weighing 25 times the sun's mass, making it part of a binary system.

"It is incredible that we hardly know of any dormant black holes, given how common astronomers believe them to be," said study coauthor Pablo Marchant, an astronomer at KU Leuven, a university in Belgium, in a news release.

The research was published in the scientific journal Nature Astronomy on Monday.

Process of elimination

To find the black hole, which can't be observed directly, the astronomers looked at 1,000 massive stars (each weighing at least eight times the mass of the sun) in the Tarantula Nebula region of the Large Magellanic Cloud, a galaxy near the Milky Way.

The discovery was made by a process of elimination, said coauthor Tomer Shenar, who was working at KU Leuven in Belgium when the study began and is now a Marie-Curie Fellow at Amsterdam University, in the Netherlands.

First, the researchers identified the stars that were part of binary systems -- stars moving around a cosmic companion. Then, they looked for binary systems where the companion was not visible, and careful analysis ultimately revealed that VFTS 243 was a dormant black hole, he explained via email.

"What we see here is a star, weighing about 25 times the mass of our Sun, moving periodically (every 10 days or so) around something 'invisible,' that we cannot see in the data," Shenar said.

"The analysis tells us that this other 'thing' must be at least 9 times more massive than our Sun. The main part of the analysis is elimination: what can weigh nine solar masses, and not emit any light? A black hole is the only possibility we have got left (this, or a fat invisible alien...)."

"There might be more in there, but only for this one we could show the presence of a black hole unambiguously," Shenar said.

The black hole was found using six years of observations by the Fibre Large Array Multi Element Spectrograph (FLAMES) instrument on ESO's Very Large Telescope. FLAMES allows astronomers to observe more than a hundred objects at once.

Black hole police

Some of the 40 study authors are known in astronomy circles as the black hole police, according to the news release, because they have debunked several other discoveries of other black holes.

The paper said that more than 10 discoveries of black hole binary systems in the past two years were disputed. However, they were confident that their discovery was not a "false alarm."

"We know what the challenges are, and we did everything in our capacity to rule out all other options," Shenar said.

The research team said they invited scrutiny of their latest findings.

"In science, you're always right until someone proves you wrong, and I cannot know that this would never happen -- I only know that none of us can spot a flaw in the analysis," Shenar said.