Astrophysicists have long believed that this star system, initially observed by the ESA’s Gaia satellite, was simply a star orbiting a black hole. However, recently, two astronomers have challenged this conclusion by finding evidence that suggests something much more peculiar: it could be a previously unseen type of star formed by dark matter. This study has been published as a preprint on arXiv (and is still awaiting peer review).
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The observed star in this system has a slightly smaller mass than the Sun (about 0.93 times the mass of the Sun) and has a chemical composition similar to our own star. Meanwhile, its companion has a much larger mass, about 11 times the mass of the Sun. These two objects orbit each other at a distance of 1.4 astronomical units, equivalent to the distance from the Sun to Mars, with a period of 188 days.
What is this invisible companion object? One possibility is that it is a black hole. Although this scenario seems to fit the observed orbit, it still faces some challenges. Black holes are formed from the deaths of massive stars, and in the case of these pairs, the Sun-like companion star would need to have formed almost simultaneously with its companion. While theoretically not impossible, it would require an extremely high level of precision for such a system to remain bound together for millions of years after the larger star has turned into a black hole.
Another scenario for this case is that the companion star could be something else, which the researchers suggest is likely a object made of dark matter.
Dark matter is an invisible form of matter that makes up a significant portion of the mass in galaxies. We still know very little about this form of matter. Most theoretical models propose that it is evenly distributed in the interstellar space of galaxies, but there is also a hypothesis that it can cluster together into masses, similar to how stars form through gravitational forces.