Vera Rubin (Observatory) aims to deliver the final verdict
Normally, black holes are extremely difficult to locate. As their name suggests, nothing, including light, can escape their gravitational grasp once it gets too close. Instead, scientists must pinpoint black holes by observing their influence on nearby objects, or, alternatively, by catching bright flares of light that are emitted when matter spirals into them.
So, to search for black holes in the distant solar system, astrophysicist Avi Loeb of Harvard, along with Harvard undergraduate Amir Siraj, developed a method to seek out flares generated when a black hole encounters small objects in the Oort Cloud, which is a vast shell of potentially trillions of icy bodies that cocoons our solar system. Occasionally, the researchers say, Oort Cloud objects like comets should interact with any black holes lurking around, producing a visible flare of light that the Vera C. Rubin Observatory could detect when it starts its 10-year Legacy Survey of Space and Time (LSST).
This isn’t an entirely new idea, however. Delivering a final verdict on whether a ninth planet is hiding in our solar neighborhood was already one of the goals for LSST. But in terms of spotting a planet-mass black hole, the groundbreaking survey happens to be perfectly suited for the job.
“Neither of us expected it to conveniently fall within the range that LSST is going to look at,” Siraj tells Astronomy. “But also, beyond just a Planet Nine black hole … we can rule out or confirm [any] planet-mass black holes all the way to the edge of the Oort Cloud.”
If LSST does end up spotting a flare from a primordial black hole masquerading as Planet Nine, another telescope of similar sensitivity could then focus on its location for much longer, likely capturing thousands more flares. But Siraj says that even if the survey doesn’t detect any flares, “we can place very tight limits on the fraction of dark matter that’s tied up in primordial black holes.”
All things considered, Loeb and Siraj expect LSST to have definitive proof of whether or not a planet-mass black hole is lurking in the outer reaches of our solar system within the first year or two of the survey. But even with slim chances of success, the researchers can’t help but hold their breath.
“It’s such a small probability, but to have a black hole within reach would be — I think exciting is an understatement,” said Siraj. “It would really open up a new field if we had a black hole in our backyard.”