Is Planet 9 Actually A Primordial Black Hole?

Conventional theory has it that Planet 9 —- our outer solar system’s hypothetical 9^th planet —- is merely a heretofore undetected planet, likely captured by our solar system at some point over its 4.6 billion year history. 

But Harvard University astronomers now raise the possibility that orbital evidence for Planet 9 could possibly be the result of a missing link in the decades-long puzzle of dark matter. That is, a hypothetical primordial black hole (PBH) with a horizon size no larger than a grapefruit, and with a mass 5 to 10 times that of Earth.

How might it be detected?

In a paper accepted for publication in The Astrophysical Journal Letters, the co-authors argue that observed clustering of extreme trans-Neptunian objects suggest some sort of massive super-earth type body lying on the outer fringes of our solar system. Perhaps as much as 800 astronomical units (Earth-Sun distances) out.

So, the authors propose that a unique wide-field survey telescope, now under construction in Chile, will soon allow them to set new limits on the possibility that Planet 9 may indeed be a PBH instead of just an ordinary planet. If they exist, such PBHs would require new physics and go a long way towards solving the mystery of the universe’s missing mass, or dark matter. 

Our paper shows that if Planet 9 is a black hole, then comets residing in the outskirts of the Solar system (in the “Oort cloud”) would impact it, Avi Loeb, Chair of Harvard University’s Dept. of Astronomy and the paper’s co-author, told me. They would then be destroyed by its strong gravitational tide and within a second of accreting onto the black hole would produce a visible flare, he says.

For large enough comets, this flare of light would be detectable by the LSST’s 8.4-meter optical telescope.

The idea is that once in the vicinity of a black hole, small cometary bodies would melt as a result of Heating from the background accretion of gas from the interstellar medium onto the black hole, Amir Siraj, the paper’s first author and an Harvard University undergraduate, noted in a statement.  

The authors calculate that they would be capable of detecting the first such accretion flare within a few months of the LSST’s operation which is now slated for first light in 2021. 

Why the LSST?

The LSST will be unique in its ability to survey the entire sky about twice per week at a remarkable level of sensitivity, Siraj told me. We calculated that the flares from the accretion of a small body onto a Planet 9 black hole would be brightest near the optical band, where LSST operates, he says. And since Planet 9’s position is unknown, Siraj notes the fact that LSST surveys the sky so quickly maximizes its chance of catching a flare.

The authors say that such brief accretion flares would be detected at a rate of at least a few per year out to a distance of some 105 AU.  And they expect to be able to rule out or confirm Planet 9 as a primordial black hole within the first two years of the LSST’s operation.

Why would our own solar system harbor such an exotic primordial black hole?

Simply by their sheer numbers in the cosmos.  The authors estimate that it might be somewhat likely that our solar system gravitationally-captured at least once such primordial black hole over the eons.    

What would the detection of such an exotic black hole mean for physics?

Loeb says that the formation of primordial black holes would definitely represent new physics. The process that made them in the early universe is not predicted by the Standard Model of particle physics and cosmology, he says.

If Planet 9 is a primordial black hole, are there likely to be others within the galaxy?

If it is a black hole, there should be fifty quadrillions like it in the Milky Way alone, says Loeb.  

Loeb says there’s nothing to lose in using the LSST to look for such primordial black hole relics. Over the past four decades, lab searches for dark matter searches consumed tens of millions of dollars, he says.

“Our paper proposes to use LSST as a dark matter experiment, searching for primordial black holes at no extra cost,” said Loeb.