Is ‘Planet Nine’ Actually A Black Hole In The Solar System? There’s Only One Way To Find Out
Is there a “Planet Nine” lurking at the fringes of the solar system? Or could it be something a whole lot more scary – a primordial black hole?
A new paper by Harvard University undergraduate Amir Siraj and theoretical astrophysicist Avi Loeb suggests that a new telescope currently being constructed in Chile could hold the key to discovering whether there is, in fact, a black hole located in our own solar system.
That would be the find of the century, arguably way more fundamental than the discovery of any “Planet Nine.”
Published this week on arXiv is “Searching for Black Holes in the Outer Solar System with LSST,” in which Loeb and Siraj propose that the Rubin Observatory in the thin mountain air at the peak of Cerro Pachón in Chile’s Elqui Valley – whose exciting 10-year survey of the sky is due for “first light” in 2022 – will be able to either rule out or confirm Planet Nine as a black hole within a year.
What and where is ‘Planet Nine?’
“Planet Nine” is speculated to be a super-Earth – a planet about five to 15 times larger than Earth – that was first theorized in 2016. Its existence would explain why objects in the Kuiper Belt – a doughnut-shaped region of the outer solar system beyond the orbit of Neptune, and home to Pluto – are clustered and aligned in a particular way. If there is an as-yet-undetected planet in the Kuiper Belt, it’s cold, dark, and way too small to see.
“Planet Nine” is considered unlikely to exist because it would be difficult for the solar system to collect enough material at such a distance from the Sun to form a super-Earth-sized planet. Which has led astronomers to think-up some even crazier-sounding theories about what, exactly, the observed “Planet Nine effect” could actually be.
One of the theories is that it is that there is a primordial black hole in our solar system. Gulp.
Why do some think that Planet Nine could be a black hole?
“Planet Nine” being a black hole is very unlikely, but no more so than “Planet Nine” existing at all, according to Siraj and Loeb. “If it exists and is not a statistical fluke, ‘Planet Nine’ is most likely a planet, not a black hole,” said lead author Siraj in an email to me last week. “There is no unambiguous evidence showing that black holes exist that are less massive than the about mass of the Sun.”
However, another recent paper showed that the probability of the solar system capturing a free-floating planet at the distance of Planet Nine could be comparable to the probability of gravitationally capturing a black hole with a similar mass.
To be clear, it probably isn’t a black hole. “No primordial black holes have been unambiguously detected, and it is unclear exactly how they would form,” says Siraj, but he thinks it’s worth checking out. “Because of the incredibly exciting implications of a black hole potentially hiding in the solar system, it is worth checking to rule it out,” says Siraj. “Or perhaps to confirm it.”
How to find Planet Nine or a black hole
The first, very expensive, way to find a black hole in the solar system would be to send 100 specially-equipped spacecraft, as proposed by Edward Written, and Scott Lawrence and Zeeve Rogoszinski at the University of Maryland, to search a vast area of space. Those spacecraft’s high-precision atomic clocks would reveal its presence as they passed through the gravitational field of any “Planet Nine” or black hole, the tell-tale sign being that they would speed up.
However, a recent paper by Loeb showed that drag and magnetic noise from the interstellar medium would likely dominate over any gravitational signal from “Planet Nine.”
“So we need another way to figure out if Planet Nine is a black hole or not, and that was the motivation for my paper,” says Siraj. Siraj and Loeb’s paper focuses on the search for the occasional flares of light that would naturally occur from collisions between Oort cloud objects and the black hole, if it exists.
What is the Rubin Observatory?
The paper suggests that the Rubin Observatory – which is now in an advanced state of construction close to the Gemini South telescope – will be able to confirm the existence, or not, of a black hole in the solar system.
The Rubin Observatory is all about wide-angle, real-time astronomy. Its 10-year “Legacy Survey of Space and Time” (LSST) survey of the sky will image the entire southern hemisphere night sky every three nights, with each image covering an area 40 times the size of the full Moon.
By both alerting astronomers to real-time events and constructing a massive data archive, it’s expected to dramatically advance astronomers’ knowledge of the cosmos.
How to find black hole in the solar system
“The LSST will be unique in its ability to survey the entire sky about twice per week at a remarkable level of sensitivity,” said Siraj. “We calculated that the flares from the accretion of a small body onto a ‘Planet Nine’ black hole would be brightest near the optical band, where LSST operates.”
That makes the Rubin Observatory ideal for confirming whether “Planet Nine” could be a black hole. “Since Planet Nine’s position is unknown, the fact that LSST surveys the sky so quickly maximizes its chance of catching a flare.”
However, there is one small problem with hunting a black hole. SpaceX Starlink and other upcoming mega-constellations of satellites are said to be particularly problematic for the Rubin Observatory’s plans to survey the night sky.
Could SpaceX Starlink and other mega-constellations jeopardize the search for a black hole?
“It certainly is possible for mega-constellations to harm such a search,” said Siraj. “They could be easily distinguished from a ‘Planet Nine black hole’ because of their large proper motions – resulting in streaks of light across the image, as opposed to a dot – but if a satellite happened to interfere with the line-of-sight to ‘Planet Nine’ at exactly the moment a flare was occurring, then that could prevent us from seeing the flare.”
Wishing you clear skies and wide eyes.
Is ‘Planet Nine’ Actually A Black Hole In The Solar System? There’s Only One Way To Find Out was originally published in ILLUMINATION on Medium, where people are continuing the conversation by highlighting and responding to this story.
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