Why astrophysicists think there's a black hole in our solar system

• In 2016, astrophysicists Konstantin Batygin and Mike Brown noticed unusual gravitational activity in the outer solar system.
• They speculated this activity was from a ninth planet situated in the Kuiper belt.
• But in late 2019, astrophysicists James Unwin and Jakub Scholtz propose Planet 9 isn't a planet but actually a primordial black hole.
• If found, that would be a black hole the size of a tennis ball.
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Following is the transcript of the video.

Narrator: Black holes are some of the most powerful and violent objects in the universe. They can warp space and time and rip entire stars to pieces. So, what if I told you that astrophysicists think there's one lurking in our very own solar system?

Reporter: Then a new discovery rocked the astronomical world to its foundation.

Narrator: Now, before you freak out, you should know that our planet isn't about to get sucked up and reappear in another galaxy. If there is a black hole, it's at least 20 times farther from Earth than the furthest planet, Neptune, in a distant region of our solar system called the extended Kuiper Belt. The belt is home to billions of icy objects, like comets and asteroids. And in 2016, astronomers Konstantin Batygin and Mike Brown noticed that some of them have highly unusual orbits. While most objects in our solar system orbit the same plane, and in the same direction, the orbit of these objects looks like this. Some of them orbit on a slight tilt. Others orbit backwards. And still others orbit perpendicular to the plane of our solar system.

Konstantin Batygin: Imagine the plane of the solar system as being your table. These orbits are stacked like books. Narrator: They concluded that this could mean only one thing. Batygin: In order to generate this population of highly inclined, perpendicular orbits, you need some distant gravitational pull.

Narrator: By their calculation, that gravitational pull is five times greater than Earth's. So they reasoned that this "something" was as large as a planet orbiting our sun. In fact, they even went so far to give it a name, Planet 9. Now, of course, if they were to confirm this discovery, it would be groundbreaking. The first new planet in our solar system since the discovery of Neptune in 1846.

Reporter: Very little is known about Neptune, the last of the giants on this tour.

Narrator: But there was a problem. No one was able to observe the planet directly. Now, that could be because finding a planet that far away is like searching for a needle in a haystack... with the lights off and only a vague idea of where the haystack even is. Or it could be because, well, that "something" is not a planet at all. In a study published in the fall of 2019, theoretical physicists Jakub Scholtz and James Unwin proposed it could actually be a black hole. Not just any black hole, but a primordial black hole.

Jakub Scholtz: Primordial black hole is a remnant from the Big Bang that came from a very dense region that almost instantly collapsed into a small black hole right during the birth of the universe.

Narrator: And they do mean small. About the size of a tennis ball. That's right; this black hole would fit in the palm of your hands. While the idea might sound far-fetched, it isn't totally out there. For over 50 years, astronomers have proposed the universe is littered with primordial black holes. And we've even observed fluctuations of starlight in our own galaxy, which primordial black holes could explain. Not only that, but Scholtz and Unwin also noticed that if a primordial black hole orbited the sun, it would have the same effect on objects in the Kuiper Belt as a large planet. But whether it's a black hole or a planet, there's another mystery.

Where did it come from? One idea is that the object was wandering through the galaxy and got caught in the sun's gravity, but for now, this is all just speculation.

James Unwin: If we do do our searches and it turns out that there is a planet in the outer solar system, this would be an absolutely incredible discovery. Such a strange planet on such an odd orbit. It would really change our understanding of the history of the solar system.

Narrator: But if it is proven that this is a black hole in our own outer solar system?

Unwin: This is completely mind-blowing. And the fact that this not only would change our understanding of solar system physics, but it also gives us a new window into astrophysics, into early-universe cosmology, and into, potentially into, the fundamentals of particle physics.

Narrator: Whether it's a planet, a black hole, or something else entirely, one thing is for sure: When we do find the source of this mysterious gravitational pull, it will be huge.