One example is the aptly named process of “spaghettiification,” often illustrated by the fable of an astronaut who ventured too close to the event horizon of a black hole – the point above which no light can escape – and fell with his head tilted forward. Although her head and feet were only a few feet apart, the difference in the gravity acting on them would be so great that she would be stretched like spaghetti.
Interestingly, the smaller the black hole, the more dramatic the effect should be. Sholtz explains that it is all about relative distances – if you are six feet tall and you fall through an event horizon three feet from the center of a primordial black hole, the discrepancy between the location of your head and feet is greater compared to to the size of the black hole. This means that you will be stretched much more than if you were to fall into a star that is a million miles wide.
“And so, curiously, they are more interesting,” says Scholtz. Spaghettiification has already been observed through a telescope, when a star came too close to a stellar black hole 215 million light-years from Earth and was torn apart (no astronauts were injured). But if there is a primordial black hole in our own solar system, it would allow astrophysicists to study this behavior – and many others – up close.
So what makes Batygin into the possibility that the long-sought ninth planet might actually be a black hole? “It’s a creative idea, and we can’t even limit its composition in the slightest,” he says. “I think it might just be my own bias because I’m a planetary science professor, but planets are a little bit more common …”
While Unwin and Scholtz are looking for a primal black hole to experiment with, Batygin is just as excited for a giant planet – citing the fact that the most common type in the galaxy are those that have roughly the same mass as planet Nine.
“Meanwhile, most of the exoplanets orbiting sun-like stars are in this strange range larger than Earth and significantly smaller than Neptune and Uranus,” he says. When scientists find the missing planet, it will be the closest to a window to the one elsewhere in the galaxy.
Only time will tell if the latest quest will be more successful than Lowell’s. But Batygin is convinced that their missions are completely different. “All the proposals are quite different in both the data they seem to want to explain and the mechanisms they use to explain it,” he says.
Either way, the search for the fabled ninth planet has already helped transform our understanding of the solar system. Who knows what else we will find before the hunt ends.
Zaria Gorvett is a senior journalist for BBC Future and Tweets @ZariaGorvett
Join a million future fans by liking us Facebook, or follow us Twitter or Instagram
If you liked this story, sign up for the weekly bbc.com features newsletter, called “The Essential List”. A hand-picked selection of stories from BBC Future Culture Work life, and To traveldelivered to your inbox every Friday.