One example is the aptly-named process of “spaghettification”, which is often illustrated by the fable of an astronaut who ventured too near a black hole’s event horizon – the point beyond which no light can escape – and fell in headfirst. Though her head and feet were just metres from each other, the difference in the gravitational forces acting on them would be so great, she would be stretched like spaghetti.
Intriguingly, the effect should be even more dramatic, the smaller the black hole is. Sholtz explains that it’s all about relative distances – if you’re two metres tall, and you’re falling through an event horizon that’s one metre from a primordial black hole’s centre, the discrepancy between the location of your head and feet is larger, compared to the size of the black hole. This means you’ll be stretched far more than if you fell into a stellar one that’s a million miles across.
“And so, peculiarly enough, they’re more interesting,” says Scholtz. Spaghettification has already been seen via a telescope, when a star got too close to a stellar black hole 215 million light years from Earth, and was ripped apart (no astronauts were harmed). But if there is a primordial black hole in our own solar system, it would provide astrophysicists with the opportunity to study this behaviour – and many others – up close.
So what does Batygin make of the possibility that the long-sought ninth planet could actually be a black hole instead? “It’s a creative idea, and we cannot constrain what its composition is even in the least bit,” he says. “I think maybe it’s just my own bias, being a planetary science professor, but planets are a little bit more common…”
While Unwin and Scholtz are rooting for a primeval black hole to experiment with, Batygin is just as keen for a giant planet – citing the fact that the most common type throughout the galaxy are those which have around the same mass as Planet Nine.
“Meanwhile most exoplanets that orbit Sun-like stars, are in this weird range of being bigger than the Earth and considerably smaller than Neptune and Uranus,” he says. If scientists do find the missing planet, it will be the closest they can get to a window into those elsewhere in the galaxy.
Only time will tell if the latest quest will be more successful than Lowell’s. But Batygin is confident that their missions are totally different. “All of the proposals are quite distinct in both the data they seem they seek to explain, as well as the mechanisms they use to explain it,” he says.
Either way, the search for the legendary ninth planet has already helped to transform our understanding of the solar system. Who knows what else we’ll find before the hunt comes to an end.
Zaria Gorvett is a senior journalist for BBC Future and tweets @ZariaGorvett
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