An example is the aptly named “spaghetti” process, which is often illustrated by the fable of an astronaut who ventured too close to the event horizon of a black hole, the point beyond which light cannot escape, and fell headlong. Although her head and feet were within a few meters of each other, the difference in the gravitational forces acting on them would be so great that she would be stretched out like spaghetti.
Interestingly, the effect should be even more dramatic the smaller the black hole is. Sholtz explains that these are relative distances: if you are two meters tall and falling through an event horizon that is one meter from the center of a primordial black hole, the discrepancy between the location of your head and your feet is older, in comparison. to the size of the black hole. This means that it will stretch much more than if it fell into a million-mile-wide star.
“And so, curiously, they are more interesting,” says Scholtz. Spaghetti has already been seen through a telescope, when a star got too close to a stellar black hole 215 million light-years from Earth and was smashed (no astronauts were harmed). But if there is a primordial black hole in our own solar system, it would give astrophysicists the opportunity to study this behavior, and many others, closely.
So what does Batygin think of the possibility that the highly sought-after ninth planet is actually a black hole? “It is a creative idea and we cannot restrict its composition even in the least,” he says. “I think maybe it’s just my own bias, being a planetary science teacher, but planets are a bit more common …”
As Unwin and Scholtz search for an early black hole to experiment with, Batygin is just as interested in a giant planet, citing the fact that the most common type in the entire galaxy are those that are roughly the same mass as Planet Nine.
“Meanwhile, most exoplanets that orbit stars similar to the Sun are in this strange range of being larger than Earth and considerably smaller than Neptune and Uranus,” he says. If scientists find the lost planet, it will be the closest they can get to a window to those in other parts of the galaxy.
Only time will tell if the latest mission will be more successful than Lowell’s. But Batygin is confident that his missions are totally different. “All the proposals are quite different both in the data that they seem to try to explain, and in the mechanisms they use to explain it,” he says.
Either way, the search for the legendary ninth planet has already helped transform our understanding of the solar system. Who knows what else we’ll find before the hunt is over.
Zaria Gorvett is a senior journalist for BBC Future and tweets @ZariaGorvett
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