Last year, scientists published a paper with an iconic diagram: a black circle representing, to scale, a 5-solar-mass black hole—and the size of the hypothetical Planet Nine if it were a black hole instead of a planet. So, if Planet Nine really is a black hole, how would we find it?
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Physicist Edward Witten, famed theorist at the Institute for Advanced Study, has an idea. All it requires is an army of small, laser-launched spacecraft with really accurate clocks.
The solar system, at present, has eight planets and a host of dwarf planets (including former planet Pluto). But astronomers have long wondered whether there could be a large ninth planet beyond Neptune that has so far eluded telescopes. Evidence for this planet stems from the strange collective motion of rocks past Neptune; they seem to move as if another massive object, five to 10 times the mass of Earth, were orbiting out there.
Telescope searches haven’t found Planet Nine, nor have they proven that it doesn’t exist. But last September, scientists James Unwin at the University of Illinois at Chicago and Jakub Scholtz at Durham University released a paper suggesting that Planet Nine could be a black hole that has stuck around since the beginning of the universe and now orbits the Sun. They realized that they’d made a provocative proposal and that their theory was unlikely, but they hoped that scientists would open their minds to looking for a ninth planet in different ways, such as with gamma-ray telescopes.