Planet Nine Exists?

 

In 1992, two astronomers who had “doggedly scanned the heavens in search of dim objects beyond Neptune” for years, according to Nasa, discovered the Kuiper Belt. This cosmic donut of frozen objects, extending just beyond the orbit of Neptune, is one of the largest features in the solar system. It’s so vast, it’s thought to contain hundreds of thousands of objects larger than 100km (62 miles) across, as well as up to a trillion comets.

Soon scientists realised that Pluto was unlikely to be the only large object in the outer reaches of the solar system – and began to question whether it was actually a planet at all. Then they found “Sedna” (around 40% of the size of Pluto), “Quaoar” (around half the size of Pluto), and “Eris” (almost the same size as Pluto). It became clear that astronomers needed a new definition.

An obscure hiding place

Of course, all this begs an obvious question – if Planet Nine is really there, why has no one seen it?

“I didn’t have a particularly strong appreciation for just how difficult would be to find Planet Nine until I started looking together with Mike using telescopes,” says Batygin. “The reason it’s such a tough search is because most astronomical surveys are not looking for a single thing.”

For example, astronomers would normally be looking for a class of objects, such as a particular kind of planet. Even if they’re rare, if you survey a wide enough expanse of space, you’re likely to find something. But hunting down a specific object such as Planet Nine is a whole different exercise. “There’s only one tiny portion of the sky that has it,” says Batygin, who explains that another factor is the slightly more prosaic challenge of booking time slots to use the right kind of telescope.

“Really, at the moment the only game in town for finding Planet Nine is the Subaru Telescope,” says Batygin. This 8.2m behemoth – located at the summit of a dormant volcano, Maunakea, in Hawaii – is capable of capturing even the weak light of distant celestial objects. This is ideal, because the shadowy planet would be so far away, it’s unlikely to be reflecting much light from the Sun.  

“We have only one machine that we can use, and we get maybe three nights on it a year,” says Batygin, who was fresh from a three-night run on the telescope the previous week. “The good news is that the Vera Rubin telescope is coming online within the next couple of years, and they are going to probably find it.” This next-generation telescope, currently under construction in Chile, will be scanning the sky systematically – photographing the entire available view – every few nights, to survey its contents.

An intriguing alternative 

However, there is one almost outrageously peculiar scenario in which the planet will never be found this way – it might not be a planet after all, but a black hole.

Unwin points out that there is zero probability of the black hole being formed from a star, since they keep their potent gravitational pull – it’s just concentrated. Even the smallest stellar blackholes have masses three times that of our Sun, so it would be like having at least three extra Suns pulling at the planets in our solar system. In short, we would definitely have noticed.

However, Unwin and Scholtz say it could be a primordial black hole, since these are thought to be substantially smaller. “Because these things are born during the early stages of the Universe, the dense regions they formed from could have been particularly small,” says Scholtz. “As a result, the mass contained in this black hole that eventually is formed out of it can be much, much less than a star – they even can be just a couple of pounds, like a chunk of rock.” This is more in line with the predicted mass of Planet Nine, which is thought to be equivalent to up to ten Earths.