Michael Guillen: Hello, Planet 9 – There’s a cosmic-sized chance we’ve got a big, new planet in our future

There’s not a more thrilling roller coaster ride than the study of our solar system. We once thought there were 11 planets, then it plummeted to nine, then further dropped to eight. But strap yourselves in because it now appears we’re heading back up again, as astronomers excitedly race to discover what they believe is a stealthy, bona fide ninth planet way bigger and way further than Pluto.

It’s enough to make your head spin.

When I was in grade school I was told there were nine planets in our solar system and that was that, no question about it. To cement the sacred fact, my teacher taught us a catchy mnemonic to help remember the planets’ proper order: Many Very Eager Monkeys Jumped Safely Under New Palms (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto).


Artist's illustration of Planet Nine, a hypothetical world that some scientists think lurks undiscovered in the far outer solar system. (R. Hurt (IPAC)/Caltech)

Artist's illustration of Planet Nine, a hypothetical world that some scientists think lurks undiscovered in the far outer solar system. (R. Hurt (IPAC)/Caltech)

The teacher never told us that early 19th century kids were taught a very different sacred fact about the planets. In 1816 “An Introduction to Astronomy,” by John Bonnycastle, avowed “the sun is now well known … to have eleven primary planets moving round him, each in its own path or orbit … Mercury, Venus, the Earth, Mars, Vesta, Juno, Pallas, Ceres, Jupiter, Saturn, and Uranus.”

Schoolkids nowadays are taught yet a different sacred fact – that there are only eight primary planets in our solar system, Pluto having been downgraded in 2006 to the vague status of a dwarf. As one science writer put it, the mnemonic du jour is now: My Very Excellent Mother Just Served Us Noodles.

And don’t think for a minute it stops there.

For decades the public has been treated to speculation about the possible existence of a Planet X orbiting beyond our sight, in the dark frontiers of the solar system. In 1976 a book called "The 12th Planet" caused a sensation by alleging evidence for Nibiru, a doomsday planet beyond Pluto that swoops in every 3,600 years and wreaks havoc on Earth. Its author is dead, but the claim and its zealous adherents are very very much alive.

Earlier this century the idea of a Planet X gained respectability when astronomers spotted irregularities in the orbits of asteroids in the Kuiper Belt, the solar system’s boondocks. It suggested the idea that the gravitational field of a massive, invisible planet was herding the orbits in an unlikely way.

In 2015 the idea’s stock soared when a pair of Cal Tech astronomers – Konstantin Batygin and Michael Brown – ran a computer simulation that predicted where and how big the surreptitious provocateur must be in order to do such a thing. They called it Planet Nine, triggering an intensifying, worldwide race to find it.

According to the latest Cal Tech calculations, published just months ago, the hypothetical planet most likely is five times Earth’s mass and four-hundred times Earth’s distance from the sun – about 40,000,000,000 miles away. That means it would take the planet about 10,000 Earth years to complete an orbit.

If these numbers are reliable, many existing optical telescopes are certainly powerful enough to spot the phantom planet – though not easily. Despite its supposedly large size, Planet  Nine’s alleged remoteness would make it extremely faint. “You can hide a very big thing in the outer solar system very easily,” quips astronomer Scott Sheppard of the Carnegie Institution for Science.


Despite the challenge, Cal Tech’s Batygin thinks it is “over 90 percent” likely that Planet Nine exists. “History shows us that it’s a bad idea to consistently say we have now reached the end of the solar system, and there’s nothing beyond what we already know.”

As for Cal Tech’s Brown, it’s fun to note he is no stranger to causing a stir. Earlier this century he advocated quite vocally for Pluto’s demotion, thereby earning himself the title of the man who killed Pluto. “All those people who are mad that Pluto is no longer a planet,” he now says, “can be thrilled to know that there’s a real planet out there still to be found.”