Astronomers can't see "Planet Nine." But it makes its presence known.
The massive hypothetical object, which supposedly looms at the edge of our solar system, has been invoked to explain the strange clustering of objects in the Kuiper belt and the unusual way they orbit the Sun.
Now Planet Nine predictors Konstantin Batygin and Mike Brown of Caltech, along with graduate student Elizabeth Bailey, offer another piece of evidence for the elusive sphere's existence: It adds "wobble" to the solar system, they say, tilting it in relation to the sun.
"Because Planet Nine is so massive and has an orbit tilted compared to the other planets, the solar system has no choice but to slowly twist out of alignment," lead author Bailey said in a statement.
Before we go any further, a caveat about Planet Nine: It's purely theoretical at this point.
Batygin and Brown predict its existence based on unusual perturbations of the solar system that aren't otherwise easily explained. (This is the same technique scientists used to find Neptune.) But the history of astronomy is rife with speculations that never bear out: The same guy who correctly predicted the existence of Neptune also believed that a planet he called Vulcan was responsible for the wobble of Mercury. That "discovery" caused the astronomy world to waste years looking for something that wasn't there (Mercury's wobble was eventually explained by the theory of general relativity).
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But the evidence offered by Batygin and Brown is compelling. When the pair announced their find in January, planetary scientist Alessandro Morbidelli of the Côte d'Azur Observatory in Nice, France, told The Washington Post: "I don't see any alternative explanation to that offered by Batygin and Brown."
"We will find it one day," he added. "The question is when."
The new research led by Bailey is based on the observation that the Sun rotates on a different axis than the orbits of the planets. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune all move around the sun in a flat, shared plane - it's as if the planets were skaters on the same rink. But that plane is tilted at a six-degree angle with respect to the sun (which, from our perspective, makes it look like the sun is tilted).
This tilt doesn't jibe with how astronomers say the solar system formed: 4.6 billion years ago, gravity started to pull together a vast, scattered cloud of gas and dust, causing it to spin into a disk. The center of the circle collapsed into a hot, dense ball of gas - the sun - and the remainder accumulated into planets. Because the sun came from the same disk as everything else, it should be spinning on the same plane. But it's not.
"It's such a deep-rooted mystery and so difficult to explain that people just don't talk about it," Brown said.
Enter Planet Nine. Based on Batygin and Brown's previous calculations, the hypothesized planet weighs five to 10 times as much as Earth and is, on average, 20 times farther from the sun than Neptune. It takes Planet Nine 10,000 to 20,000 years to orbit the sun.
Planet Nine also orbits at an even more dramatic angle than the rest of the planets: about 30 degrees. That fact, combined with its distant location and massive size, could mean that its angular momentum puts the rest of the solar system off-kilter.
"It's an argument that is basically like an asymmetrical seesaw or a dolly," Batygin told Astronomy Magazine. "Planet Nine has a really long orbit, so it can assert quite a bit of torque on the inner planets without having to apply so much force. Planet Nine has as much angular momentum as the entire solar system combined, because its orbit is so big."
Over the course of several billion years, that could be enough to explain the six-degree tilt we now see.
The study was posted in the online research database ArXiv, and will be published in the Astrophysical Journal this year. The scientists also presented their findings at the American Astronomical Society Division of Planetary Sciences's annual meeting in Pasadena, Calif., this week.
According to Batygin, the research is a "consistency check" for their theories about Planet Nine. It provides another piece of evidence for the planet's existence, but it doesn't change our understanding of it or make it any easier to find. He, Bailey and Brown also don't know why the planet's orbit would be so off-kilter in the first place - perhaps it was knocked askew by Jupiter, or was influenced by the gravitational pull of other stars.
Most of all, they need to find the darn thing. Scientists have a sense of its orbital path, but that path is thousands of years long. Even with some of the most powerful telescopes scouring the skies, it could be a very long wait.