• This artistic rendering provided by California Institute of Technology shows the distant view from Planet Nine back towards the sun. The planet is thought to be gaseous, similar to Uranus and Neptune. Hypothetical lightning lights up the night side. Scientists reported Wednesday, Jan. 20, 2016, they finally have "good evidence" for Planet X, a true ninth planet on the fringes of our solar system. (R. Hurt/Infrared Processing and Analysis Center/Courtesy of California Institute of Technology via AP)

New evidence points to giant 9th planet on solar system edge

Wednesday, January 20, 2016

Astronomers say they've found evidence that our solar system might hold another giant planet, hidden in the dark, distant badlands far beyond Neptune's orbit.

This so-called Planet Nine, described in the Astronomical Journal, would likely have roughly 10 times the mass of Earth and circle the sun in 10,000 to 20,000 Earth years. Neptune, the farthest known planet today, makes its round trip in a mere 165 years.

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If it is found to exist by powerful telescopes on Earth, Planet Nine would rewrite our definition of the solar system and help solve some mysteries about its violent past.

"If this planet is out there, it's really a big deal," said Gregory Laughlin, an astronomer at UC Santa Cruz who was not involved in the paper. "It's a major, major discovery if it turns out to be true."

Scientists have been wondering whether a "Planet X" exists in the dim regions far beyond the known planets, but the idea has remained largely speculative. Scientists and layfolk alike have had to content themselves with just eight planets, cut down from nine after Pluto was demoted to dwarf in 2006.

Here's the evidence for Planet Nine at the edge of the solar system

Caltech scientists Konstantin Batygin and Mike Brown make the case for Planet Nine, a giant planet tracing a bizarre, highly elongated orbit in the outer solar system.

That started to change in March 2014, when a pair of astronomers announced that they'd discovered a new dwarf planet, 2012 VP113, beyond the well-populated edge of the icy Kuiper belt, whose main mass stretches from Neptune's orbit around 30 astronomical units (or 30 times the Earth-Sun distance) out to 50 astronomical units.

It wasn't the only such object: Sedna, an icy 600-mile-wide rock discovered in 2003, also boasted a far-out orbit, and it seemed to be making its closest approach to the sun at a similar angle as 2012 VP113. This could be highly coincidental - or it could mean that a giant planet was lurking out there in the dark, influencing both their movements.

Caltech astronomer Mike Brown, one of the main players responsible for Pluto's demotion, saw this paper and was intrigued by the implication. He went down the hall to see astrophysicist Konstantin Batygin.

"I walked in and said, 'Look, this is real. We have to figure out what is going on,' " Brown said.

Batygin, the study's lead author, hadn't been thinking much about the solar system or about invisible planets - but the data was intriguing.

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"These two Kuiper belt objects are really outliers. … Their orbits do not hug the orbit of Neptune, and hugging the orbit of Neptune is kind of a unifying feature among the vast population of Kuiper belt objects," Batygin said. "Neptune could not have gravitationally kicked them out to an orbit with which it doesn't interact."

Brown and Batygin set out to nail down an explanation, pooling their complementary skill sets. The researchers studied half a dozen far-out Kuiper belt objects, in the hopes of finding the gravitational fingerprint of a planet left on the shapes of their orbits.

They began to see a strange pattern: All the objects' orbits were tilted at about the same angle, dipping roughly 30 degrees downward relative to Earth and the other planets. Also, their orbits — including their perihelia, the points at which each object came closest to the sun — were all clustered fairly close together, rather than being randomly distributed.

Brown was baffled by these orbits, which often crossed each other. There's no way that these crossing paths should remain stable, he thought — unless there were something else out there that was massive enough to shepherd these objects along, keeping them in line.

Using complex computer simulations, the scientists calculated that there's only a 0.007% possibility that these tightly clustered orbits arose by chance.

"We would have to get monstrously lucky to have the solar system that we have," Batygin said. "The chances of this are better than winning the Powerball, but they're not tremendously better."

The scientists concluded that there must be something influencing their movements; such a massive planet would have an orbit whose closest approach was on the other side of the sun from the perihelia of the objects it was shepherding. This giant planet is so far out, however, that the closest it gets to the sun is a whopping 200 astronomical units.

"I understand a lot about how the observations are taken and how the solar system is arranged. … He understands a lot about how gravity is pushing things around," Brown said of Batygin. "And it really took both of us to put the whole picture together."

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The model neatly explained the orbits of Sedna, as well as 2012 VP113. But the true power of their prediction was that it actually described the behavior of objects that the scientists did not set out to explain.

The model showed that there must be a weird class of objects that actually were moving vertically - that is, perpendicular to Earth and the other known planets. At first, this seemed patently ridiculous to the researchers, until they looked it up and found that there were, in fact, several known objects moving vertically that perfectly matched their prediction.

"That's when my jaw hit the floor," Brown said. "That was not only a prediction we weren't working on, this wasn't even a phenomenon that we even knew existed until we looked at the data."

Planet Nine probably would be somewhere around 10 Earth masses, making it a tiny gas giant, known as a mini-Neptune. In fact, this miniature status probably had something to do with its current banishment to the interplanetary boonies, said Scott Sheppard, an astronomer at the Carnegie Institute for Science in Washington, who was not involved in the paper.

"It was probably the runt of the family," said Sheppard, who co-wrote the paper that originally piqued Brown's interest. Planet Nine may have formed closer to the sun, but it was probably hurled out of the area thanks to the gravitational influence of one of the larger gas giants, such as Jupiter.

Discovery of new dwarf planet hints at other objects in solar system

Discovery of new dwarf planet hints at other objects in solar system

If this planet exists, it could shed light on the solar system's tumultuous early years and help explain why the solar system looks the way it does today, scientists said.

It also could help explain why our solar system's demographics look quite different from those around the stars observed by NASA's Kepler spacecraft. Kepler has found that planets with a few to several Earth masses — super-Earths and mini-Neptunes — are common around other stars, and yet our home system seems to be missing these usual characters.

If telescopes do prove that Planet Nine exists, Brown said, then our home in the galaxy doesn't look quite so different after all.

Planet Nine, if discovered where it's predicted to be, would follow recent astronomical tradition. The last and most distant planet discovered in our solar system, Neptune, was found thanks to the mathematical predictions of Frenchman Urbain Le Verrier in 1846; German astronomer Johann Gottfried Galle found it within a day of receiving Le Verrier's calculations.

Finding this distant world, however, probably will be much harder than finding Neptune. At its closest point, 200 astronomical units from the sun, it would be roughly 18.6 billion miles away, where little sunlight can reach. It will take powerful telescopes such as the Keck Observatory and Japan's Subaru Telescope (both on Mauna Kea in Hawaii) to find Planet Nine, if it's really out there.

In some ways, the scientists noted, it might be easier to see planets around other, far more distant stars, simply because you know where to look — directly at the star. In the case of Planet Nine, scientists currently don't know exactly where it is in its potentially 20,000-year orbit - which means there's a lot of celestial ground to cover.

Sheppard, who along with Chadwick Trujillo had proposed the idea of a hidden planet beyond the main Kuiper belt in 2014, said that his certainty that the planet exists has gone from about 50% to 60%. The best way to prove it exists for sure? To catch it in action.

"I think it would actually be a mind-blowing event, to know that something bigger than the Earth in our solar system exists and we haven't known about it until now," said Sheppard, who was not involved in the current paper. "I think that's a pretty amazing thing."

Sheppard said he's focusing on finding more small objects whose orbital shapes may bear the mark of Planet Nine in the hopes that it will help researchers home in on where the mysterious world might be.

"We hope when we get their orbits nailed down, that they'll lead us to this giant planet," he said.

Brown, for his part, thinks the data might already be in hand — that there might be the tiniest of perturbations in the orbits of planets like Mars and Saturn, which could show up in data already gathered from the many missions sent to those planets. The trick is in teasing such faint information out of the noise.

Planet Nine might help redeem Brown, known as the "Pluto killer" in the eyes of former schoolchildren who remember (or have since learned of) the dwarf planet's demotion.

"I hope so. My daughter is the one who told me I need to do this," Brown said. "Even before we started this, she said, 'Daddy, what you need to do is go find a new planet so that people will no longer be sad about Pluto.'"