Planet Lurking Beyond Pluto

In 2003, Caltech astronomer Mike Brown and his team discovered a new addition to the solar system: a small icy world called Sedna. It was a bit of an oddball—taking an extremely elongated orbit that sent it well beyond Neptune—but astronomers suspected it wasn’t alone.

After over a decade of searching, Sedna finally has a friend. Yesterday, astronomers announced they’ve found a similar body—dubbed a sednoid after the original finding—in the same neighborhood. The discovery, published in Nature, suggests the so-called inner Oort cloud may be real. The inner Oort cloud sits outside the Kuiper belt, which lies just beyond Neptune’s orbit, but inside the outer Oort cloud. All three of these regions are filled with swarms of ice balls smaller than our moon.

Also striking were the similar orbits of the two sednoids, which are reviving speculation that there may be a much bigger, unseen planet lurking in the outer reaches of our solar system.

The new sednoid, called 2012 VP113, was discovered by Chadwick A. Trujillo and Scott Sheppard of the Gemini Observatory in Hawaii. Its closest approach to the sun is 7.4 billion miles, which is 80 times farther than Earth’s proximity. The team estimates it’s about 450 kilometers in diameter—probably large enough and round enough to be considered a dwarf planet.

Finding 2012 VP113 is especially sweet for Trujillo, who was on the team that discovered Sedna. “It’s really satisfying to see that we were right and there is a whole huge population of bodies like Sedna,” Trujillo says. “2012 VP113 is the smoking gun” that confirms the inner Oort cloud, says Meg Schwamb, an astronomer at Academia Sinica in Taipei who searched for sednoids with Brown during her PhD.

The orbits of both Sedna and 2012 VP113 are oriented away from the plane of the other planets at a suspiciously similar angle. It could just be coincidence, but it could also hint at a massive planet many times the size of Earth hiding in the inner Oort cloud whose gravity shepherds sednoids into similar orbits.

We’ve been there before. Astronomers have invoked the idea of a so-called “Planet X” in the outer solar system many times and for different reasons, dating back to the mid–1800s when American astronomer Percival Lowell became convinced that an unseen planet was causing disturbances in the orbits of Uranus and Neptune. For a few decades, astronomers thought Pluto fit the bill, but in 1978 they learned it was too small to cause the disturbances. Then, in 1992, NASA’s Voyager revealed that Neptune was slightly more massive than previously thought, explaining the planet’s orbit.

Scientists have also toyed with the idea of an unseen body to explain the dynamics of comets. In 1984, physicist Richard Muller suggested that a small star roughly one light year away could be tugging on comets, tossing them into the inner solar system and causing periodic mass extinctions. That idea fizzled when paleontologists found out that comet-caused extinctions weren’t as regular as they’d thought. (That didn’t stop conspiracy theorists from convincing themselves that not only did such a planet exist, but NASA was covering up an impending cataclysmic collision.)

Still, most comets seem to come from the outer Oort cloud, so astronomers kept around the idea of a small star or a gas giant to explain it.

Just two weeks ago, that hypothesis took a huge blow. Kevin Luhman, a Penn State astronomer, announced that NASA’s Wide-Field Infrared Survey Explorer hadn’t turned up anything like it.

So if a gas planet isn’t there, what could explain the similar orbits of Sedna and 2012 VP113? Perhaps, say Trujillo and Shepherd, an unseen rocky super Earth could be the cause. “It would be too small and too faint to be detected by WISE, so it is plausible that it exists,” Luhman says.

Trujillo and Sheppard simulated the presence of a super Earth some 23 billion miles from the sun, and their results suggest the super Earth hypothesis is valid. But no telescope is powerful enough to detect such a planet out there. Ramon Brasser, who studies the outer solar system at Acadmia Sinica, doesn’t think there’s a super Earth out beyond Neptune, calling it “a low-probability event.”

Schwamb says “the jury is still out” on this latest revival of a Planet X. The same goes for a lot of what we know about the Oort cloud. She compares our understanding of it today to what we knew about the Kuiper belt in 1992, when astronomers had just discovered the second object out there. (Pluto was the first.) “I think we’re just at the beginning of learning about this region,” she says.

Planet X would explain why the angles of Sedna and 2013 VP113′s orbits are similar, but it doesn’t explain how they ended up in the inner Oort cloud in the first place. The most accepted theory suggests that they started out as comets that were boosted into higher orbits when the sun came close to another star.

While that’s unlikely in our rather sparse sector of space, there are a couple of ways this could happen. Our sun—like most every star—was born from a cloud of gas that spawned a cluster of swirling stars. In those early years, close encounters would have been common. Or at another point in its distant past, the sun may have been traversing more crowded parts of the galaxy.

2012 VP113 doesn’t settle this debate. “We really need to find several more of these objects to say anything significant,” Sheppard says. Astronomers are hoping a new generation of telescopes with wide fields of view—like the upcoming Large Synoptic Survey Telescope—will speed up the search.

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