Planet-hunters scouring the heavens have found thousands of distant worlds around other stars, but astronomers may have overlooked one lurking much closer to home.
Scientists searching for glimmers of light beyond Pluto say they've discovered a new dwarf planet — and that its movements hint that an invisible giant planet far larger than Earth may inhabit the solar system's mysterious frontier.
The new dwarf planet, dubbed 2012 VP113 and described in a study published in Thursday's edition of the journal Nature, helps confirm the existence of an "inner Oort cloud" in an interplanetary no man's land that was once thought to be barren but could be teeming with rocky objects.
"We had high hopes, and our hopes were confirmed," said Scott Sheppard, an astronomer at the Carnegie Institution for Science in Washington, who co-wrote the paper with Chadwick Trujillo of the Gemini Observatory in Hawaii.
The find could lead scientists to rewrite our understanding of the fringes of our solar system, experts said.
2012 VP113 measures about 280 miles across and at its closest comes to within about 80 astronomical units of the sun, or about 7.4 billion miles. (One astronomical unit is the distance between Earth and the sun.) That's far beyond the Kuiper belt, an icy field of debris that sits beyond Neptune's orbit between 30 and 55 astronomical units. (The Kuiper belt's largest known object is dwarf planet Pluto, whose farthest point from the sun is 49 AU.)
While the new dwarf planet is a long way out, it's still not far enough to be considered part of the Oort cloud, a hypothesized spherical shell of icy debris that surrounds the solar system's disc of planets. This cloud is thought to stretch a mind-blowing 5,000 to 100,000 astronomical units from the sun and is believed to seed the inner solar system with long-period comets.
2012 VP113's orbit, by comparison, is probably a few hundred astronomical units long, placing it inside what scientists thought was an empty doughnut of space between the Oort cloud and the Kuiper belt.
That assumption began to change with the 2003 discovery of Sedna, a roughly 600-mile-wide rock big enough to qualify as a dwarf planet that was found near the inner edge of this no man's land. Scientists were puzzled: Was Sedna a fluke, or was it part of a population of rocky bodies in that supposedly empty area — an inner Oort cloud?
Sheppard and Trujillo sought to answer this question by looking for more Sedna-like objects in this area; if they could find more, they'd know that Sedna wasn't an anomaly.
Spotting such distant, dim objects is not easy. Unlike stars, rocks don't make their own light, so astronomers have to settle for faint, moving glints of sunlight reflected off these distant bodies. That means the sun's rays have to travel all the way out to the interplanetary fringe and then come all the way back to telescopes on Earth. As a result, the signal is incredibly weak. Adding to the challenge is the fact that such distant objects appear to move incredibly slowly.
To do this difficult job, the astronomers used the Dark Energy Camera (DECam) on the National Optical Astronomy Observatory's 4-meter telescope in Chile, an instrument chosen because its large field of view allowed them to search vast swaths of sky for faint objects. The pair scanned the sky looking for dim, slow-moving bodies.
The scientists collected about 100 gigabytes of data each night that had to be finely combed through, Sheppard said. It could take two months to go through a few nights' data. After months of analysis, the scientists picked up an intriguing signal.
"It was the slowest-moving thing I'd seen in the discovery process, so immediately I knew it was interesting," Sheppard said.
After finding the dwarf planet, they returned to use the telescope multiple times to make sure it really was there and to determine its orbit.
"It's a very time-consuming process because you've got to keep observing the object over and over," Sheppard said. "But it's definitely worth it to find these very interesting objects."
The scientists noticed something else that seemed too odd to be a coincidence: Both Sedna and 2012 VP113 seemed to be making their closest approach to the sun at similar angles. That could mean that there's a giant planet out there, tugging at both of their orbits in the same way. If so, this ghost planet could have a size of anywhere from 1 to 20 Earth masses, Sheppard said.
"It's not a complete explanation, but it's a possible explanation," said UCLA astronomer David Jewitt, who first discovered Kuiper belt objects and was not involved in the new paper. "The trouble with that is, how massive and where is it and would we have seen it?"
Astronomers will have to find far more of these distant objects and catch enough of them traveling in the same direction before they can say whether or not a giant planet is lurking in the inner Oort cloud, Jewitt said.
Sheppard and Trujillo's search covered about 220 moons' worth of sky. Extrapolating their findings to the rest of the heavens, they estimated that there could be about 1,000 objects in the inner Oort cloud with diameters of at least 1,000 kilometers, or about 620 miles, including some that could be as large as Mars.
An inner Oort cloud would be valuable to study, scientists said, because these objects are so far away from the gravitational pull of either the planets or the stars that they're like a dynamic "fossil" of interplanetary movement in the early solar system.
In fact, the orbits of inner Oort cloud objects could help reveal whether the sun was born in a cluster of stars that later grew apart, leaving little trace of their origins. These neighboring stars' gravitational jostlings could have left a mark on the icy denizens of the inner Oort cloud.
"The mark from those early stellar encounters [are] like fingerprints at a crime scene," astronomer Megan Schwamb of the Institute of Astronomy and Astrophysics in Taiwan wrote in a commentary on the study.
There are plenty of theories to explain how these rocky bodies may have gotten to the inner Oort cloud. Perhaps they were shot out from the central solar system during its early upheavals, for instance, or a passing star early in the sun's history yanked them into their present position. It's even possible they were exoplanets beyond our solar system, nabbed from other stars.
Future telescopes should be able to help settle whether a super-Earth exists by finding more dwarf planets like Sedna and 2012 VP113, scientists said.
"With greater sky coverage and depth than that obtained by Trujillo and Sheppard, we will find more of these distant bodies lurking in the shadows and will begin to unravel the origin of the inner Oort cloud," Schwamb wrote.