Astronomers have found a dwarf planet far beyond the orbit of Pluto and can only guess how it got there.
The diminutive world, provisionally called "2012 VP 113" by the international Minor Planet Center, is estimated to be about 280 miles (450 km) in diameter, less than half the size of a neighboring dwarf planet named Sedna discovered a decade ago.
Sedna and VP 113 are the first objects found in a region of the solar system previously believed to be devoid of planetary bodies.
The proverbial no-man's land extended from the outer edge of the Kuiper Belt, home to the dwarf planet Pluto and more than 1,000 other small icy bodies, to the comet-rich Oort Cloud, which orbits the sun some 10,000 times farther away than Earth.
"When Sedna was discovered 10 years ago it kind of redefined what we thought about the solar system," astronomer Scott Sheppard of the Carnegie Institution of Washington D.C. said in an interview.
Nothing in the appearance of the modern-day solar system can account for Sedna and VP 113's existence, say astronomers who published their findings on Wednesday in the journal Nature.
Sedna's 11,400-year orbit takes it only as close as 76 times the distance that Earth orbits the sun. VP 113's closest approach is 80 times as far as Earth's orbit of the sun - roughly twice as far as the Kuiper Belt.
"In the current architecture of the solar system, Sedna and 2012 VP113 should not be there," writes astronomer Megan Schwamb, of the Academia Sinica in Taipei, Taiwan, in a separate Nature article.
Computer simulations provide a few potential scenarios.
Lead researcher Chad Trujillo favors the idea that a sibling star forming in the same stellar nursery as the sun gravitationally elbowed some Oort Cloud residents inward as it flew by.
Sheppard suggests that another planet at least as massive as Earth got bumped out of the solar system, taking some Kuiper Belt bodies with it along the way.
That renegade planet or planets actually may still be lurking in the farthest reaches of the solar system, too dim and remote to be detected by currently available telescopes and cameras, Sheppard said.
A third option is that the sun has a companion, something five- to 10 times the mass of Earth, whose gravity is pinning Sedna, VP 113 and potentially millions of other dwarf-like planets in unusual and distant orbits.
"With our discovery of one more object, we can't rule out one theory or another," Trujillo said.
More residents of the region, now known as the inner Oort Cloud, soon may make their presence known.
Astronomers are working to confirm six other Sedna-like objects found last year. That requires imaging the mini-planets several times over a year or longer to measure how much they have moved relative to background stars.
"They're really hard to find," Trujillo said.
Astronomers suspect there may be 150 million Sedna-like dwarf planets measuring between 31 and 5,000 miles (50 and 8,000 km) in diameter, a larger population than the Kuiper Belt objects.