When we divide up the solar system, we usually do it in three broad categories: the planets, the Kuiper Belt, and the Oort Cloud. We're all familiar with the eight plants: four inner, rocky worlds and four outer, gaseous ones. Outside of that, some 30-50 times farther from the Sun than the Earth lies the Kuiper Belt. Its nearest member to us, Pluto, is giant ball of ice. Other members of this region are likely the same. Beyond that, we expect there to be vast stretches of empty space. And, tens of thousands of times farther from the Sun than the Earth, we've hypothesized the presence of the Oort Cloud, an isotropic population of objects which may outnumber all the other members of the solar system combined.
Just over a decade ago, we discovered Sedna, an object way out in that empty space beyond the Kuiper Belt. Sedna's orbit is incredibly elongated. At its closest approach (perihelion) it reaches about 76 AU from the Sun. One astronomical unit, or AU, is the distance from the Earth to the Sun (about 150 million kilometers). When Sedna reaches its farthest point from the Sun (aphelion) it's at a distance of 937 AU. Wow!
The detection of Sedna raised many questions. Is there an entire population of objects between the Kuiper Belt and the Oort Cloud? Or was Sedna's amazingly-elongated orbit evidence that it was accidentally knocked into the region? We couldn't be sure.
Now astronomers have detected a new object, provisionally named 2012 VP113. It shares characteristics with Sedna, such as its elongated orbit. But 2012 VP113 has a perihelion distance of about 80 AU, meaning it stays farther from the Sun than any object we've ever detected. At aphelion, it strays as far as 450 AU from the Sun.
One object is an anomaly; two is the start of a trend. Could there be a population of objects between the Kuiper Belt and the traditional notion of the Oort Cloud? Our simulations don't suggest so, but observations always trump theory. The orbits for Sedna and 2012 VP113 more closely resemble those predicted for the Oort Cloud than those observed in the Kuiper Belt, so perhaps they are the brightest two objects in some sort of transitional cloud.
Where do things go from here? Two objects may be the start of a trend, but we'll need many more to begin to draw firm conclusions about this new, elusive region of space. In the meantime, the theorist of the world have plenty of work to do!