A mysterious dwarf planet named Sedna, orbiting the sun far beyond Neptune, has been a loner since its discovery 11 years ago.
Astronomers suspected that the solar system might host more like it. And if there were more, they reasoned, what stories they might tell of the early solar system and of the cosmic nursery that the sun shared with its stellar siblings.
Now, Sedna is a loner no longer.
Astronomers say they have discovered a sister object, in a similarly unusual orbit spanning comparable distances to those of Sedna. The find takes Sedna firmly out of the oddball category. And it raises the stakes for astronomers trying to explain how the two objects came to occupy such similar, unusual orbits.
Possible explanations range from the presence of a planet up to 10 times the size of Earth even farther out, which acts as a shepherd for a large number of Sedna-like objects, to close encounters between the sun and one or more siblings early on, born from the same vast nebula of primordial dust and gas.
If astronomers could find several dozen of these Sedna-like dwarf planets, "the payoff would be pretty large," says Scott Kenyon, who studies the formation of planetary systems at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
"You would really know a lot about the early history of the solar system, and it's not just the architecture of the solar system," says Dr. Kenyon, who was not involved in the discovery. "This is a big deal, from the point of view of understanding where we come from."
Dubbed 2012 VP113, the object is estimated to be 280 miles wide. It has a strikingly elliptical orbit that brings it to within 80 astronomical units of the sun, or 80 times the distance between Earth and the sun. From there it travels out to a distance of 226 AU. That translates into a 4,000-year round-trip with each orbit.
Even at its closest approach, 2012 VP113 falls far short of reaching the outer edge of the Kuiper Belt, a collection of small primitive bodies that includes the dwarf planet Pluto. The Kuiper Belt extends from just beyond Neptune to about 50 AU from the sun.
Astronomers are assigning 2012 VP113 to the inner Oort Cloud. The cloud, as a whole, is yet another extended population of primitive objects that are one of the solar system's reservoirs for comets. The outer part of the cloud, which extends out to nearly a light-year from the sun, is vulnerable to disturbance from passing stars, whose gravity can dislodge icy objects in otherwise stable orbits and send them hurtling toward the inner solar system.
The inner Oort Cloud, whose existence Sedna and VP12 VP113 confirm, is too close to the sun to be disturbed by passing stars. And it's too far from the giant outer planets to get disrupted by their gravitational interactions.
Hence the mystery: What caused Sedna's and now 2012 VP113's orbits so become so stretched, when by all rights they should be virtually circular?
The two objects "could not have formed on these eccentric orbits because to accumulate mass, you have to pretty much be on a circular orbit," says Scott Sheppard, an astronomer with the Carnegie Institution for Science in Washington. Otherwise, the likelihood of growth-stunting collisions with other objects is too high.
The distant, elongated orbits also make these objects extremely difficult to find, says Dr. Sheppard, who along with Chadwick Trujillo, an astronomer at the Gemini Observatory in Hawaii, report their discovery in Thursday's issue of the journal Nature.
These objects can only be detected at their closest approach to the sun. Even then, they are remarkably dim. They are visible only for about 20 years before fading as they make their way back out to aphelion, or the point at which an orbit is farthest from the sun – where they spend most of their 4,000-plus years (Sedna's orbit carries it from 76 AU to 937 AU and back, giving it an orbital period of 11,400 years).
"That suggests that there's a huge population of these objects out there that we just can't see," Sheppard says.
The duo made its discovery using a 4-meter telescope at the Cerro Tololo Inter-American Observatory in Chile's Atacama Desert. Once they had identified 2012 VP113's apparent motion against background stars, they followed up with observations using the Carnegie Institution's 6.5-meter Magellan telescope at the Las Campanas Observatory, also in Chile.
The larger telescope was used to measure the object's color and record additional positions to determine its orbit. Slightly reddish in hue, the object is mostly ice and likely formed in the region of the gas giants, rather than farther out in the Kuiper Belt, Dr. Trujillo and Sheppard posit.
In the years since Sedna's discovery, researchers have developed three possible explanations for the bizarre orbits of these objects.
One invokes the influence of passing stars early in the solar system's history, which disrupted the orbits of Sedna and 2012 VP113 sufficiently to send them into new orbits.
Another explanation posits that a star passed close enough to the sun early on to exchange either planets or Kuiper Belt-like objects, which then assumed the elongate orbits.
Finally, some hold that a rogue planet Earth's size or larger got ejected from the main region of planet formation, and either disrupted Sedna's and 2012 VP113's orbits on its way out, or settled into a distant orbit around the sun, allowing it to shepherd these and other dwarf planets thought to be orbiting along a Sedna-like trajectory.
Each hypothesis makes its own prediction about the number of objects and the nature of their orbits – predictions that must await additional discoveries for rigorous tests.
The passing-star scenario may hold the most promise for probing the sun's likely environment during the first 10 million years of its existence. A good estimate of the number of Sedna-like objects would yield an estimate of the number of interactions with other stars.
Most of the planet-forming action takes place within 100 to 200 AU of a star, Kenyon explains. If the sun was born in a molecular cloud that yielded only three stars, the odds of any of them coming within even 300 AU are vanishingly small.
If the sun formed in a cluster like the one in the the nebula in Orion's sword, which has 10,000 stars, or the double cluster in Perseus, with 100,000 stars, the odds of encounters skyrocket. So do the odds of the cluster hosting one or more hot, massive stars, which emit copious amounts of ionizing radiation. This radiation could ionize the nebula that would provide the material for planets.
Understanding Sedna-like objects allows researchers to estimate the probability of an interaction and how close any interactions were. These, in turn, can lead to estimates of the number of stars sharing the sun's nursery.
So far, Trujillo and Sheppard have scanned just over a thousandth of the night sky. But they have additional candidates in the analysis pipeline, Sheppard says, and the search for new ones continues.
If astronomers can find even 10 of these objects, he adds, that should be a big help in winnowing the field of competing hypotheses.