Most Centauri Dreams readers are hardly going to be surprised by the idea that a large number of objects exist well outside the orbit of Pluto and, indeed, outside the Kuiper Belt itself. The search for unknown planets or even a brown dwarf that might perturb cometary orbits in the Oort Cloud has occupied us for some time, with the latest analysis of WISE findings showing that nothing larger than Jupiter exists out to a distance of 26,000 AU. Objects of Saturn size or larger are ruled out within 10,000 AU, according to the work of Kevin Luhman (Penn State) and team, whose study probed deeply into the Wide-field Infrared Survey Explorer’s results. For more on all this, see WISE: New Stars and Brown Dwarfs.
But the evidence for objects big enough to perturb the local neighborhood does persist, even if we have to scale down our expectations as to its size. A new paper in Nature reports the discovery of 2012 VP113, a dwarf planet that joins Sedna in orbiting entirely beyond the Kuiper Belt’s outer edge, which is normally defined at 50 AU. The object at perihelion does not approach closer than 80 AU, making it more distant than Sedna itself. The work of Scott Sheppard and Chadwick Trujillo (Carnegie Institution for Science, Washington), the paper goes on to suggest a larger inner Oort Cloud population and the possibility of perturbed orbits there. Are the orbits of objects like 2012 VP113 and Sedna telling us something about larger bodies in this region?
The researchers used the NOAO 4-meter telescope in Chile in conjunction with the Dark Energy Camera (DECam), a high-performance, wide-field CCD imager, a combination that offers a wide field of view in the search for faint objects in large areas of sky. They also used the Magellan 6.5-meter instrument at Las Campanas Observatory to help determine the orbit of the newfound object. As with Sedna, we find that the orbit of 2012 VP113 takes it well outside the Kuiper Belt. In fact, given that their orbits extend at aphelion out to hundreds of AU, it is only the fact that both are currently near their closest approach to the Sun that has made them detectable.
Image: Artist’s rendering of the Oort cloud and the Kuiper belt. Credit: NASA.
Sedna, it appears, is not unique, and we can continue to infer from this the existence of the so-called inner Oort Cloud, extending out to about 1500 AU, where numerous objects with sizes larger than 1000 kilometers may exist. Sheppard and Trujillo, basing their estimate on the amount of sky searched, believe that 900 objects in this category may be found there, with a total inner Oort Cloud population probably larger than both the Kuiper Belt and the main asteroid belt.
The problem of distance is such that most would not be visible with current technology. However, says Sheppard, “Some of these inner Oort cloud objects could rival the size of Mars or even Earth. The search for these distant… objects beyond Sedna and 2012 VP113 should continue, as they could tell us a lot about how our solar system formed and evolved.”
2012 VP113’s orbit brings it to as close as 80 AU, outside Sedna’s perihelion. Interestingly, this finding indicates at least the possibility of a much larger planet, perhaps ten times the size of the Earth, orbiting in the inner Oort and influencing the orbits of both Sedna and the newfound object. The possibility remains that a ‘super-Earth’ or somewhat larger object at hundreds of AU, and thus well within the inner Oort, could be influencing the orbital configurations of objects like 2012 VP113 and Sedna. And based on Kevin Luhman’s WISE data studies, the existence of such a planet would not be inconsistent with what WISE is capable of telling us. If this unseen world is just several Earth masses in size, it’s going to be tricky to find, although locating more small objects being gravitationally influenced by it could eventually help us pin its orbit down.
Image: This is an orbit diagram for the outer solar system. The Sun and Terrestrial planets are at the center. The orbits of the four giant planets, Jupiter, Saturn, Uranus and Neptune, are shown by purple solid circles. The Kuiper Belt, including Pluto, is shown by the dotted light blue region just beyond the giant planets. Sedna’s orbit is shown in orange while 2012 VP113’s orbit is shown in red. Both objects are currently near their closest approach to the Sun (perihelion). They would be too faint to detect when in the outer parts of their orbits. Notice that both orbits have similar perihelion locations on the sky and both are far away from the giant planet and Kuiper Belt regions. Credit: Scott Sheppard / Carnegie Institution for Science.
Meanwhile, the inner edge of the Oort Cloud seems to be fairly well defined. From the paper:
Although our survey was sensitive to objects from 50 AU to beyond 300 AU, no objects were found with perihelion distances between 50 AU and 75 AU, where objects are brightest and easiest to detect. This was true for the original survey that found Sedna and the deeper follow-up survey… If the inner Oort cloud objects had a minimum perihelion of 50 AU and followed a size distribution like that of the large end of all known small-body reservoir distributions…, there would be only a 1% chance of finding 2012 VP113 and Sedna with perihelion greater than 75 AU and no objects with perihelion less than 75 AU. Therefore, we conclude that there are few (although probably not zero) inner Oort cloud objects in the 50-75 AU region. Some stellar encounter models that include the capture of extrasolar material predict a strong inner edge to the perihelion distribution of objects, which is consistent with our observations.
A few words about this: Sheppard and Trujillo distinguish between the inner Oort out to 1500 AU and an outer Oort Cloud, assuming that beyond 1500 AU objects are more subject to interstellar influences. One theory of inner Oort Cloud object formation is that Sedna and its ilk are captured extrasolar planetesimals lost in encounters with stars in the Sun’s birth cluster. Primordial close encounters with other stars may be implicated, but only further discovery of other inner Oort Cloud objects will provide the information needed to make this call about our system’s evolution.
The paper is Sheppard and Trujillo, “A Sedna-like body with a perihelion of 80 astronomical units,” Nature 507 (27 March, 2014), 471-474..