The discovery of a second resident in a region of the Solar System called the inner Oort cloud prompts fresh thinking about this no-man's-land between the giant planets and the reservoir of comets of long orbital period. See Letter p.471
A decade after its discovery, Sedna1 still remains one of the strangest objects in the Solar System. This remote icy body is on a highly eccentric orbit that extends to about 1,000 astronomical units (AU; 1 AU is the mean Earth–Sun distance) and has a perihelion (point of closest approach to the Sun) of 76 AU. Its orbit is well beyond the reach of Neptune, which is located at 30 AU, and is a long way from the edge of the Solar System where the Oort cloud, the reservoir of long-orbital-period comets, resides at about 10,000 AU. Although other potential candidates have been found2,3,4, Sedna had remained the solitary confirmed member of a proposed inner Oort cloud beyond 70 AU. On page 471 of this issue, Trujillo and Sheppard report5 the discovery of an object, called 2012 VP113, which joins Sedna as the second confirmed member of the inner Oort cloud. The finding solidifies the existence of a population of icy bodies probably ranging in size from a few to a thousand kilometres.
To all intents and purposes, in the current architecture of the Solar System, Sedna and 2012 VP113 should not be there. These objects are in a no-man's-land between the giant planets and the Oort cloud where nothing in the known configuration of the modern-day Solar System could have emplaced them. Effectively frozen in place and untouched as the Solar System evolved to its present state, their orbits preserve the dynamical signature of whatever event scattered these bodies to such distances and detached them from the giant-planet region. With Sedna's discovery in 2003, several formation mechanisms were put forth, each predicting different orbital distributions of inner Oort cloud objects. But with only a single object, it was impossible to unambiguously distinguish between the different hypotheses.
Proposed mechanisms to explain the origin of the inner Oort cloud include the scattering of planetesimals (the building blocks of planet formation) by a distant planet beyond Neptune that may have been ejected from our Solar System, or by the passage of a single star, located at between 500 and 1,000 AU, early on in the Solar System's history6,7. The preferred mechanism is the gravitational scattering of leftover planetesimals from close encounters between the nascent Sun and other members of its birth star cluster, which would have contained between 10 and 1,000 stars6,8,9,10. Most stars are born in stellar nurseries that dissolve within a few million years. At the present age of the Solar System, the Sun and other stars of its birth cluster would be spread across the Galaxy. There is no direct way to identify whether the Sun was ever in a stellar nursery or to find its siblings. But indirectly, the mark from those early stellar encounters, like fingerprints at a crime scene, could provide evidence for the sculpted distribution of orbits in the inner Oort cloud.
Trujillo and Sheppard's detection of 2012 VP113, which was made using the Dark Energy Camera (DECam) at the Cerro Tololo Inter-American Observatory 4-metre telescope in Chile, brings us one step closer to reading the dynamical record of the inner Oort cloud. With its farthest distance from the Sun at 452 AU and coming no closer into the Solar System than 80 AU, 2012 VP113 is placed well within the expected inner Oort cloud outside the Kuiper belt, a disk-shaped region of small icy bodies that lies beyond Neptune's orbit (Fig. 1).
It is still true that, with two objects, we cannot unambiguously identify the origin of the inner Oort cloud, but Trujillo and Sheppard find that the orbits of 2012 VP113 and Sedna are consistent with models for the Sun's birth cluster and constraints from previous ground-based, wide-field surveys11. This suggests that Sedna and 2012 VP113 are the tip of the iceberg for this population of distant inner Oort cloud objects. Most objects in the inner Oort cloud would reside at distances farther than Sedna and 2012 VP113. For only an extremely small fraction of their orbits around the Sun would these inner Oort objects be bright enough to be detected in current ground-based surveys.
But there is more to the authors' study than described so far. Trujillo and Sheppard noticed a peculiarity with the orbits of Sedna and 2012 VP113. The two objects have similar values for one of their orbital parameters: the angle between the point of perihelion and where the orbit crosses the plane of the Solar System. Interestingly, the most distant Kuiper-belt objects, with orbital semimajor axes greater than 150 AU and perihelia beyond Neptune, also seem to have values for such angles comparable to those of Sedna and 2012 VP113. Such clustering of orbital angles seems to be unexplainable by the gravitational influence of Neptune alone. This result may be the first hint we have of an identifiable signature of the inner Oort cloud's formation mechanism on the orbits of closer-in Solar System bodies. If true, any formation mechanism proposed for the origin of Sedna and 2012 VP113 will need to explain this orbital structure. More detections of objects both in the Kuiper belt and the inner Oort cloud will be needed to confirm this result.
Our knowledge of the inner Oort cloud is, in many ways, in the same state as the study of the Kuiper belt was in the 1990s, when the first Kuiper-belt objects were discovered — 62 years after Pluto's detection. 2012 VP113 is the smoking gun for the existence of the inner Oort cloud. We now know that Sedna is not alone. The prospects of future detections using next-generation instruments and facilities, such as the Hyper Suprime-Cam on the 8.2-m Subaru telescope in Hawaii and the 8.4-m Large Synoptic Survey Telescope in Chile, are promising. 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.
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Monthly Notices of the Royal Astronomical Society (2016)
Space Science Reviews (2015)
Monthly Notices of the Royal Astronomical Society (2015)