Sunday, September 8, 2013

Drifter in the Dark

The Inner Oort Cloud (IOC) is a region of space that lies beyond the Kuiper Belt. Objects presently known to be part of the IOC have highly elongated orbits that never come close enough to the Sun for their orbits to be dynamically influenced by gravitational interactions with Neptune. These objects have semi-major axis  ≥ 200 AU (average distance from Sun) and perihelion ≥ 44 AU (minimum distance from Sun). Due to their great distance and faintness, only a few IOC objects are presently known - 2000 CR105 (Gladman et al. 2002) with perihelion 44.1 AU and semi-major axis 228.8 AU; Sedna (Brown et al. 2004) with perihelion 76.2 AU and semi-major axis 542.7 AU; and 2004 VN112 (Becker et al. 2008) with perihelion 47.3 AU and semi-major axis 343.3 AU.

These four panels illustrate the orbit of Sedna - an IOC object which lies in the farthest reaches of the Solar System. Each panel, moving clockwise from the upper left, successively zooms out to place Sedna in context with the Solar System.

One or more mechanisms triggered by external perturbations must have produced the large perihelion distances and highly elongated orbits of the IOC objects. Proposed scenarios include interactions with a massive planet-sized body in the outer Oort cloud; the passage of a solar-mass star within a few 100 AUs from the Sun; stellar encounters that took place while the Sun was still a member of its natal star cluster; and the capture of extrasolar planetesimals from low-mass stars during the early evolution of the solar system.

Chen et al. (2013) report the discovery of 2010 GB174, a likely new member of the IOC. This object was discovered as part of the Next Generation Virgo Cluster Survey (NGVS). 2010 GB174 is estimated to have an orbit with perihelion ~ 48 AU and semi-major axis ~350 AU. At the far end of its highly elongated orbit, 2010 GB174 swings out as far as ~655 AU from the Sun. Based on certain assumptions, 2010 GB174 is estimated to be ~300 km in size. Sedna, the largest known IOC object, has an estimated diameter of ~1000 km.

The very fact that 2010 GB174 was detected at all points towards a large IOC population far beyond the Kuiper belt. In fact, based solely on the discovery of Sedna alone, it has been estimated with reasonable certainty that 15 to 90 Sedna-sized or bigger objects on Sedna-like orbits reside in the IOC (Schwamb et al. 2009). For comparison, the total number of objects Sedna-sized or larger in the Kuiper belt is ∼5 to 8, indicating that there may be ~10 times more mass residing in the IOC than in the Kuiper belt.

- Chen et al. (2013), “Discovery of a New Member of the Inner Oort Cloud from The Next Generation Virgo Cluster Survey”, arXiv:1308.6041 [astro-ph.EP]
- Gladman et al. (2002), “Evidence for an Extended Scattered Disk”, Icarus 157 pp. 269-279
- Brown et al. (2004), “Discovery of a candidate inner Oort cloud planetoid”, arXiv:astro-ph/0404456
- Becker et al. (2008), “Exploring the Outer Solar System with the ESSENCE Supernova Survey”, ApJ 682 L53
- Schwamb et al. (2009), “A Search for Distant Solar System Bodies in the Region of Sedna”, arXiv:0901.4173 [astro-ph.EP]