Hills Cloud - Oort Cloud Objects (OCOs)

Oort Cloud Objects (OCOs)

Apart from long-period comets, only four known objects have orbits which suggest that they may belong to the Oort cloud: 90377 Sedna, 2000 CR₁₀₅, 2006 SQ₃₇₂ and 2008 KV₄₂. The first two, unlike scattered disc objects, have perihelia outside the gravitational reach of Neptune, and thus their orbits cannot be explained by perturbations from the gas giant planets. If they formed in their current locations, their orbits must originally have been circular; otherwise accretion (the coalescence of smaller bodies into larger ones) would not have been possible because the large relative velocities between planetesimals would have been too disruptive. Their present-day elliptical orbits can be explained by a number of hypotheses:

1. These objects could have had their orbits and perihelion distances "lifted" by the passage of a nearby star when the Sun was still embedded in its birth star cluster.
2. Their orbits could have been disrupted by an as-yet-unknown planet-sized body within the Oort cloud.
3. They could have been scattered by Neptune during a period of particularly high eccentricity or by the gravity of a far larger primordial trans-Neptunian disc.
4. They could have been captured from around smaller passing stars.

Of these, the stellar disruption and "lift" hypothesis appears to agree most closely with observations. Some astronomers prefer to refer to Sedna and 2000 CR₁₀₅ as belonging to the "extended scattered disc" rather than to the inner Oort cloud.