Background to Sedna
Sedna was discovered by Mike Brown and Chad Trujillo, of Caltech and Gemini Observatory, respectively. The survey uses a wide-field telescope on Palomar Mountain to hunt for bright Kuiper Belt Objects (KBOs). The orbit has semimajor axis/eccentricity/inclination = a/e/i = 532AU/0.857/11.9.

Why is Sedna Interesting?

1 It is a Detached KBO. Its perihelion (closest approach to the Sun) is at 76 AU. This means that it is effectively beyond the scattering influence of Neptune. This is unlike the Classical KBOs, and unlike the Scattered KBOs. It is similar, dynamically, to 2000 CR105 (for which a/e/i = 227AU/0.805/22.7) which has perihelion at 44 AU, also outside Neptune's reach, and which has been discussed in papers by Gladman et al (Icarus 157, 269, 2002) and Emelyanenko et al (Monthly Notices RAS, 338, 443, 2003). Other objects have larger aphelia than Sedna's 990 AU (e.g. Kuiper Belt Object 2000 OO67, with aphelion at 1010 AU) and many comets travel to larger distances. Sedna is interesting because of its perihelion distance.

2 Sedna is a 1000 km scale KBO. An object this large cannot have formed by accretion in the tenuous regions of the protoplanetary disk corresponding to its current location. Sedna must have formed elsewhere, presumably amongst the planets or in the Kuiper Belt, and been scattered out. Lastly, its perihelion was lifted out of the range of Neptune.

The orbit and the size attest to an early epoch in which strong gravitational scattering events rearranged the small bodies of the solar system.

Is Sedna an Oort Cloud Comet?
From the Classical Oort Cloud - no. The latter consists of objects whose orbits are so large (50,000 AU) that passing stars and galactic tides can alter their properties. Sedna doesn't travel very far out (1000 AU) and is effectively immune to external forces. Also, the inclinations of both Sedna and 2000 CR105 are small (12 and 23 degrees, respectively). These objects know where the plane of the solar system lies. Oort Cloud orbits are random with inclinations all the way up to 180 degrees.

So What Is It?
Sedna could be a member of a substantial population of bodies trapped between the Kuiper Belt and Oort Cloud. These would have been emplaced at early times and unseen until recently. 2000 CR105 and Sedna are "just the tip of the iceberg", as they say. The scientific interest lies in how these objects had their perihelia lifted out of the planetary region.

Recent numerical work suggests that the detached KBOs, as a class, may be explained by tiny perturbations acting over long timescales from the giant planets (i.e. not just Neptune).

Is Sedna = Planet X?
No. Planet X is a term invoked by Percival Lowell in the beginning of the 20th Century, when he thought that a planet massive enough to perturb Neptune might exist at large distances. Sedna, although big relative to most other KBOs, is too puny to measurably perturb Neptune (or anything else for that matter). Its mass is roughly one thousandth that of the Earth.

Why "Sedna"?
A shocking mythological tale of bestiality (with a dog and a ...bird), murder and filicide. Absolutely nothing to do with the Kuiper Belt, but that's OK.

Summary
Sedna = 2003 VB12 is an exciting new object whose large perihelion distance - beyond the reach of Neptune - is nearly unique amongst Kuiper Belt Objects. It has probably followed a dynamical path different from those of most KBOs and different from the Classical Oort Cloud comets. Its large size indicates that it was formed closer to the Sun and scattered outwards, probably at early times.

David Jewitt: Last updated Mar 2004

Comet	Jewitt	Kuiper