Year in Space Calendar

Space Topics: Trans-Neptunian Objects

90377 Sedna

Not a Kuiper Belt Object
Formerly Known As 2003 VB12

Credit: Michael Brown

Sedna, formerly known as 2003 VB12, is a member of a whole new class of solar system objects. It’s not a comet, nor is it really a member of the Kuiper belt. It’s been called an “inner Oort cloud” object, but the reality is that we don’t know much about the part of the solar system where Sedna spends most of its 12,000-year orbital period.

Basic Facts

Diameter: unknown, probably 1,600-2,200 kilometers (about 12-17% of Earth)
Mass: unknown
Bulk density: unknown, probably about 2 grams per cubic centimeter (roughly half-rock, half-ice)
Average orbital radius: 525.6 astronomical units (semi-major axis)
Orbital eccentricity: 0.855
Orbital inclination: 11.9 degrees
Orbital period: 12,050 years
Rotational period: 10 hours

The solar system is divided into regions depending on distance to the Sun as measured in astronomical units (AU), the average Sun-Earth distance; one astronomical unit is equal to 150 million kilometers (93 million miles). Starting from the inside, the terrestrial planet region is the home of Mercury, Venus, Earth, and Mars stretching to nearly two astronomical units. After the asteroid belt (two to four astronomical units), the outer planets dominate the scene all the way to the orbit of Neptune (five to 30 astronomical units). Then comes the transneptunian region with the Kuiper belt (35 to 47 astronomical units) and the scattered disk (35 to more than 100 astronomical units). Sedna orbits far beyond that, but still nearer to the Sun than the Oort cloud.

The view from Sedna
The view from Sedna
The view from Sedna
An observer on Sedna would gaze upon the entire solar system in one tiny patch of the sky, with the Sun at the center a point of light barely brighter than the other stars. Credit: NASA, ESA, and A. Schaller

In this distant region, it is extremely difficult, if not impossible, to find faint solar system objects. Brightness falls as the square of the distance: the same object viewed at twice the distance from Earth will be four times fainter. The Sun’s light is about 900 times fainter at Neptune than on Earth. Sunlight reflected by distant Kuiper belt objects must then travel all the way back to Earth for astronomers to see it, and the brightness again falls as the square of the distance. So for distant objects with no internally generated light, apparent brightness falls as the distance to the fourth power. An object 100 astronomical units away is roughly 100 million times fainter than an object at only two astronomical units away. This is why we can easily find sub-kilometer-sized objects near Earth, but we have to strain our telescopic eyes to see 100-kilometer-sized objects in the Kuiper belt.

Unlike virtually every other known trans-Neptunian object, Sedna never even comes close to Neptune. Its perihelion (closest approach to the Sun) is at a record-breaking 76 astronomical units; and it was discovered near perihelion, so it spends most of its time much farther away.  Sedna’s orbit barely knows that Neptune is even there. The Kuiper belt and scattered disk have been sculpted by gravitational interactions with Neptune, but the origin of Sedna’s orbit is a mystery. One common hypothesis is that Sedna was emplaced on such an unusual orbit by a very close passage of a star. This would imply that perhaps the solar system formed in a stellar cluster with many very close neighbors that later moved away. (We see stars forming in such clusters today and the clusters do eventually dissolve.) Perhaps there was another Neptune-size planet that got kicked out of the solar system, dragging Sedna to its distant realm.

Sedna's orbit
Sedna's orbit
Sedna is a trans-Neptunian object with a very unusual elliptical orbit. Although presently in a position close to (but not in) the Kuiper belt (see red dot in upper right panel, compared to the field of light blue dots), its 12,000-year path takes it far beyond the Kuiper belt (lower right panel). Still, this long orbit is tiny compared to the orbits theorized for comets in the much more distant Oort cloud (lower left panel). Credit: Michael Brown, Chad Trujillo, and David Rabinowitz

For most of its orbit Sedna would be simply too faint to see. Sedna's discovery was enabled by its current position near its orbital perihelion.  However, scientists don't think that this was a lucky catch of an extremely unusual object.  It is much more likely that Sedna is just one of many Sedna-like objects out there waiting to be discovered. Most of them are too faint to find in the near future, but eventually we should be able to discover more objects with Sedna-like orbits that will teach us more about the inner Oort cloud.

Sedna’s surface is very red and shows no clear signs of any surface ices.  Initially, scientists thought that Sedna had a slow rotation period, taking weeks to turn about on its axis. This is unusually long and may have indicated that a moon has slowed Sedna’s rotation, just as Charon slowed Pluto to a 6-day rotation period. However, more observations proved that the rotation period was a more typical 10 days and all searches for a moon have failed. This is unfortunate, as a moon would be useful in determining the mass of Sedna.


Brown, M. E., Trujillo, C. A. & Rabinowitz, D. L., 2004. "Discovery of a Candidate Inner Oort Cloud Planetoid." The Astrophysical Journal, 617, Issue 1, pp. 645-649

Gaudi, B. Scott; Krzysztof Z. Stanek, Joel D. Hartman, Matthew J. Holman, Brian A. McLeod, 2005. "On the Rotation Period of (90377) Sedna". The Astrophysical Journal 629: L49-L52. 

Alessandro Morbidelli and Harold F. Levison, 2004. "Scenarios for the Origin of the Orbits of the Trans-Neptunian Objects 2000 CR105 and 2003 VB12 (Sedna)" The Astronomical Journal, 128, pp 2564-2576.