90377 Sedna

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90377 Sedna
Sedna is located in the center of the green circle
Sedna is located in the centre of the green circle
Discovery A
Discoverer M. Brown,
C. Trujillo,
D. Rabinowitz
Discovery date November 14, 2003
designations B
2003 VB12
Category Trans-Neptunian object
Orbital elements C
Epoch September 26, 1990 ( JD 2448160.5)
Eccentricity (e) 0.855
Semi-major axis (a) 78629.540 G m (525.606 AU)
Perihelion (q) 11392.832 Gm (76.156 AU)
Aphelion (Q) 145866.248 Gm (975.056 AU)
Orbital period (P) 4401380 d (12050.32 a)
Mean orbital speed 1.04 km/ s
Inclination (i) 11.934 °
Longitude of the
ascending node (Ω)
Argument of
perihelion (ω)
Mean anomaly (M) 357.455°
Physical characteristics D
Dimensions 1180–1800 km
Mass 1.7-6.1×1021 kg
Density 2.0? g/ cm³
Surface gravity 0.33-0.50 m/s²
Escape velocity 0.62-0.95 km/s
Rotation period 0.42 d (10 h) 1
Spectral class (red) B-V=1.24; V-R=0.78
Absolute magnitude 1.6
Albedo ( geometric) >0.2?
Mean surface
below 33 K

90377 Sedna is a trans-Neptunian object, discovered by Michael Brown ( Caltech), Chad Trujillo ( Gemini Observatory) and David Rabinowitz ( Yale University) on November 14, 2003. At the time of its discovery it was the most distant observed natural solar system body. Sedna may qualify as a dwarf planet pending the detailed definition of that category by the International Astronomical Union.

General information

Sedna was discovered during a survey conducted with the Samuel Oschin telescope at Palomar Observatory near San Diego, California (USA) using Yale's 160 megapixel Palomar Quest camera and was observed within days on telescopes from Chile, Spain, and the USA ( Arizona, and Hawaii). NASA's orbiting Spitzer Space Telescope was also pointed toward the object, but could not detect it — putting an upper-bound on its diameter at roughly three-quarters that of Pluto.

The object is named after Sedna, the Inuit goddess of the sea, who was believed to live in the cold depths of the Arctic Ocean. Before Sedna was officially named it had provisional designation 2003 VB12.

Orbital characteristics

Panels showing the location of Sedna in relation to other astronomical objects. Image courtesy of NASA / JPL-Caltech / R. Hurt
Panels showing the location of Sedna in relation to other astronomical objects. Image courtesy of NASA / JPL-Caltech / R. Hurt

Sedna has a highly elliptical orbit, with its aphelion estimated at 975  AU and its perihelion at about 76.16 AU. At its discovery it was approaching perihelion at about 90 AU from the Sun. It was the farthest from the Sun that any solar system object had up to then been observed, although some objects like long-period comets originally observed at closer distances were most likely further from the Sun than Sedna but too dim to be observed. Eris was later detected at 97 AU.

Sedna's orbit takes about 12000 years. It will reach perihelion in 2075 or 2076.

When first discovered, Sedna was believed to have an unusually long rotational period (20 to 50 days). A search was thus made for a natural satellite, the most likely cause for such a long rotation, but investigation by the Hubble Space Telescope in March 2004 observed no such object orbiting the planetoid. New measurements from the MMT telescope suggest a much shorter rotation period, only about 10 hours, rather typical for bodies of its size.

A study done by Hal Levison and Alessandro Morbidelli of the Observatoire de la Côte d'Azur (OCA) in Nice, France, suggested that the most likely explanation for Sedna's orbit was that it had been perturbed by a close (~800 AU) pass by another star in the first 100 million years or so of the solar system's existence, possibly one of the other stars that formed out of the same collapsing nebula as the Sun. They proposed another, less probable scenario that managed to explain Sedna's orbit very well —Sedna could have formed around a brown dwarf about 20 times less massive than the Sun and have been captured by the solar system when the brown dwarf passed through it.

Another possible explanation, advanced by Gomez, involves a hypothetical distant ‘planet’ (a planetary-sized companion in the inner Oort cloud). Recent simulation show that Sedna's orbit characteristics could be explained by perturbations by a Neptune-mass object at 2000 AU (or less), a Jupiter-mass at 5000AU or even an Earth-mass object at 1000AU.

Another object, 2000 CR105, has an orbit similar to Sedna's but a bit less extreme: perihelion is 44.3 AU, aphelion is 394 AU, and the orbital period is 3240 years. Its orbit may have resulted from the same processes that produced Sedna's orbit.

Physical characteristics

Sedna compared to Eris, Pluto, 2005 FY9, 2003 EL61, Orcus, Quaoar, Varuna, and Earth.
Sedna compared to Eris, Pluto, 2005 FY9, 2003 EL61, Orcus, Quaoar, Varuna, and Earth.

Sedna has an estimated diameter of between 1180 and 1800 kilometres (730 to 1120 miles). At the time of its discovery it was the largest object found in the solar system since Pluto was discovered in 1930. It is now generally believed to be the 5th largest known trans-Neptunian object after Eris, Pluto, 2003 EL61, and 2005 FY9. The planetoid is so far from the Sun that the temperature never rises above 33 kelvins (−240 °C; −400 °F).

Observations from Chile show that Sedna is one of the reddest objects in the solar system, nearly as red as Mars. Unlike Pluto and Charon, Sedna appears to have very little methane ice or water ice on its surface; Chad Trujillo and his colleagues at the Gemini Observatory in Hawaii suggest that Sedna's dark red colour is caused by a hydrocarbon sludge, or tholin, like that found on 5145 Pholus. Its surface is homogeneous in colour and spectrum; this is probably because Sedna, unlike objects nearer the sun, is rarely impacted by other bodies, which would expose bright patches like that on 8405 Asbolus.

Sedna's and Triton's spectra have been recently compared suggesting the following common model of the surface: 24% Triton tholin, 7% amorphous carbon, 26% methanol ice with 33% methane.


The discoverers have argued that Sedna is actually the first observed body belonging to the Oort cloud, saying that it is too far out to be considered a Kuiper belt object. Because it is a great deal closer to the Sun than was expected for an Oort cloud object, and has an inclination roughly in line with the planets and the Kuiper belt, they described the planetoid as being an inner Oort cloud object, situated in the disc reaching from the Kuiper belt to the spherical part of the cloud.

A number of explanations have been put forward since, including a passing star and a distant, planet-sized object.

Sedna, together with a few other objects discovered since (e.g. 2000 CR105), prompted suggestions of a new category of distant objects named Extended scattered disc (E-SDO), detached objects, Distant Detached Objects (DDO) or Scattered-Extended in the formal classification by Deep Ecliptic Survey.

The last classification, introduces a formal distinction between Scattered-Near objects (which could be scattered by Neptune) e.g. Eris from Scattered-Extended objects like Sedna. The distinction is made formally, using the orbital elements (see Tisserand's parameter).

The discovery of Sedna had also resurrected the question of which astronomical objects should be considered planets and which should not. On March 15, 2004, articles in the popular press reported that "the tenth planet has been discovered". This question was answered under the new International Astronomical Union definition of a planet, adopted on August 24, 2006. Sedna may be made a candidate for consideration as a dwarf planet. It is not, however, considered to be a planet.

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