90377 Sedna

Encyclopedia : 9 : 90 : 903 : 90377 Sedna

**90377 Sedna**
Orbital elements
Eccentricity (orbit)>e
Inclination>i	°
Perihelion>q	AU
Argument of perihelion>ω	°
Semi-major axis>a	AU
Longitude of the ascending node>ω	°
Aphelion>Q	AU
Mean anomaly>M	°
Orbital period>P	years
Mean motion>n	°/day
Time of perihelion passage>TP

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, though this has now been exceeded by 2003 UB₃₁₃. Sedna is described as a cold planetoid, perhaps as large as two-thirds the size of Pluto.

Contents

1 General information
2 Orbital characteristics
3 Physical characteristics
4 Classification
5 References
6 See also
7 External links

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.

Because of its cold, distant nature, and because all other planets of the Solar system are named after (Roman and Greek) gods, the scientists who discovered it unofficially named it 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. As of September 28, 2004, the International Astronomical Union has officially accepted the name "Sedna".

Orbital characteristics

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. 2003 UB₃₁₃ was later detected at 97 AU.

Sedna's orbit takes about 12050 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.

Sedna is located in the center of the green circle.

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 (~800AU) 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. Alessandro Morbidelli and Harold F. Levison Scenarios for the Origin of the Orbits of the Trans-Neptunian Objects 2000 CR105 and 2003 VB12 (Sedna) The Astronomical Journal, (2004) 128, pp 2564-2576. [Preprint] 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 shows that Sedna's orbit characteristics could be explained by perturbations by a Neptune-mass object at 2000AU (or less), a Jupiter-mass at 5000AU or even a Earth-mass object at 1000AU. Rodney S. Gomes, John J. Matese, and Jack J. Lissauer A Distant Planetary-Mass Solar Companion May Have Produced Distant Detached Objects To appear in Icarus (2006). [Preprint]

Another object, 2000 CR₁₀₅, has an orbit similar to Sedna's but a bit less extreme: perihelion is 45 AU, aphelion is 415 AU, and the orbital period is 3420 years. Its orbit may have resulted from the same processes that produced Sedna's orbit.

Physical characteristics

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 4th largest known trans-Neptunian object after 2003 UB₃₁₃, Pluto, and 2005 FY₉. The planetoid is so far from the Sun that the temperature never rises above 23 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 color is caused by a hydrocarbon sludge, or tholin, like that found on 5145 Pholus. Its surface is homogenous 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. - 404 error as of access date

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.

Classification

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 describe 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.

However, others believe that its inclination and size qualify it as a Kuiper Belt object. These astronomers suggest that previous assumptions about the outer edge of the Kuiper Belt should be revised outward.

Writing in Nature, astronomers Scott J. Kenyon and Benjamin C. Bromley consider Sedna's origin and argue that a passing star probably scattered Sedna from the Sun's Kuiper belt into its observed orbit. They estimate the probability that Sedna was captured from the outer disk of the passing star at about 10%.

The discovery of Sedna has 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", and some believe its claim to planethood is comparable to that of Pluto. Sedna's discoverers (like others) call it a planetoid, but because the two objects are similar in size and composition, a permanent decision to classify Sedna as a planetoid could re-open questions about whether Pluto should also bear that classification. Isaac Asimov suggested the term mesoplanet be used for planetary objects intermediate in size between Mercury and 1 Ceres, which would include both Pluto and Sedna.

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

Artist's rendition showing Sedna's size in relation to other objects. Image courtesy of NASA / JPL-Caltech

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References

[Discovery of a candidate inner Oort cloud planetoid] (Brown et.al., submitted to ApJ Letters, 16 March 2004)
Hurt, Robert. [Artist's Concept of Sedna]. California, USA: Infrared Processing and Analysis Center. 15 March 2004.
Hurt, Robert. [Sedna's Orbit Comparisons]. California, USA: Infrared Processing and Analysis Center. 15 March 2004.
Hurt, Robert. [Sedna's Size Comparisons]. California, USA: Infrared Processing and Analysis Center. 15 March 2004.
JPL. [Most Distant Object in Solar System Discovered]. Press release: Jet Propulsion Laboratory. 15 March 2004.
Whitehourse, David. [Sedna has no moon, say astronomers]. BBC News. 14 April 2004.

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