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Sedna is the unofficial name given to 2003 VB12, a Trans-Neptunian object discovered by Mike
Brown (Caltech), Chad Trujillo (Gemini Observatory) and David Rabinowitz
(Yale University) on November 14, 2003. Its discovery was the farthest distance at which any
natural object in the solar system has ever been observed. Sedna is
described as a cold planetoid, perhaps as large as 2/3 the size of Pluto.
General information
Sedna was discovered during a survey conducted with the Samuel Oschin Telescope at Palomar Observatory near San
Diego, California (USA) 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-fourths that of Pluto.
Because of its cold, distant nature, and because all other extraterrestrial planets are named after (Roman) 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. By the International Astronomical Union naming conventions for asteroids, the temporary designation is 2003 VB12 until its orbit can be
established well enough; only then do the discoverers have the right to suggest a name. As the discoverers have already announced
that they will suggest the name Sedna, it is commonly called by that name now. It could potentially change later; for instance,
William Herschel originally named Uranus Georgium Sidus ("King George's Star"), but this was
deemed inconsistent with astronomical convention. For much the same reason, the humorous suggestion that the new planetoid should
be named "Mickey" (in reference to Mickey Mouse's dog Pluto) also seems unlikely to gain much support.
Characteristics
Sedna has a highly elliptical orbit, with
its aphelion estimated at 942 AU and its perihelion at about 76.1 AU. At its
discovery, it was about 90 AU from the Sun, approaching perihelion. This is the
furthest from the sun that any object has been observed, although some objects like long-period comets originally observed at
closer distances may currently be further from the sun than Sedna but are too dim to be observable. For comparison, the average
distance of Pluto from the Sun is about 40 AU. Sedna's orbit takes
about 11,487 years. It will reach perihelion in 2075 or 2076. It
was believed that the long rotational period of Sedna (between 20-50 days) was caused by a natural satellite, however, investigation by the Hubble Space Telescope in March 2004 observed no such object orbiting the planetoid, and the precise
cause remains a mystery.
Sedna has an estimated diameter of between 1180 and 1800 kilometers (730 to 1470
miles) and may be the largest object found in the solar system since Pluto
was discovered in 1930. The planetoid is so far from the Sun that the temperature never
rises above -240°C (-400°F).
According to Brown, the Sun appears so small from Sedna that it is smaller than the head of a pin held an arm-length away.
Observations from Chile show that Sedna is one of the reddest objects in the solar system, nearly as red as Mars.
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. Instead, these astronomers suggest
that previous assumptions about the outer edge of the Kuiper Belt should be revised outward.
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 on planethood is comparable to that of Pluto. Sedna's discoverers (like others) call
it a planetoid, but because the two objects are similar in location, size and composition, a permanent decision to classify Sedna
as a planetoid could re-open questions about whether Pluto should also bear that classification.
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Artist's concept of Sedna. Image courtesy of NASA / JPL-Caltech / R. Hurt
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Panels showing the location of Sedna in relation to other astronomical objects. Image courtesy of NASA / JPL-Caltech / R.
Hurt
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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,
March 16, 2004)
- Hurt, Robert. Artist's Concept of Sedna . California, USA:
Infrared Processing and Analysis Center. March 15, 2004.
- Hurt, Robert. Sedna's Orbit Comparisons . California, USA:
Infrared Processing and Analysis Center. March 15, 2004.
- Hurt, Robert. Sedna's Size Comparisons . California, USA:
Infrared Processing and Analysis Center. March 15, 2004.
- JPL. Most Distant Object in Solar System Discovered
. Press release:
Jet Propulsion Laboratory. March 15, 2004.
- Whitehourse, David. Sedna has no moon, say astronomers . BBC News. April 14, 2004.
See also
External links
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