90377 Sedna

2003 VB12 (informally, Sedna)

Sedna is located in the center of the green circle.
Orbital characteristics
Orbit type	Trans-Neptunian object
Semimajor axis	509.1 AU
Eccentricity	0.8506
Orbital period	11,487 Years
Inclination	11.932°
Physical characteristics
Diameter	1180-1800 km
Albedo	>0.2?
History
Discoverer	M. Brown, C. Trujillo, D. Rabinowitz, 2002

Sedna (2003 VB12), is the unofficial name given to 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 furthest 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 is believed to rotate approximately once every 40 days, and this long rotation may be caused by a natural satellite.

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 could be completely blocked out with the head of a pin held an arm-length away.

Observations from Chile show that Sedna is one of the most red 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.

File:Ssc2004-05b medium.jpg Artist's concept of Sedna. Image courtesy of NASA / JPL-Caltech / R. Hurt

File:Trans-Neptunian object 2003 VB12.Sedna.orbit comparisons.jpg Panels showing the location of Sedna in relation to other astronomical objects. Image courtesy of NASA / JPL-Caltech / R. Hurt

File:56755main sedna-comp-330-264.jpg Artist's rendition showing Sedna's size in relation to other objects. Image courtesy of NASA / JPL-Caltech

References

1. Discovery of a candidate inner Oort cloud planetoid (Brown et.al., submitted to ApJ Letters, March 16, 2004)
2. New Scientist.com. Myster of far-out planetoid deepens. United Kingdom: New Scientist. March 16, 2004
3. BBC News. Astronomers discover 'new planet'. United Kingdom: British Broadcasting Corporation News. March 15, 2004.
4. CNN. Scientists: Most distant object in solar system found. Atlanta, Georgia, USA: Cable News Network. March 15, 2004.
5. Hurt, Robert. Artist's Concept of Sedna. California, USA: Infrared Processing and Analysis Center. March 15, 2004.
6. Hurt, Robert. Sedna's Orbit Comparisons. California, USA: Infrared Processing and Analysis Center. March 15, 2004.
7. Hurt, Robert. Sedna's Size Comparisons. California, USA: Infrared Processing and Analysis Center. March 15, 2004.
8. JPL. Most Distant Object in Solar System Discovered. Press release: Jet Propulsion Laboratory. March 15, 2004.
9. Sheehy, Kate. Tenth planet a new 'Star'. New York, USA: New York Post. March 15, 2004.

See also

• asteroid
• Kuiper belt
• Oort cloud
• planet
• Planet X - a hypothetical planet beyond Pluto which does not match the characteristics of Sedna
• plutino
• protoplanetary disc - Sedna's orbit appears to imply a similar origin to the planets and asteroids.
• Quaoar
• Trans-Neptunian object

External links

• NASA's Sedna page
• Mike Brown's Sedna page
• Official press release by NASA JPL/SCC