90377 Sedna is a trans-Neptunian object, discovered by Mike Brown (Caltech), Chad Trujillo (Gemini Observatory) and David L. 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 two-thirds the size of Pluto.

General information

Sedna is located in the center of the green circle.

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-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 28th 2004, the International Astronomical Union has officially accepted the name "Sedna".

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 solar system 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 kilometres (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).

Observations from Chile show that Sedna is one of the reddest objects in the solar system, nearly as red as Mars.

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 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. Another, less probable, scenario they proposed that managed to explain Sedna's orbit very well was that Sedna could have formed around a brown dwarf about 20 times less massive than the Sun and was captured by our Solar System when the brown dwarf passed through it.

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.

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.

Writing in Nature, astronomers Scott J. Kenyon and Benjamin C. Bromley consider Sedna's origin and show that a passing star probably scattered Sedna from the Sun's Kuiper belt into its observed orbit. They estimate the likelihood that Sedna was captured from the outer disk of the passing star at about 10 percent (Nature 432, 598–602 (02 December 2004); doi:10.1038/nature03136).

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.

Artist's concept of Sedna. 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

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 (http://www.gps.caltech.edu/~mbrown/papers/ps/sedna.pdf) (Brown et.al., submitted to ApJ Letters, March 16, 2004)
2. Hurt, Robert. Artist's Concept of Sedna (http://www.spitzer.caltech.edu/Media/releases/ssc2004-05/ssc2004-05b.shtml). California, USA: Infrared Processing and Analysis Center. March 15, 2004.
3. Hurt, Robert. Sedna's Orbit Comparisons (http://www.spitzer.caltech.edu/Media/releases/ssc2004-05/ssc2004-05d.shtml). California, USA: Infrared Processing and Analysis Center. March 15, 2004.
4. Hurt, Robert. Sedna's Size Comparisons (http://www.spitzer.caltech.edu/Media/releases/ssc2004-05/ssc2004-05c.shtml). California, USA: Infrared Processing and Analysis Center. March 15, 2004.
5. JPL. Most Distant Object in Solar System Discovered (http://www.spitzer.caltech.edu/Media/releases/ssc2004-05/release.shtml). Press release: Jet Propulsion Laboratory. March 15, 2004.
6. Whitehourse, David. Sedna has no moon, say astronomers (http://news.bbc.co.uk/1/hi/sci/tech/3625233.stm). BBC News. April 14, 2004.

See also

• asteroid
• Kuiper belt
• Oort cloud
• planet
• definition of 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.
• 50000 Quaoar – a planetoid beyond Pluto's orbit, discovered in 2002 AD.
• Trans-Neptunian object – an object beyond Neptune's orbit

External links

• NASA's Sedna page (http://www.nasa.gov/vision/universe/solarsystem/planet_like_body.html)
• Mike Brown's Sedna page (http://www.gps.caltech.edu/~mbrown/sedna/)
• Official press release by NASA JPL/SCC (http://www.spitzer.caltech.edu/Media/releases/ssc2004-05/release.shtml)
• Astronomy Picture of the Day 04 July 2004 – Artist's rendering of view from Sedna (http://antwrp.gsfc.nasa.gov/apod/ap040604.html)
• Scenarios for the Origin of the Orbits of the Trans-Neptunian Objects 2000 CR105 and 2003 VB12 (Sedna) (http://www.boulder.swri.edu/~hal/CR105.html) by Alessandro Morbidelli and Harold F. Levison
• MPEC 2004-S73: Comment on the naming of Sedna (http://cfa-www.harvard.edu/mpec/K04/K04S73.html)