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Figure 1 - Artist's impression of Sedna - NASA/JPL-Caltech/R. Hurt (SSC-Caltech) |
A Brief Overview of Sedna
Sedna is a celestial body which is known as a ‘trans-Neptunian object’, that is; a minor planet which lies out-with the orbit of the gas giant Neptune. The largest of these objects are classified as ‘dwarf-planets’, though not enough is known about Sedna at present to give it that definition, as it can only be observed using very powerful telescopes with sophisticated imaging technology so many details about it remain a mystery. Sedna’s orbit is unusual as it follows a large ellipse, passing closest to the Sun at a distance of 76.361 Astronomical Units, where 1AU is the average distance from the Earth to the Sun. Sedna is at it’s furthest from the Sun, or it’s aphelion, at a distance of 960.78 AU. Unlike most other large trans-Neptunian objects Sedna’s orbit is so distant that it is almost unaffected by Neptune’s gravity and can appear to behave as a ‘detached object’. A year on Sedna is equivalent to 11,809 Earth years.
As Sedna is so distant from the Sun, or any other source of radiation, it is extremely cold and is estimated to have an average temperature of 12 K, or -261.15oC. At Sedna’s distance the Sun would appear to be a very small point of light in the sky, similar to how stars appear on Earth.
The Discovery of Sedna
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Figure 2 - The large red elipse shows the path of Sedna's orbit, while the cluster of coloured circles show the orbits of the major planets - NASA/Caltech |
Sedna was first discovered by astronomers Mike Brown (Caltech), Chad Trujillo (Gemini Observatory) and David Rabinowitz (Yale) using the Samuel Oschin Telescope at the Palomar Observatory in California on the 14th of November, 2003. The discovery was officially announced by the team on the 15th of March, 2004, and at the time was the coldest and most distant object known in the Solar System. The team cited Sedna as the first tangible evidence of the hypothesised Oort Cloud, a field of comets and other icy bodies thought to sit at the edge of our Solar System. The team’s initial observations were confirmed by further observations made in November/December 2003 at the Cerro Tololo Inter-American Observatory in Chile, and at the W. M. Keck Observatory in Hawaii. The team also examined previous observational data made at the Palomar Observatory and also by NASA’s Near Earth Asteroid Tracking project.
The name ‘Sedna’ was chosen in honour of the Inuit goddess of the sea, who the Inuit people believe lives at the bottom of the Arctic Ocean. She is regarded as vengeful, and is worshiped by hunters who believe they must appease her so she will provide animals for the hunt.
Observing Sedna
As mentioned above, the distances involved in observing Sedna make gathering useful information difficult. Much of what we know about Sedna has been mathematically postulated and not directly observed. This has resulted in large error margins in many of its physical characteristics. For example, Sedna’s mass is predicted to be between 1.8 and 4.3x1021 kg.
Although Sedna’s orbit has only been observed in part, it is possible to use computer models to predict its entire path through the Solar System. It is thought that Sedna will pass at its closest to Earth in 2075 and 2076 before it begins its journey outward from the Sun. Though Sedna is listed on NASA’s list of ‘solar system exploration targets’, there are no formal plans to send probes to investigate the minor planet.
Study of Sedna’s reflected light reveals that it is one of the reddest objects in the Solar System, second only to Mars. This suggests that Sedna may be covered in a layer of hydrocarbon ‘sludge’.
At present, Sedna is formally classified by the Minor Planet Centre as a ‘scattered object’, though this is an issue of some controversy within the astronomical society. Many prominent astronomers feel that Sedna should be in a class of its own due to its highly elliptical orbit and extreme distance from the Sun. The discovery of Sedna sparked new life in the debate over what constitutes a planet. Currently, a planet is defined as “is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit” – IAU Resolution: Definition of a Planet.
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