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90377 Sedna is a trans-Neptunian object and a likely dwarf planet discovered by Michael Brown (Caltechmarker), Chad Trujillo (Gemini Observatorymarker) and David Rabinowitz (Yale Universitymarker) on November 14, 2003. It is currently 88 AU from the Sun, about three times as distant as Neptune. For most of its orbit Sedna is farther from the Sun than any other known dwarf planet candidate.

Discovery

What came to be known as Sedna was discovered during a survey conducted with the Samuel Oschin telescope at Palomar Observatorymarker near San Diegomarker, Californiamarker (USAmarker) using Yale's 160 megapixel Palomar Quest cameramarker and was observed within days on telescopes from Chilemarker, Spainmarker, and the USAmarker (Arizonamarker, and Hawaiimarker). NASA'smarker orbiting Spitzer Space Telescope was later pointed toward the object, putting an upper-bound on its diameter at roughly three-quarters that of Pluto (less than 1,600 km).

The object is named after Sedna, the Inuit goddess of the sea, who was believed to live in the cold depths of the Arctic Oceanmarker. Before Sedna was officially named it had provisional designation .

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 and about 89.6 AU from the Sun. At the time of its discovery it was the most distant object in the solar system yet observed; although the orbits of some objects—like long-period comets—extend further than that of Sedna, they are basically too dim to be observed except near perihelion. Eris was later detected at 97 AU.
Panels showing the location of Sedna in relation to other astronomical objects.


Sedna's precise orbital period is not yet known, but it is calculated at between 10.5 and 12.0 thousand years. It should reach perihelion in late 2075 to mid 2076. Sedna will overtake Eris as the farthest known spheroid orbiting the Sun in 2114.

A study done by Hal Levison and Alessandro Morbidelli of the Observatoire de la Côte d'Azurmarker (OCA) in Nice, Francemarker, suggested that the most likely explanation for Sedna's orbit was that it had been perturbed by a close (~800 AU) 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.They proposed another scenario, which may explain Sedna's orbit more accurately: 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.

It has also been proposed that Sedna's orbit is the result of influence by and in resonance with Nemesis, a theorized dim companion to the Sun which has been proposed to be responsible for the periodicity of mass extinctions on earth from cometary impacts, the lunar impact record, and the common orbital elements of a number of long period comets.

Another possible explanation, advanced by Gomes, involves perturbations of Sedna's orbit by a hypothetical planetary-sized body in the inner Oort cloud. Recent simulations show that Sedna's orbital characteristics could be explained by perturbations by a Neptune-mass object at 2000 AU (or less), a Jupiter-mass at 5000 AU, or even an Earth-mass object at 1000 AU.

Another object, , has an orbit similar to Sedna's but a bit less extreme: perihelion is 44.3 AU, aphelion is 394 AU, and the orbital period is 3240 years. Its orbit may have resulted from the same processes that produced Sedna's orbit.

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 MMTmarker telescope suggest a much shorter rotation period, only about 10 hours, rather typical for bodies of its size.

Physical characteristics

Artist's impression of 90377 Sedna.
Sedna has an absolute magnitude (H) of 1.6, and it is estimated to have an albedo of 0.16 to 0.30, thus giving it a diameter between 1,200 and 1,600 km. At the time of its discovery it was the largest object found in the solar system since the discovery of Pluto in 1930. It is now generally believed to be the fifth largest known trans-Neptunian object after Eris, Pluto, , and . In 2004, the discoverers placed an upper limit of 1,800 kilometers on its diameter, but by 2007 it was revised downward to being less than 1,600 after observations from the Spitzer Space Telescope.

Sedna is so far from the Sun that the temperature never rises above 33 kelvin (−240 °C; −400 °F).

Observations from Chilemarker 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 Hawaiimarker suggest that Sedna's dark red color is caused by a hydrocarbon sludge, or tholin, like that found on 5145 Pholus. Its surface is homogeneous in colour and spectrum; this may be because Sedna, unlike objects nearer the sun, is rarely impacted by other bodies, which would expose bright patches like that on 8405 Asbolus.

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.

Amateur astronomers using advanced software and long exposures have been able to detect Sedna.

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 scattered disk 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 described 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. By 2006 it was considered a detached object.

A number of explanations for Sedna's unusual orbit have been put forward since, including the gravitational influence of a passing star, Nemesis a theoretical dim companion to the sun, or a distant, planet-sized object.

Sedna, together with a few other objects (e.g. ), prompted suggestions of a new category of distant objects named extended scattered disc (E-SDO), detached objects, distant detached objects (DDO) or scattered-extended in the formal classification by the Deep Ecliptic Survey.

The last classification, introduces a formal distinction between scattered-near objects (which could be scattered by Neptune) such as Eris from scattered-extended objects like Sedna. The distinction is made formally, using the orbital elements (see Tisserand's parameter).

The discovery of Sedna 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 a tenth planet had been discovered. This question was answered under the new International Astronomical Union definition of a planet, adopted on August 24, 2006. It is unknown whether or not Sedna is in hydrostatic equilibrium. If, as currently suspected, it is, then it would qualify as a dwarf planet. Sedna has a Stern–Levison parameter estimated at between 8 and 6 times that of Pluto,Stern–Levison parameter (using unlikely highest estimated mass) = ((7 ) / (5.9736 ))^2 / 12,059 yr = 1.14
(Sedna 1.14 ) / (Pluto 1.95 ) = 5.8 and therefore cannot be considered to have cleared the neighborhood of its orbit, even though no other objects have yet been discovered in its vicinity.

See also



References

  1. At present, though, Eris is farther from the Sun than Sedna.
  2. Lowell DES Perihelion Epoch = 2000.0 + (2479283.2278 − 2451545.0)/365.25 = 2075.9431 = (2076-1-1 - 20.7768 days) = 2075-12-11 ( Julian Date Converter)
  3. Horizons
  4. (Original Preprint)
  5. Staff. " Evidence Mounts For Companion Star To Our Sun." SpaceDaily, April 25, 2006. Accessed November 27, 2009.
  6. Jewitt, David, Morbidelli, Alessandro, & Rauer, Heike. (2007). Trans-Neptunian Objects and Comets: Saas-Fee Advanced Course 35. Swiss Society for Astrophysics and Astronomy. Berlin: Springer. ISBN 3540719571.
  7. Lykawka, Patryk Sofia & Mukai, Tadashi. (2007). Dynamical classification of trans-neptunian objects: Probing their origin, evolution, and interrelation. Icarus Volume 189, Issue 1, July , Pages 213–232. .
  8. Staff. " Evidence Mounts For Companion Star To Our Sun." SpaceDaily, April 25, 2006. Accessed June 10, 2008.
  9. 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
  10. Evidence for an Extended Scattered Disk?
  11. Jewitt, D., A. Delsanti, The Solar System Beyond The Planets in Solar System Update : Topical and Timely Reviews in Solar System Sciences , Springer-Praxis Ed., ISBN 3-540-26056-0 (2006) Preprint of the article (pdf)
  12. Elliot, J. L., S. D. Kern, K. B. Clancy, A. A. S. Gulbis, R. L. Millis, M. W. Buie, L. H. Wasserman, E. I. Chiang, A. B. Jordan, D. E. Trilling, and K. J. Meech The Deep Ecliptic Survey: A Search for Kuiper Belt Objects and Centaurs. II. Dynamical Classification, the Kuiper Belt Plane, and the Core Population. The Astronomical Journal, 129 (2006), pp. preprint.


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