90377 Sedna is a trans-Neptunian object and likely dwarf planet discovered by Michael Brown (Caltech), Chad Trujillo (Gemini Observatory) and David Rabinowitz (Yale University) on November 14, 2003. As of 2010[update], it is 87.4 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 currently known dwarf planet candidate. Eris, the largest known dwarf planet, is currently farther from the Sun than Sedna, though it is near its aphelion, or maximum distance from the Sun, while Sedna is nearing its perihelion, or minimum distance.
Sedna's exceptionally long and elongated orbit, taking roughly 12,000 years to complete, and distant perihelion (76 AU) have led to much speculation as to its origin. The Minor Planet Center currently places Sedna in the scattered disc, a group of objects sent into highly elongated orbits by the gravitational influence of Neptune. However, Sedna never comes close enough to Neptune to have been scattered by it, leading some astronomers to conclude that it is in fact the first known member of the inner Oort cloud. Mike Brown claims that Sedna is the most scientifically important new object yet discovered in the regions beyond Neptune, as understanding its peculiar orbit is likely to yield valuable information about the origin and early evolution of the Solar System.
Discovery and naming
Sedna (provisionally designated 2003 VB12) was discovered during a survey begun in 2001 with the Samuel Oschin telescope at Palomar Observatory near San Diego, California (USA) using Yale's 160 megapixel Palomar Quest camera. An object was observed to move by 4.6″ over 3.1 hours relative to stars on November 14, 2003, which indicated a distance of about 100 AU. Follow-up observations in November–December 2003 with the SMARTS telescope at the Cerro Tololo Inter-American Observatory in Chile as well as with the Tenagra IV telescope at the W. M. Keck Observatory in Hawaii revealed that the object was moving along a distant highly eccentric orbit. Later the object was identified on older images made by the Samuel Oschin telescope as well on images from Near Earth Asteroid Tracking consortium.
Due to the object's undoubtedly frigid surface temperatures, the team named it after Sedna, the Inuit goddess of the sea, who was believed to live in the cold depths of the Arctic Ocean. Brown also suggested to the International Astronomical Union's (IAU) Minor Planet Center that any future objects discovered in Sedna's orbit should also be named from Inuit mythologies. The team made the name "Sedna" public before the object had been officially numbered. Brian Marsden, the head of the Minor Planet Center, complained that such an action was a violation of protocol, and that some members of the IAU might vote against it. However, no objection was raised as to the name itself, and no competing names were suggested. The MPC formally accepted the name in September, 2004, and also considered that, in similar cases of extraordinary interest, it might allow names to be announced in future before they were officially numbered.
Orbit and rotation
Barring comets, Sedna has the longest orbital period of any known object in the Solar System, calculated at between 11,800 and 12,100 years. This represents a best-fit solution, as Sedna has only been observed over a brief part of its orbital arc. Its orbit is extremely elliptical, with an aphelion estimated at 960 AU and and a perihelion at about 76 AU. At its discovery it was approaching perihelion 89.6 AU from the Sun, and was the most distant object in the Solar System yet observed. Although the orbits of some long-period comets extend farther than that of Sedna, they are too dim to be observed except near perihelion. Eris was later detected at 97 AU. Even with Sedna nearing perihelion, the Sun would appear merely as a bright star in its sky; its angular diameter would be too small to resolve a disc, and it would be only 100 times brighter than a full Moon on Earth.
Sedna should reach perihelion in late 2075[n 2] to mid 2076. It will overtake Eris as the farthest presently known dwarf planet candidate in 2114.
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. Subsequent measurements from the MMT telescope suggest a much shorter rotation period, only about 10 hours, rather typical for bodies of its size.
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. Mike Brown and colleagues now believe it to be the fifth largest known trans-Neptunian object after Eris, Pluto, Makemake, and Haumea. 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 km after observations from the Spitzer Space Telescope. As Sedna has no known moons, determining its mass is very difficult. However, if the above estimates for its diameter are coupled with Pluto's density of 2.0 g/cm3, the resultant estimated mass range is 1.8–4.3 x 1021 kg.[n 1]
Observations from the SMARTS telescope show that in visible light Sedna is one of the reddest objects in the Solar System, nearly as red as Mars. Chad Trujillo and his colleagues at the Gemini Observatory in Hawaii suggest that Sedna's dark red colour 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.
Trujillo et al. have placed upper limits of 60% for methane ice in Sedna's surface composition and 70% for water ice. Barucci, Cruikshank et al. compared Sedna's spectrum with that of Triton and detected weak absorption bands belonging to the methane and nitrogen ices. They suggested the following common model of the surface: 24% Triton-type tholins, 7% amorphous carbon, 10% nitrogen, 26% methanol and 33% methane. The detection of methane and water ices was confirmed in 2006 by the mid-infrared photometry by Spitzer Space Telescope. The presence of nitrogen on the surface suggests the possibility that, at least for a short time, Sedna may possess an atmosphere. A 200-year period exists around its perihelion during which Sedna's surface temperature may rise above the 35.6 K (−237.4°C or −395.3°F) boundary required for nitrogen to shift from solid to gas. However, its deep red spectrum is indicative of high concentrations of organic material on its surface, and its weak methane absorption bands indicate that methane on Sedna's surface is ancient, rather than freshly fallen. This means that Sedna is too cold for methane to evaporate from its surface and then fall back as snow, as happens on Triton and probably Pluto.
In their paper announcing the discovery of Sedna, Mike Brown and his colleagues described it as the first observed body belonging to the Oort cloud, the hypothetical cloud of comets believed to exist nearly a light-year from the Sun. They observed that, unlike scattered disc objects such as Eris, Sedna's perhihelion (76 AU) is too distant for it to have been scattered by the gravitational influence of Neptune. 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.
If Sedna formed in its current location, the Sun's original protoplanetary disc must have extended as far as 11 billion km into space. Also, Sedna's orbit must have been circular, otherwise accretion (the coalescence of smaller bodies into a larger one) would not have been possible because the large relative velocities between planetesimals would have been too disruptive; so it must have been tugged out of its original orbit into its current eccentricity. In their initial paper, Brown, Rabinowitz et al. suggested three possible gravitational causes for Sedna's orbit: an unseen planet beyond the Kuiper belt, a single passing star, or one of the young stars embedded with the Sun in the stellar cluster in which it formed. These remain the most widely accepted hypotheses among astronomers today.
The passing star hypothesis has been advanced by both Alessandro Morbidelli and Scott J. Kenyon. A study by Morbidelli and Hal Levison 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.
Brown and his team considered the possibility that Sedna's orbit resulted from the Sun's formation in an open cluster which gradually disassociated over time to be the most likely of the three proposed scenarios. Computer simulations by Julio A. Fernandez and Adrian Brunini suggest that multiple close passes by young stars in such a cluster would pull many objects into Sedna-like orbits.
The trans-Neptunian planet hypothesis has been advanced in several forms by a number of astronomers, including Gomes and Patryk Lykawka. One scenario 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. Computer simulations by Patryk Lykawka have suggested that Sedna's orbit may have been caused by a body roughly the size of Earth, ejected outward by Neptune early in the Solar System's formation and currently in an elongated orbit between 80 and 170 AU from the Sun. 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.
Morbidelli and Kenyon have even suggested that Sedna may not have originated in our Solar System at all, but instead was captured by the Sun from a passing star, specifically a brown dwarf about 20 times less massive than the Sun.
Sedna's highly elliptical orbit meant that the probability of its detection was roughly one in 60, suggesting that, unless its discovery was a fluke, another 40–160 Sedna-sized objects should exist within its region. Another object, 2000 CR105, has an orbit similar to Sedna's but less extreme: perihelion is 44.3 AU, aphelion is 394 AU, and the orbital period is 3240 years; it may have been affected by the same processes as Sedna. Each of the proposed mechanisms for Sedna's extreme orbit would leave a distinct mark on the structure and dynamics of any wider population, so gaining a larger sample of such objects could help in determining which scenario is most likely. " I call Sedna a fossil record of the earliest Solar System", said Brown in 2006. "Eventually, when other fossil records are found, Sedna will help tell us how the Sun formed and the number of stars that were close to the Sun when it formed." A 2007-2008 survey by Brown, Rabinowitz and Megan Schwamb attempted to locate another member of Sedna's hypothetical population. Although the survey was sensitive to movement out to 1000 AU and discovered the large and distant object 2007 OR10, it detected no new bodies in Sedna-like orbits. Subsequent simulations incorporating the new data suggested that a total of about 40 Sedna-sized objects probably exist in this region.
The Minor Planet Center, which officially catalogs the objects in the Solar System, classifies Sedna as a scattered object. However, this grouping is heavily questioned, and many astronomers have suggested that it, together with a few other objects (e.g. 2000 CR105), be placed in 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 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, which mandated that a planet must have cleared the neighborhood around its orbit. Sedna has a Stern–Levison parameter estimated at between 8 × 10−5 and 6 × 10−3 times that of Pluto,[n 3] and therefore cannot be considered to have cleared the neighborhood, even though no other objects have yet been discovered in its vicinity. It is unknown whether or not Sedna is in hydrostatic equilibrium. If it is, as is currently suspected, then it would qualify as a dwarf planet, though the IAU's 2008 decree that only those TNOs with an absolute magnitude brighter than +1 be allowed into the category would appear to exclude Sedna for the time being.
Sedna's perihelion will be reached within this century, after which it will move back out and farther away from the Sun again for another estimated 12 thousand years. Though an exploration target within the Solar System, NASA is not considering any type of mission at this time.
* Planets beyond Neptune
1. ^ a b c Taking Brown's estimates for the diameter of 1,200–1,600 km and assuming Pluto's density of 2.0
1. ^ "Discovery Circumstances: Numbered Minor Planets (90001)-(95000)". IAU: Minor Planet Center. http://www.cfa.harvard.edu/iau/lists/NumberedMPs090001.html. Retrieved 2008-07-23.
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