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==Exploration==
==Exploration==
Sedna's perihelion will be reached within this century,<ref name=jpldata/> after which it will move back out and farther away from the Sun again for another ~6,000 years. Though an exploration target within the Solar System,<ref>
Sedna's perihelion will be reached within this century,<ref name=jpldata/> after which it will move back out and farther away from the Sun again for another ~12,000 years. Though an exploration target within the Solar System,<ref>
{{cite web
{{cite web
|url=http://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=1504
|url=http://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=1504

Revision as of 07:15, 14 November 2010

For other uses, see Sedna (disambiguation).
90377 Sedna
An image of Sedna seen through an Earth-based telescope: it is a faint point of light.
Discovery image of Sedna (identified by the yellow arrow)
Discovery[1]
Discovered byMichael E. Brown,
C. Trujillo,
D. Rabinowitz
Discovery dateNovember 14, 2003
Designations
Designation
90377 Sedna
Pronunciation/ˈsɛdnə/ SED-nə
Named after
Sedna
2003 VB12
Trans-Neptunian object
detached object[2]
Orbital characteristics[3]
Epoch 2010-Jul-23 (JD 2455400.5)
Aphelion1.437×1014 m
(960.78 AU)
Perihelion1.142 3×1013 m
(76.361 AU)
7.757 6×1013 m
(518.57 AU)
Eccentricity0.8527
around 4,313,319 d
(11,809 yr)
1.04 km/s
358.01°
Inclination11.927°
144.26°
311.02°
Physical characteristics
Dimensions1,200–1,600 km[4]
<1,600 km[5]
Mass1.8–4.3 x 1021 kg[a]
Mean density
2.0? g/cm3[a]
0.33–0.50 m/s2
0.62–0.95 km/s
0.42 d (10 h)[3][6]
Albedo0.16–0.30[4]
Temperature~12 K (see note w here)
Spectral type
(red) B-V=1.24; V-R=0.78[7]
21.1[8]
20.5 (Perihelic)[9]
1.58[3]

90377 Sedna is a trans-Neptunian object, discovered in 2003, which currently lies about three times as far from the Sun as Neptune. However, its farthest orbital distance from the Sun is estimated to be 960 astronomical units (AU), and thus it is, for the majority of its orbit, the most distant known object in the Solar System after long-period comets.[b]

Roughly two-thirds the size of Pluto, Sedna is hypothetically large enough to be rounded by its own gravity, and thus would qualify as a dwarf planet under current definitions. However, its distance from the Sun makes determining its shape difficult. Spectroscopy has revealed that Sedna's surface composition is similar to that of some other trans-Neptunian objects, being largely a mixture of water, methane and nitrogen ices with tholins. Its surface is one of the reddest in the Solar System.

Sedna's exceptionally long and elongated orbit, taking approximately 12,000 years to complete, and distant point of closest approach to the Sun, at 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, this classification has been contested, as 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. Others speculate that it might have been tugged into its current orbit by a passing star, perhaps one within the Sun's birth cluster, or even that it was captured from another star system. Another hypothesis suggests that its orbit may be evidence for a large planet beyond the orbit of Neptune. Astronomer Mike Brown, who co-discovered Sedna as well as the dwarf planets Eris, Haumea, and Makemake, believes it to be the most scientifically important trans-Neptunian object found to date, as understanding its peculiar orbit is likely to yield valuable information about the origin and early evolution of the Solar System.[10]

Discovery and naming

Sedna (provisionally designated 2003 VB12) was discovered by Mike Brown (Caltech), Chad Trujillo (Gemini Observatory) and David Rabinowitz (Yale University) on November 14, 2003. The discovery formed part of a survey begun in 2001 with the Samuel Oschin telescope at Palomar Observatory near San Diego, California using Yale's 160 megapixel Palomar Quest camera. On that day, an object was observed to move by 4.6 arcseconds over 3.1 hours relative to stars, which indicated that its distance was 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 precovery images made by the Samuel Oschin telescope as well as on images from the Near Earth Asteroid Tracking consortium. These previous positions expanded its known orbital arc and allowed a more precise calculation of its orbit.[11]

"Our newly discovered object is the coldest most distant place known in the Solar System," said Mike Brown on his website, "so we feel it is appropriate to name it in honour of Sedna, the Inuit goddess of the sea, who is thought to live at the bottom of the frigid Arctic Ocean."[12] Brown also suggested to the International Astronomical Union's (IAU) Minor Planet Center that any future objects discovered in Sedna's orbital region should also be named after entities in arctic mythologies.[12] The team made the name "Sedna" public before the object had been officially numbered.[13] 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.[14] However, no objection was raised as to the name itself, and no competing names were suggested. The IAU's Committee on Small Body Nomenclature formally accepted the name in September 2004,[15] and also considered that, in similar cases of extraordinary interest, it might in future allow names to be announced before they were officially numbered.[13]

Orbit and rotation

The orbit of Sedna lies well beyond these objects, and extends many times their distances from the Sun
The orbit of Sedna (red) set against the orbits of Jupiter (orange), Saturn (yellow), Uranus (green), Neptune (blue), and Pluto (purple)

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.[2] This represents a best-fit solution, as Sedna has only been observed over a brief part of its orbital arc. Its orbit is extremely eccentric, with an aphelion estimated at 960 AU and a perihelion at about 76 AU. At its discovery it was approaching perihelion at 89.6 AU[16] from the Sun, and was the most distant object in the Solar System yet observed. Eris was later detected by the same survey at 97 AU. Although the orbits of some long-period comets extend farther than that of Sedna, they are too dim to be discovered except when approaching perihelion in the inner Solar System. Even as Sedna nears its perihelion in late 2075[c] to mid 2076,[9] the Sun would appear merely as a bright star in its sky: with an angular diameter too small to resolve as a disc, it would be only 100 times brighter than a full Moon on Earth.[17]

When first discovered, Sedna was believed to have an unusually long rotational period (20 to 50 days).[17] It was initially speculated that Sedna's rotation was slowed by the gravitational pull of a large binary companion, similar to Pluto's moon Charon.[12] A search for such a satellite by the Hubble Space Telescope in March 2004 found nothing,[18][e] and subsequent measurements from the MMT telescope suggest a much shorter rotation period, only about 10 hours, rather typical for bodies of its size.[19]

Physical characteristics

Sedna, a red, icy globe, is barely lit by a distant Sun
Artist's impression of 90377 Sedna

Sedna has an absolute magnitude (H) of 1.6,[3] and it is estimated to have an albedo of 0.16 to 0.30,[4] thus giving it a diameter between 1,200 and 1,600 km.[4] 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.[4][20] In 2004, the discoverers placed an upper limit of 1,800 km on its diameter,[21] but by 2007 this was revised downward to less than 1,600 km after observation by the Spitzer Space Telescope.[5] 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.[a]

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.[12] Chad Trujillo and his colleagues suggest that Sedna's dark red colour is caused by a surface coating of hydrocarbon sludge, or tholin, formed from simpler organic compounds after long exposure to ultraviolet radiation.[22] 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 of fresh icy material like that on 8405 Asbolus.[22] Sedna and two other very distant objects ((87269) 2000 OO67 and 2006 SQ372) share their colour with outer classical Kuiper belt objects and the centaur 5145 Pholus, possibly suggesting a similar region of origin.[23]

Trujillo and colleagues have placed upper limits in Sedna's surface composition of 60% for methane ice and 70% for water ice.[22] The presence of methane further supports the existence of tholins on Sedna's surface, as they are produced by irradiation of methane.[24] Barucci and colleagues compared Sedna's spectrum with that of Triton and detected weak absorption bands belonging to methane and nitrogen ices. From these observations, they suggested the following model of the surface: 24% Triton-type tholins, 7% amorphous carbon, 10% nitrogen, 26% methanol and 33% methane.[25] The detection of methane and water ices was confirmed in 2006 by Spitzer Space Telescope mid-infrared photometry.[24] 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) minimum required for nitrogen to sublimate from solid to gas.[25] However, its deep red spectral slope 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 deposited. 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 on Pluto.[24]

Origin

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 perihelion (76 AU) is too distant for it to have been scattered by the gravitational influence of Neptune.[11] 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.[26][27]

If Sedna formed in its current location, the Sun's original protoplanetary disc must have extended as far as 11 billion km into space.[28] Also, Sedna's initial orbit must have been circular, otherwise its formation by the accretion of smaller bodies into a whole would not have been possible, as the large relative velocities between planetesimals would have been too disruptive. Therefore, it must have been tugged into its current eccentric orbit by a gravitational interaction with another body.[29] In their initial paper, Brown, Rabinowitz and colleagues suggested three possible candidates for the perturbing body: 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.[11]

Mike Brown and his team favoured the hypothesis that Sedna was lifted into its current orbit by a star from the Sun's birth cluster, arguing that Sedna's aphelion of about 1,000 AU, which is relatively close compared to those of long period comets, is not distant enough to be affected by passing stars at their current distances from the Sun. They propose that Sedna's orbit is best explained by the Sun's having formed in an open cluster of several stars that gradually disassociated over time.[11][30][31] That hypothesis has also been advanced by both Alessandro Morbidelli and Scott J. Kenyon.[32][33] 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.[11] 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 (approximately 800 AU) pass by another star in the first 100 million years or so of the Solar System's existence.[32][34]

EarthMoonCharonCharonNixNixKerberosKerberosStyxStyxHydraHydraPlutoPlutoDysnomiaDysnomiaErisErisNamakaNamakaHi'iakaHi'iakaHaumeaHaumeaMakemakeMakemakeMK2MK2XiangliuXiangliuGonggongGonggongWeywotWeywotQuaoarQuaoarSednaSednaVanthVanthOrcusOrcusActaeaActaeaSalaciaSalacia2002 MS42002 MS4File:10 Largest Trans-Neptunian objects (TNOS).png
Artistic comparison of Pluto, Eris, Makemake, Haumea, Gonggong (2007 OR10), Sedna, Quaoar, Orcus, 2002 MS4, and Salacia.

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 2,000 AU (or less), a Jupiter-mass at 5,000 AU, or even an Earth-mass object at 1,000 AU.[31][35] 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.[36] Mike Brown's various sky surveys have not detected any Earth-sized objects out to a distance of about 100 AU. However, it is possible that such an object may have been scattered out of the Solar System after the formation of the inner Oort cloud.[37]

It has been suggested that Sedna's orbit is the result of influence by a large binary companion to the Sun, thousands of AU distant. One such hypothetical companion is Nemesis, a dim companion to the Sun which has been proposed to be responsible for the supposed 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.[35][38] However, to date, no direct evidence of Nemesis has been found.[39] John J. Matese and Daniel P. Whitmire, longtime proponents of the possibility of a wide binary companion to the Sun, have suggested that an object of five times the mass of Jupiter lying at roughly 7850 AU from the Sun could produce a body in Sedna's orbit.[40]

Morbidelli and Kenyon have also suggested that Sedna did not originate in our Solar System, but was captured by the Sun from a passing extrasolar planetary system, specifically that of a brown dwarf about 20 times less massive than the Sun.[32][33]

Population

the Sun appears merely as a point of light, distended by dust. The surface of Sedna is red ice, dimly glimmering in the noontime sunlight
Artist's conception of the surface of Sedna, with the Milky Way and Sun above

Sedna's highly elliptical orbit means that the probability of its detection was roughly one in 60, suggesting that, unless its discovery was a fluke, another 40–120 Sedna-sized objects should exist within its region.[4] Another object, 2000 CR105, has a similar but less extreme orbit: it has a perihelion of 44.3 AU, an aphelion of 394 AU, and an orbital period of 3,240 years. It may have been affected by the same processes as Sedna.[32]

Each of the proposed mechanisms for Sedna's extreme orbit would leave a distinct mark on the structure and dynamics of any wider population. If a trans-Neptunian planet was responsible, all such objects would share roughly the same perihelion (~80 AU). If Sedna were captured from another planetary system that rotated in the same direction as the Solar System, then Sedna's population would all possess relatively low inclinations and range from 100–500 AU. If it rotated in the opposite direction, then two populations would form, one with low inclinations and one with high. The gravity of perturbing stars would produce a wide range of perihelia and inclinations, each dependent on the number and angle of such encounters.[37]

Gaining a larger sample of such objects could therefore help in determining which scenario is most likely.[41] "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."[10] 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 1,000 AU and discovered the dwarf planet candidate 2007 OR10, it detected no new bodies in Sedna-like orbits.[41] Subsequent simulations incorporating the new data suggested that a total of about 40 Sedna-sized objects probably exists in this region.[41]

Classification

The Minor Planet Center, which officially catalogs the objects in the Solar System, classifies Sedna as a scattered object.[42] 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 objects (E-SDO),[43] detached objects,[44] distant detached objects (DDO)[35] or scattered-extended in the formal classification by the Deep Ecliptic Survey.[45]

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 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 to be much less than 1,[d] and therefore cannot be considered to have cleared the neighborhood, even though no other objects have yet been discovered in its vicinity. To qualify as a dwarf planet, Sedna must be shown to be in hydrostatic equilibrium. It is not bright enough to conclusively prove this by the absolute magnitude threshold of +1 specified by the IAU naming guidelines,[46] so other evidence will have to be acquired. However, it remains bright enough that it is expected to be a dwarf planet.[47]

Exploration

Sedna's perihelion will be reached within this century,[3] after which it will move back out and farther away from the Sun again for another ~12,000 years. Though an exploration target within the Solar System,[48] NASA is not considering any type of mission at this time.[49]

Notes

  • ^ Taking Brown's estimates for the diameter of 1,200–1,600 km and assuming Pluto's density of 2.0 (<0.26 Eris)
  • ^ As of 2010, Sedna is 87.3 AU from the Sun;[8] Eris, the largest known dwarf planet, is currently farther from the Sun than Sedna, though it is near its aphelion, while Sedna is nearing its perihelion. Sedna will overtake Eris as the farthest presently known spherical minor planet in 2114.[9]
  • ^ Lowell DES Perihelion Epoch = 2479285.0598 = 2075-12-13
  • ^ The Stern-Levison parameter (Λ) as defined by Alan Stern and Harold F. Levison in 2002 determines if an object will eventually clear its orbital neighbourhood of small bodies. It is defined as the object's fraction of solar mass (i.e., the object's mass divided by the sun's mass) squared, divided by its semi-major axis to the 3/2 power, times a constant 1.7×1016.[50](see equation 4) If an object's Λ is greater than 1, then that object will eventually clear its neighbourhood, and it can be considered for planethood. Using the unlikely highest estimated mass for Sedna of 7×1021 kg, Sedna's Λ is (7×1021/1.9891×1030)2 / 5193/2 × 1.7×1016 = 1.8×10−5. This is much less than 1, so Sedna is not a planet by this criterion.
  • ^ The HST search found no satellite candidates to a limit of about 500 times fainter than Sedna (Brown and Suer 2007).[4]

References

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