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90377 Sedna

Sedna seen through Hubble
Sedna as imaged by the Hubble Space Telescope
Discovered by Michael Brown
Chad Trujillo
David Rabinowitz
Discovery date November 14, 2003
MPC designation (90377) Sedna
Named after
2003 VB12
trans-Neptunian object
detached object
Orbital characteristics[3]
Epoch 2014-Dec-09.0 (JD 2457000.5)
Aphelion ≈ 936 AU (Q)[4]
1.4×1011 km
0.015 ly
Perihelion 76.0917±0.0087 AU (q)
1.1423×1010 km
524.4±1.0 AU (a)
7.7576×1010 km
Eccentricity 0.85491±0.00029
≈ 11400 yr[4][1]
Average orbital speed
1.04 km/s
Inclination 11.92864°
144.545° (Ω)
311.29°±0.014° (ω)
Physical characteristics
Dimensions 995±80 km
(thermophysical model)
1060±100 km
(standard thermal model)[6]
Sidereal rotation period
10.3 h ± 30%[3][7]
Albedo 0.32±0.06[6]
Temperature ≈ 12 K (see note)
Spectral type
(red) B−V=1.24; V−R=0.78[8]
20.5 (perihelic)[10]

90377 Sedna is a large minor planet in the outer reaches of the Solar System that was, as of 2015, at a distance of about 86 astronomical units (AU) from the Sun, about three times as far as Neptune. 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 among Solar System objects. It is most likely a dwarf planet.

For most of its orbit it is even farther from the Sun than at present, with its aphelion estimated at 937 AU[4] (31 times Neptune's distance), making it one of the most distant known objects in the Solar System other than long-period comets.[2][3]

Sedna's exceptionally long and elongated orbit, taking approximately 11,400 years to complete, and distant point of closest approach to the Sun, at 76 AU, have led to much speculation about 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, because 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 (an open 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 Michael Brown, co-discoverer of Sedna and the dwarf planets Eris, Haumea, and Makemake, thinks that it is the most scientifically important trans-Neptunian object found to date, because understanding its unusual orbit is likely to yield valuable information about the origin and early evolution of the Solar System.[12]


  • History 1
    • Discovery 1.1
    • Naming 1.2
  • Orbit and rotation 2
  • Physical characteristics 3
  • Origin 4
  • Population 5
  • Classification 6
  • Exploration 7
  • Notes 8
  • References 9
  • External links 10



Sedna (provisionally designated 2003 VB12) was discovered by Michael 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 Cerro Tololo Inter-American Observatory in Chile as well as with the Tenagra IV telescope at the Keck Observatory on Mauna Kea in Hawaii revealed that the object was moving along a distant highly eccentric orbit. Later, the object was precovered on older 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.[13]


"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 honor of Sedna, the Inuit goddess of the sea, who is thought to live at the bottom of the frigid Arctic Ocean."[14] 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.[14] The team made the name "Sedna" public before the object had been officially numbered.[15] Brian Marsden, the head of the Minor Planet Center, said that such an action was a violation of protocol, and that some members of the IAU might vote against it.[16] However, no objection was raised to the name, and no competing names were suggested. The IAU's Committee on Small Body Nomenclature formally accepted the name in September 2004,[17] and also considered that, in similar cases of extraordinary interest, it might in the future allow names to be announced before they were officially numbered.[15]

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)

Sedna has the longest orbital period of any known large object in the Solar System,[3] calculated at around 11,400 years.[4][1] Its orbit is extremely eccentric, with an aphelion estimated at 937 AU[4] and a perihelion at about 76 AU. This perihelion is the largest of that of any known Solar System object until the discovery of 2012 VP113.[18][19] When Sedna was discovered it was 89.6 AU[20] from the Sun approaching perihelion, and was the most distant object in the Solar System yet observed. Eris was later detected by the same survey at 97 AU. Only 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 mid 2076,[10][4] the Sun would appear merely as an extremely bright star-like pinpoint in its sky, 100 times brighter than a full moon on Earth (for comparison, the Sun appears from Earth to be roughly 400,000 times brighter than the full Moon), and too far away to be visible as a disc to the naked eye.[21]

When first discovered, Sedna was thought to have an unusually long rotational period (20 to 50 days).[21] 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.[14] A search for such a satellite by the Hubble Space Telescope in March 2004 found nothing,[22][5] and subsequent measurements from the MMT telescope suggest a much shorter rotation period of about 10 hours; rather typical for a body of its size.[24]

Physical characteristics

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

Sedna has a V-band absolute magnitude (H) of about 1.8, and it is estimated to have an albedo of about 0.32, thus giving it a diameter of approximately 1,000 km.[6] At the time of its discovery it was the intrinsically brightest object found in the Solar System since Pluto in 1930. In 2004, the discoverers placed an upper limit of 1,800 km on its diameter,[25] but by 2007 this was revised downward to less than 1,600 km after observation by the Spitzer Space Telescope.[26] In 2012, measurements from the Herschel Space Observatory suggested that Sedna's diameter was 995 ± 80 km, which would make it smaller than Pluto's moon Charon.[6] Because Sedna has no known moons, determining its mass is currently impossible without sending a space probe.

Observations from the SMARTS telescope show that in ultraviolet radiation.[27] 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.[27] Sedna and two other very distant objects – 2006 SQ372 and (87269) 2000 OO67 – share their color with outer classical Kuiper belt objects and the centaur 5145 Pholus, suggesting a similar region of origin.[28]

Trujillo and colleagues have placed upper limits in Sedna's surface composition of 60% for methane ice and 70% for water ice.[27] The presence of methane further supports the existence of

  • Media related to at Wikimedia Commons
  • How Sedna and family were captured in a close encounter with a solar sibling (9 Jun 2015)
  • NASA's Sedna page (Discovery Photos)
  • Mike Brown's Sedna page

External links

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  • ^ "Solar System Exploration: Missions to Dwarf Planets". NASA. Retrieved 11 November 2010. 
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  • ^  
  • ^ Tancredi, G.; Favre, S. (2008). "Which are the dwarfs in the solar system?" (PDF). Asteroids, Comets, Meteors. Retrieved 2011-01-05. 
  • ^ Malhotra, 2010. "On the Importance of a Few Dwarf Planets". Bulletin of the American Astronomical Society, Vol. 41
  • ^ Rabinowitz, Schaefer, Tourtellotte, 2011. "SMARTS Studies of the Composition and Structure of Dwarf Planets". Bulletin of the American Astronomical Society, Vol. 43
  • ^ Barucci, M.; Morea Dalle Ore, C.; Alvarez-Candal, A.; De Bergh, C.; Merlin, F.; Dumas, C.; Cruikshank, D. (2010). "(90377) Sedna: Investigation of surface compositional variation". The Astronomical Journal 140 (6): 6.  
  • ^ Michael E. Brown. "The Dwarf Planets". California Institute of Technology, Department of Geological Sciences. Archived from the original on 29 February 2008. Retrieved 2008-02-16. 
  • ^ S. Alan Stern and Harold F. Levison; Levison (2002). "Regarding the criteria for planethood and proposed planetary classification schemes" (PDF). Highlights of Astronomy 12: 205–213, as presented at the XXIVth General Assembly of the IAU–2000 [Manchester, UK, 7–18 August 2000].  
  • ^ J. L. Elliot; S. D. Kern; K. B. Clancy; Gulbis, A. A. S.; Millis, R. L.; Buie, M. W.; Wasserman, L. H.; Chiang, E. I.; Jordan, A. B.; et al. (2006). "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 (2): 1117.  
  • ^ "The Solar System Beyond The Planets". Solar System Update : Topical and Timely Reviews in Solar System Sciences. Springer-Praxis Ed. 2006.  
  • ^ Gladman, Brett (2001). "Evidence for an Extended Scattered Disk?". Observatoire de la Cote d'Azur. Retrieved 2010-07-22. 
  • ^ IAU: Minor Planet Center (2008-07-02). "List of Centaurs and Scattered-Disk Objects". Central Bureau for Astronomical Telegrams, Harvard-Smithsonian Center for Astrophysics. Retrieved 2008-07-02. 
  • ^ "A new object at the edge of our Solar System discovered". 26 March 2014. 
  • ^ "JPL Small-Body Database Browser: (2012 VP113)" (2013-10-30 last obs). Jet Propulsion Laboratory. Retrieved 2014-03-26. 
  • ^ a b c Schwamb, Megan E.; Brown, Michael E.; Rabinowitz, David L. (2009). "A Search for Distant Solar System Bodies in the Region of Sedna". The Astrophysical Journal Letters 694 (1): L45–L48.  
  • ^ Grand Theft Sedna: how the sun might have stolen a mini-planet
  • ^ Sun Accused of Stealing Planetary Objects from Another Star
  • ^ John J. Matese, Daniel P. Whitmire and Jack J. Lissauer (2006). "A Widebinary Solar Companion as a Possible Origin of Sedna-like Objects". Earth, Moon, and Planets 97 (3–4): 459–470.  
  • ^ "Nemesis is a myth". Max Planck Institute. 2011. Retrieved 2011-08-11. 
  • ^ J. G. Hills (1984). "Dynamical constraints on the mass and perihelion distance of Nemesis and the stability of its orbit".  
  • ^ Staff (April 25, 2006). "Evidence Mounts For Companion Star To Our Sun". SpaceDaily. Archived from the original on 7 January 2010. Retrieved November 27, 2009. 
  • ^ a b Megan Schwamb (2007). "Searching for Sedna's Sisters: Exploring the inner Oort cloud" (PDF). Cal Tech. Retrieved 2010-08-06. 
  • ^ P. S. Lykawka and T. Mukai (2008). "An Outer Planet Beyond Pluto and the Origin of the Trans-Neptunian Belt Architecture". Astronomical Journal 135 (4): 1161.  
  • ^ a b c Rodney S. Gomes, John J. Matese, and Jack J. Lissauer (2006). "A distant planetary-mass solar companion may have produced distant detached objects". Icarus 184 (2): 589–601.  
  • ^ "The Challenge of Sedna". Harvard-Smithsonian Center for Astrophysics. Retrieved 2009-03-26. 
  • ^ a b Scott J. Kenyon, Benjamin C. Bromley (2 December 2004). "Stellar encounters as the origin of distant Solar System objects in highly eccentric orbits". Nature 432 (7017): 598–602.  
  • ^ a b c d Alessandro Morbidelli, Harold F. Levison (2004). "Scenarios for the Origin of the Orbits of the Trans-Neptunian Objects 2000 CR105 and 2003 VB12 (Sedna)". The Astronomical Journal 128 (5): 2564–2576.  
  • ^ a b "Transneptunian Object 90377 Sedna (formerly known as 2003 VB12)".  
  • ^ Mike Brown (2004). "Sedna and the birth of the solar system". Bulletin of the American Astronomical Society (American Astronomical Society Meeting 205) 36 (127.04): 1553.  
  • ^ Scott S. Sheppard, D. Jewitt (2005). "Small Bodies in the Outer Solar System" (PDF). Frank N. Bash Symposium. The University of Texas at Austin. Retrieved 2008-03-25. 
  • ^ S. Alan Stern (2005). "Regarding the accretion of 2003 VB12 (Sedna) and like bodies in distant heliocentric orbits". The Astronomical Journal (Astronomical Journal) 129 (1): 526–529.  
  • ^ Lykawka, Patryk Sofia; Mukai, Tadashi (2007). "Dynamical classification of trans-neptunian objects: Probing their origin, evolution, and interrelation". Icarus 189 (1): 213–232.  
  • ^ Trans-Neptunian Objects and Comets: Saas-Fee Advanced Course 35. Swiss Society for Astrophysics and Astronomy. Berlin: Springer. 2007. p. 86.  
  • ^ Hussmann, Hauke; Sohl, Frank; Spohn, Tilman (November 2006). "Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects" (PDF).  
  • ^ a b M. A. Barucci; D. P. Cruikshank; E. Dotto; Merlin, F.; Poulet, F.; Dalle Ore, C.; Fornasier, S.; De Bergh, C. (2005). "Is Sedna another Triton?". Astronomy & Astrophysics 439 (2): L1–L4.  
  • ^ a b c J. P. Emery; C. M. Dalle Ore; D. P. Cruikshank; Fernández, Y. R.; Trilling, D. E.; Stansberry, J. A. (2007). "Ices on 90377 Sedna: Conformation and compositional constraints" (pdf). Astronomy and Astrophysics 406 (1): 395–398.  
  • ^ Sheppard, Scott S. (2010). "The colors of extreme outer Solar System objects". The Astronomical Journal 139 (4): 1394–1405.  
  • ^ a b c  
  • ^ John Stansberry, Will Grundy, Mike Brown, Dale Cruikshank, John Spencer, David Trilling, Jean-Luc Margot (2008). "Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope". In M. Antonietta Barucci, Hermann Boehnhardt, Dale P. Cruikshank. The Solar System Beyond Neptune (pdf). University of Arizona Press. pp. 161–179.  
  • ^ W. M. Grundy, K. S. Noll, D. C. Stephens (2005). "Diverse Albedos of Small Trans-Neptunian Objects". Icarus (Lowell Observatory, Space Telescope Science Institute) 176 (1): 184–191.  
  • ^ B. Scott Gaudi, Krzysztof Z. Stanek, Joel D. Hartman, Matthew J. Holman, Brian A. McLeod (CfA) (2005). "On the Rotation Period of (90377) Sedna". The Astrophysical Journal 629 (1): L49–L52.  
  • ^ a b Michael E. Brown. "The largest Kuiper belt objects". In M. Antonietta Barucci, Hermann Boehnhardt, Dale P. Cruikshank. The Solar System Beyond Neptune (pdf). University of Arizona Press. pp. 335–345.  
  • ^ "Hubble Observes Planetoid Sedna, Mystery Deepens". Hubblesite, STScI-2004-14. 2004. Retrieved 2010-08-30. 
  • ^ a b "Hubble Observes Planetoid Sedna, Mystery Deepens; Long View from a Lonely Planet". Hubblesite, STScI-2004-14. 2004. Retrieved 2010-07-21. 
  • ^ "AstDys (90377) Sedna Ephemerides 2003-11-14". Department of Mathematics, University of Pisa, Italy. Retrieved 2008-05-05. 
  • ^ a b C. A. Trujillo; S. S. Sheppard (2014). "A Sedna-like body with a perihelion of 80 astronomical units". Nature 507: 471–474.  
  • ^ Chadwick A. Trujillo, M. E. Brown, D. L. Rabinowitz; Brown; Rabinowitz (2007). "The Surface of Sedna in the Near-infrared". Bulletin of the American Astronomical Society 39: 510.  
  • ^ "MPC 52733" (PDF). Minor Planet Center. 2004. Retrieved 2010-08-30. 
  • ^ Walker, Duncan (2004-03-16). "How do planets get their names?". BBC News. Retrieved 2010-05-22. 
  • ^ a b "MPEC 2004-S73 : Editorial Notice". IAU Minor Planet Center. 2004. Retrieved 2010-07-18. 
  • ^ a b c d Brown, Mike. "Sedna". Caltech. Archived from the original on 25 July 2010. Retrieved 2010-07-20. 
  • ^ a b c d e f Mike Brown, David Rabinowitz, Chad Trujillo (2004). "Discovery of a Candidate Inner Oort Cloud Planetoid". Astrophysical Journal 617 (1): 645–649.  
  • ^ a b Cal Fussman (2006). "The Man Who Finds Planets". Discover. Archived from the original on 16 June 2010. Retrieved 2010-05-22. 
  • ^ "AstDys (136199) Eris Ephemerides". Department of Mathematics, University of Pisa, Italy. Archived from the original on 4 June 2011. Retrieved 2011-05-05. 
  • ^ a b c d e   Horizons
  • ^ a b "AstDys (90377) Sedna Ephemerides". Department of Mathematics, University of Pisa, Italy. Retrieved 2011-05-05. 
  • ^ Stephen C. Tegler (2006-01-26). "Kuiper Belt Object Magnitudes and Surface Colors". Northern Arizona University. Retrieved 2006-11-05. 
  • ^ "Case of Sedna's Missing Moon Solved". Harvard-Smithsonian Center for Astrophysics. 2005-04-05. Retrieved 2005-04-07. 
  • ^ a b c d e Pál, A.; Kiss, C.; Müller, T. G.; Santos-Sanz, P.; Vilenius, E.; Szalai, N.; Mommert, M.; Lellouch, E.; Rengel, M.; Hartogh, P.; Protopapa, S.; Stansberry, J.; Ortiz, J. -L.; Duffard, R.; Thirouin, A.; Henry, F.; Delsanti, A. (2012). ""TNOs are Cool": A survey of the trans-Neptunian region. VII. Size and surface characteristics of (90377) Sedna and 2010 EK139". Astronomy & Astrophysics 541: L6.  
  • ^ Kaib, Nathan A.; Becker, Andrew C.; Jones, R. Lynne; Puckett, Andrew W.; Bizyaev, Dmitry; Dilday, Benjamin; Frieman, Joshua A.; Oravetz, Daniel J.; Pan, Kaike; Quinn, Thomas; Schneider, Donald P.; Watters, Shannon (2009). "2006 SQ372: A Likely Long-Period Comet from the Inner Oort Cloud".  
  • ^ a b c d e f   (Solution using the Solar System Barycenter and barycentric coordinates. Select Ephemeris Type:Elements and Center:@0) (saved Horizons output file 2011-Feb-04). In the second pane "PR=" can be found, which gives the orbital period in days (4.15E+06, which is ~11400 Julian years).
  • ^ a b c d e "JPL Small-Body Database Browser: 90377 Sedna (2003 VB12)" (2012-10-16 last obs). Retrieved 2014-03-30. 
  • [52]


    The Minor Planet Center, which officially catalogs the objects in the Solar System, classifies Sedna as a scattered object.[53] 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),[54] detached objects,[55] distant detached objects (DDO),[41] or scattered-extended in the formal classification by the Deep Ecliptic Survey.[56]

    The discovery of Sedna resurrected the question of which astronomical objects should be considered planets and which should not. On March 15, 2004, articles on Sedna 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,[6] and therefore cannot be considered to have cleared the neighborhood, even though no other objects have yet been discovered in its vicinity. To be a dwarf planet, Sedna must be in hydrostatic equilibrium. It is bright enough, and therefore large enough, that this is expected to be the case,[58] and several astronomers have called it one.[59][60][61][62][63]


    Sedna will come to perihelion around 2075–2076.[4] This close approach to the Sun provides an opportunity for study that will not occur again for 12,000 years. Although Sedna is listed on NASA's Solar System exploration website,[64] NASA is not known to be considering any type of mission at this time.[65] It was calculated that a flyby mission to Sedna could take 24.48 years using a Jupiter gravity assist, based on launch dates of 6 May 2033 and 23 June 2046. Sedna would be 77.27 or 76.43 AU from the Sun when the spacecraft arrives.[66]


    1. ^ a b Given the orbital eccentricity of this object, different epochs can generate quite different heliocentric unperturbed two-body best-fit solutions to the orbital period. Using a 1950 epoch, Sedna has a 12,100-year period,[2] but using a 2010 epoch Sedna has an 11,800-year period.[3] For objects at such high eccentricity, the Sun's barycentric coordinates are more stable than heliocentric coordinates.[5] Using JPL Horizons, the barycentric orbital period is approximately 11,400 years.[4]
    2. ^ As of 2014, Sedna was about 86.3 AU from the Sun;[9] Eris, the most massive known dwarf planet, and 2007 OR10, the largest object in the Solar System without a name, are currently farther from the Sun than Sedna at 96.4 AU and 87.0 AU, respectively.[11] Eris is near its aphelion (furthest distance from the Sun), whereas Sedna is nearing its 2076 perihelion (closest approach to the Sun).[10] Sedna will overtake Eris as the farthest known large object in the Solar System in 2114, but the probable dwarf planet 2007 OR10 has recently overtaken Sedna and will overtake Eris by 2045.[10]
    3. ^ a b Small Solar System bodies such as (308933) 2006 SQ372, 2005 VX3, (87269) 2000 OO67, 2002 RN109, 2007 TG422, and several comets (such as the Great Comet of 1577) have larger heliocentric orbits. But only (308933) 2006 SQ372, (87269) 2000 OO67, and 2007 TG422 have a perihelion point further than Jupiter's orbit, so it is debatable whether or not most of these objects are misclassified comets.
    4. ^ a b Different programs using different epochs and/or data sets will produce slightly different dates for Sedna's perihelion. Using a 2014 epoch, the JPL Small-Body Database has a perihelion date of 2076.[3] Using a 1990 epoch the Lowell DES has perihelion on 2479282.9591 (2075-12-11) As of 2010, the JPL Horizons (using numerical integration) indicated a perihelion date of 2076-Jul-18.[10]
    5. ^ The HST search found no satellite candidates to a limit of about 500 times fainter than Sedna (Brown and Suer 2007).[23]
    6. ^ 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.[57](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 2×1021 kg, Sedna's Λ is (2×1021/1.9891×1030)2 / 5193/2 × 1.7×1016 = 1.44×106. This is much less than 1, so Sedna is not a planet by this criterion.


    1. ^ "Discovery Circumstances: Numbered Minor Planets (90001)–(95000)". IAU: Minor Planet Center. Retrieved 2008-07-23. 
    2. ^ a b  
    In 2014, astronomers announced the discovery of

    Acquiring a larger sample of such objects would help in determining which scenario is most likely.[50] "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."[12] 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 likely dwarf planet 2007 OR10, it detected no new sednoid.[50] Subsequent simulations incorporating the new data suggested about 40 Sedna-sized objects probably exist in this region, with the brightest being about Eris's magnitude (−1.0).[50]

    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 (about 80 AU). If Sedna were captured from another planetary system that rotated in the same direction as the Solar System, then all of its population would have orbits on relatively low inclinations and have semi-major axes ranging from 100–500 AU. If it rotated in the opposite direction, then two populations would form, one with low and one with high inclinations. The perturbations from passing stars would produce a wide variety of perihelia and inclinations, each dependent on the number and angle of such encounters.[43]

    Sedna's highly elliptical orbit means that the probability of its detection was roughly 1 in 80, which suggests that, unless its discovery was a fluke, another 40–120 Sedna-sized objects would exist within the same region.[13][23] 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.[38]

    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, Antares, the Sun and Spica above


    Morbidelli and Scott Jay Kenyon have also suggested that Sedna did not originate in the Solar System, but was captured by the Sun from a passing extrasolar planetary system, specifically that of a brown dwarf about 1/20th the mass of the Sun (M).[38][39][48][49]

    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 that 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.[41][44] However, to date no direct evidence of Nemesis has been found, and many lines of evidence (such as crater counts), have thrown its existence into doubt.[45][46] 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 5 MJ lying at roughly 7,850 AU from the Sun could produce a body in Sedna's orbit.[47]

    The trans-Neptunian planet hypothesis has been advanced in several forms by a number of astronomers, including Rodney 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 traits could be explained by perturbations by a Neptune-mass object at 2,000 AU (or less), a Jupiter-mass (MJ) at 5,000 AU, or even an Earth-mass object at 1,000 AU.[37][41] 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.[42] 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.[43]

    Comparison of Sedna with the other largest trans-Neptunian objects and with Earth (all to scale)

    Mike Brown and his team favored 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 having formed in an open cluster of several stars that gradually disassociated over time.[13][36][37] That hypothesis has also been advanced by both Alessandro Morbidelli and Scott Jay Kenyon.[38][39] 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.[13] 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.[38][40]

    If Sedna formed in its current location, the Sun's original protoplanetary disc must have extended as far as 75 AU into space.[34] Also, Sedna's initial orbit must have been approximately circular, otherwise its formation by the accretion of smaller bodies into a whole would not have been possible, because 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.[35] 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.[13]

    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 thought 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.[13] 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.[32][33]


    Models of internal heating via radioactive decay suggest that Sedna might be capable of supporting a subsurface ocean of liquid water.[31]


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