90377 Sedna

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90377 Sedna
note)
(red) B−V=1.24; V−R=0.78[11]
20.8 (opposition)[12]
20.5 (perihelic)[13]
1.83±0.05[14]
1.3[2]

Sedna (

trans-Neptunian objects. Within the range of uncertainties, it is tied with the dwarf planet Ceres in the asteroid belt as the largest dwarf planet not known to have a moon. Its diameter is roughly 1,000 km (most likely in between those of Ceres and Saturn's moon Tethys). Owing to its lack of known moons, the Keplerian laws
of planetary motion cannot be employed for determining its mass, and the precise figure as yet remains unknown.

Sedna's orbit is

fly-by mission to Sedna near its perihelion through a Jupiter gravity assist could be completed in 24.5 years.[16]

Due to its exceptionally elongated orbit, the dwarf planet takes approximately 11,400 years, over 11 millennia, to return to the same point in its orbit around the Sun. The IAU initially considered Sedna to be a member of the scattered disc, a group of objects sent into high-eccentricity orbits by the gravitational influence of Neptune. Several astronomers who worked in the associated field contested this classification, however, as even its perihelion is far too distant for it to have been scattered by any of the currently known planets. This has led some astronomers to informally refer to it as the first known member of the inner Oort cloud. The dwarf planet is also the prototype of a new orbital class of objects named after itself, the sednoids, which include 2012 VP113, Leleākūhonua, and 2021 RR205, all celestial bodies with extremely elongated orbits.

The astronomer Michael E. Brown, co-discoverer of Sedna, believes that studying Sedna's unusual orbit could yield valuable information on the origin and early evolution of the Solar System.[17][18] It might have been perturbed into its orbit by one or more stars within the Sun's birth cluster, or captured from a nearby wandering star, or to have been sent into its present orbit through a close gravitational encounter with the hypothetical 9th planet, some time during the solar system's formation. The statistically unusual clustering to one side of the solar system of the aphelions of Sedna and other similar objects is speculated to be the evidence for the existence of a planet beyond the orbit of Neptune, which would by itself orbit on the opposing side of the Sun.[19][20][21]

History

Discovery

Sedna (

Keck Observatory on Mauna Kea in Hawaii. Combined with precovery observations taken at the Samuel Oschin telescope in August 2003, and by the Near-Earth Asteroid Tracking consortium in 2001–2002, these observations allowed the accurate determination of its orbit. The calculations showed that the object was moving along a distant and highly eccentric orbit, at a distance of 90.3 AU from the Sun.[22][19] Precovery images have since been found in the Palomar Digitized Sky Survey dating back to 25 September 1990.[2]

Naming

Brown initially nicknamed Sedna "

Quaoar, would be easily pronounceable by English speakers.[23] Brown further justified his choice of naming by stating that the goddess Sedna's traditional location at the bottom of the Arctic Ocean reflected Sedna's large distance from the Sun.[24] He suggested to the International Astronomical Union's (IAU) Minor Planet Center that any objects discovered in Sedna's orbital region in the future should be named after mythical entities in Arctic mythologies.[24]

The team made the name "Sedna" public before the object had been officially numbered, which caused some controversy among the community of amateur astronomers.

Committee on Small Body Nomenclature accepted the name in September 2004,[27] and considered that, in similar cases of extraordinary interest, it might in the future allow names to be announced before they were officially numbered.[25]

Sedna has no symbol in the astronomical literature, as

Inuktitut: ᓴᓐᓇ Sanna, the modern pronunciation of Sedna's name.[29]

Orbit and rotation

See also: List of Solar System objects most distant from the Sun
A large oval represents the orbit of Sedna around the offset Sun and smaller, more circular planetary orbits
The orbit of Sedna set against the orbits of outer Solar System objects (top and side views, Pluto's orbit is purple, Neptune's is blue)
A grid chart showing smoothly varying brightness over time
The 10,000 year apparent magnitudes of Sedna and two other sednoids

Sedna has the longest

perihelion of 76.19 AU. Near aphelion, Sedna is one of the coldest places in the Solar System, located far past the termination shock, where temperatures never exceed −240°C (−400°F) due to its extreme distance.[32][33] At aphelion, Sun as viewed from Sedna is a particularly bright star in the otherwise black sky, being about 45% as bright as the full moon as seen from Earth.[34] Its perihelion was the largest for any known Solar System object until the discovery of the sednoid 2012 VP113.[35][36] At its aphelion, Sedna orbits the Sun at a meagre 377 m/s,[37] 1.3% that of Earth's average orbital speed.[38]

When Sedna was first discovered, it was 89.6 AU[39] away from the Sun, approaching perihelion, and was the most distant object in the Solar System observed. Sedna was later surpassed by Eris, which was detected by the same survey near its aphelion at 97 AU. Because Sedna is near perihelion as of 2024, both Eris and Gonggong are farther from the Sun, at 96 AU and 89 AU respectively, than Sedna at 84 AU, despite both of their semi-major axes being shorter than Sedna's.[40][41][12] The orbits of some long-period comets extend further than that of Sedna; they are too dim to be discovered except when approaching perihelion in the inner Solar System. As Sedna nears its perihelion in mid-2076,[6][b] the Sun will appear merely as a very bright pinpoint in its sky, the G-type star too far away to be visible as a disc to the naked eye.[42]

When first discovered, Sedna was thought to have an unusually long rotational period (20 to 50 days).[43] 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.[24] However, a search for such a satellite by the Hubble Space Telescope in March 2004 found no such objects.[43][c] Subsequent measurements from the MMT telescope showed that Sedna in reality has a much shorter rotation period of about 10 hours, more typical for a body its size. It could rotate in about 18 hours instead, but this is thought to be unlikely.[10]

Physical characteristics

Sedna is a spherical shape at lower left with a crescent glow from the distant Sun at upper right
Artist's visualization of Sedna. Sedna has a reddish hue.

Sedna has a

stellar occultation by Sedna in 2013 produced similar results on its diameter, giving chord lengths 1025±135 km and 1305±565 km.[9] The size of this object suggests it could have undergone differentiation and may have a sub-surface liquid ocean and possibly geologic activity.[47]

As Sedna has no known moons, the direct determination of its mass is as yet impossible without either sending a

space probe, or perhaps locating a nearby object which is gravitationally perturbed by the planetoid. It is the largest trans-Neptunian Sun-orbiting object not known to have a natural satellite.[48] Observations from the Hubble Space Telescope in 2004 were the only published attempt to find a satellite,[49][50] and it is possible that a satellite could have been lost in the glare from Sedna itself.[51]

Observations from the SMARTS telescope show that Sedna, in

centaur 5145 Pholus, suggesting a similar region of origin.[53]

Trujillo and colleagues have placed upper limits on Sedna's surface composition of 60% for methane ice and 70% for water ice.

near-infrared spectrometer, finding indications of tholins and water ice on the surface.[55]

In 2022, low resolution near-infrared (0.7–5 μm) spectroscopic observations by the James Webb Space Telescope (JWST) revealed the presence of significant amounts of ethane ice (C2H6) and of complex organics on the surface of Sedna. The JWST spectra also contain evidence of presence of small amounts of ethylene (C2H4), acetylene (C2H2) and possibly carbon dioxide (CO2). On the other hand little evidence of presence of methane (CH4) and nitrogen ices was found at variance with the earlier observations.[56]

The possible presence of nitrogen on the surface suggests that, at least for a short time, Sedna may have a tenuous atmosphere. During a 200-year period near perihelion, the maximum temperature on Sedna should exceed 35.6 K (−237.6 °C), the transition temperature between alpha-phase solid N2 and the beta-phase seen on Triton. At 38 K, the N2 vapor pressure would be 14 microbar (1.4 Pa). The weak methane absorption bands indicate that methane on Sedna's surface is ancient, as opposed to being freshly deposited. This finding indicates that Sedna's surface never reaches a temperature high enough for methane on the surface to evaporate and subsequently fall back as snow, which happens on Triton and probably on Pluto.[47]

Origin

In their paper announcing the discovery of Sedna, Brown and his colleagues described it as the first observed body belonging to the

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.[57][58]

If Sedna formed in its current location, the Sun's original

passing star, or one of the young stars embedded with the Sun in the stellar cluster in which it formed.[19]

Brown and his team favored the hypothesis that Sedna was lifted into its current orbit by a star from the Sun's

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.[19] A study by Morbidelli and 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.[63][65]

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 Rodney Gomes and Patryk Lykawka. One scenario involves perturbations of Sedna's orbit by a hypothetical planetary-sized body in the inner Oort cloud. In 2006, simulations suggested that Sedna's orbital traits could be explained by perturbations of a Jupiter-mass (MJ) object at 5,000 AU (or less), a Neptune-mass object at 2,000 AU, or even an Earth-mass object at 1,000 AU.[62][66] Computer simulations by Patryk Lykawka have indicated 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.[67] Brown's various sky surveys have not detected any Earth-sized objects out to a distance of about 100 AU. It is possible that such an object may have been scattered out of the Solar System after the formation of the inner Oort cloud.[68]

Caltech researchers Konstantin Batygin and Brown have hypothesized the existence of a super-Earth planet in the outer Solar System, Planet Nine, to explain the orbits of a group of extreme trans-Neptunian objects that includes Sedna.[21][69] This planet would be perhaps six times as massive as Earth.[70] It would have a highly eccentric orbit, and its average distance from the Sun would be about 15 times that of Neptune (which orbits at an average distance of 30.1 astronomical units (4.50×109 km)). Accordingly, its orbital period would be approximately 7,000 to 15,000 years.[70]

Morbidelli and Kenyon have suggested that Sedna did not originate in the Solar System, but was captured by the Sun from a passing extrasolar

main-sequence star 80 percent more massive than the Sun, which, owing to its larger mass, may now be a white dwarf. In either case, the stellar encounter had likely occurred within 100 million years after the Sun's formation.[63][72][73] Stellar encounters during this time would have minimal effect on the Oort cloud's final mass and population since the Sun had excess material for replenishing the Oort cloud.[63]

Population

Main article: Sednoid
Three overlapping ovals represent the orbits
Orbit diagram of Sedna, 2012 VP113, and Leleākūhonua with 100 AU grids for scale

Sedna's highly elliptical orbit, and thus a narrow temporal window for detection and observation with currently available technology, means that the probability of its detection was roughly 1 in 80. Unless its discovery were a fluke, it is expected that another 40–120 Sedna-sized objects with roughly the same orbital parameters would exist in the outer solar system.[19][44]

In 2007, astronomer Megan Schwamb outlined how each of the proposed mechanisms for Sedna's extreme orbit would affect 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 was 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 to 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.[68]

A larger sample of objects with Sedna's extreme perihelion may help in determining which scenario is most likely.[74] "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."[17] 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 Gonggong, it detected no new sednoid.[74] 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).[74]

In 2014, Chad Trujillo and Scott Sheppard announced the discovery of 2012 VP113,[36] an object half the size of Sedna, a 4,200-year orbit similar to Sedna's, and a perihelion within Sedna's range of roughly 80 AU;[75] they speculated that this similarity of orbits may be due to the gravitational shepherding effect of a trans-Neptunian planet.[76] Another high-perihelion trans-Neptunian object was announced by Sheppard and colleagues in 2018, provisionally designated 2015 TG387 and now named Leleākūhonua.[77] With a perihelion of 65 AU and an even more distant orbit with a period of 40,000 years, its longitude of perihelion (the location where it makes its closest approach to the Sun) appears to be aligned with the directions of both Sedna and 2012 VP113, strengthening the case for an apparent orbital clustering of trans-Neptunian objects suspected to be influenced by a hypothetical distant planet, dubbed Planet Nine. In a study detailing Sedna's population and Leleākūhonua's orbital dynamics, Sheppard concluded that the discovery implies a population of about 2 million inner Oort Cloud objects larger than 40 km, with a total mass in the range of 1×1022 kg (several times the mass of the asteroid belt and 80% the mass of Pluto).[78]

Sedna was recovered from Transiting Exoplanet Survey Satellite data in 2020, as part of preliminary work for an all-sky survey searching for Planet Nine and other as-yet-unknown trans-Neptunian objects.[79]

Classification

The discovery of Sedna renewed the old question of just which

Stern–Levison parameter is estimated to be much less than 1.[d] The IAU also adopted dwarf planet as a term for the largest non-planets (despite the name) that, like planets, are in hydrostatic equilibrium and thus can display planet-like geological activity, yet have not cleared their orbital neighborhoods.[81] Sedna is bright enough, and therefore large enough, that it is expected to be in hydrostatic equilibrium.[82] Hence, astronomers generally consider Sedna a dwarf planet.[55][83][84][85][86][87]

Beside its physical classification, Sedna is also categorized according to its orbit. The Minor Planet Center, which officially catalogs the objects in the Solar System, designates Sedna only as a trans-Neptunian object (as it orbits beyond Neptune),

2000 CR105, be placed in a new category of distant objects named extended scattered disc objects (E-SDO),[90] detached objects,[91] distant detached objects (DDO),[66] or scattered-extended in the formal classification by the Deep Ecliptic Survey.[92]

Exploration

Sedna will come to perihelion around July 2076.

heliopause, the point at which the solar wind gives way to the interstellar particle wind, examining Sedna's surface would provide unique information on the effects of interstellar radiation, as well as the properties of the solar wind at its farthest extent.[93] It was calculated in 2011 that a flyby mission to Sedna could take 24.48 years using a Jupiter gravity assist, based on launch dates of 6 May 2033 or 23 June 2046. Sedna would be either 77.27 or 76.43 AU from the Sun when the spacecraft arrives near the end of 2057 or 2070, respectively.[16] Other potential flight trajectories involve gravity assists from Venus, Earth, Saturn, and Neptune as well as Jupiter.[94] Research at the University of Tennessee has also examined the potential for a lander.[95]

Appearance in video games

Sedna is featured in the popular online multiplayer game Warframe developed and published by Digital Extremes as a dwarf planet with several playable mission locations named after mythological aquatic creatures, such as Selkie and Kelpie from Scottish mythos, and Adaro. The dwarf planet is populated by the Grineer Faction, mostly from spaceships in orbit, but with a few bases on and inside the surface. This dwarf planet is where players will encounter the boss Kela de Thaym who, once, defeated, will reward the player with parts used to craft the Warframe Saryn, among other rare weapon modifications not found anywhere else. Sedna is one of the last bodies the player will reach, as the game's progression through the solar system is tied to the planet's distance from the Sun.[citation needed]

Notes

  1. ^
    barycenter (Sun+Jupiter) generates solutions that are more stable than heliocentric solutions.[31] Using JPL Horizons, the barycentric orbital period is consistently about 11,388 years, with a variation of 2 years over the next two centuries.[5]
  2. ^ a b Different programs using different epochs and/or data sets will produce slightly different dates for Sedna's perihelion as they generate instantaneous unperturbed 2-body solutions. Using a 2020 epoch, the JPL Small-Body Database has a perihelion date of 9 March 2076.[2] Using a 1990 epoch the Lowell DES has perihelion on 2479285.9863 (14 December 2075). As of 2021, the JPL Horizons (using much more accurate numerical integration) indicates a perihelion date of 18 July 2076.[6]
  3. ^ The HST search found no satellite candidates to a limit of about 500 times fainter than Sedna (Brown and Suer 2007).[44]
  4. ^ 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.[80](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 = 1.44×10−6. This is much less than 1, so Sedna is not a planet by this criterion.

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