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90377 Sedna
For other uses, see Sedna (disambiguation).
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]
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 longperiod comets.[lower-alpha 2][lower-alpha 3]
Sedna has an exceptionally long and elongated orbit, taking approximately 11,400 years to complete and a distant
point of closest approach to the Sun at 76 AU. These facts
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.
1.2 Naming
Astronomer Michael E. Brown, co-discoverer of Sedna
and the dwarf planets Eris, Haumea, and Makemake,
thinks that it is the most scientifically important transNeptunian object found to date, because understanding its unusual orbit is likely to yield valuable information about the origin and early evolution of the Solar
“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]
2 Orbit and rotation
Sedna has the longest orbital period of any known
large object in the Solar System,[lower-alpha 3] calculated at
Sedna (provisionally designated 2003 VB12 ) was discov- around 11,400 years.[4][lower-alpha 1] Its orbit is extremely
ered by Michael Brown (Caltech), Chad Trujillo (Gemini eccentric, with an aphelion estimated at 937 AU[4] and a
Artist’s impression of Sedna
The orbit of Sedna (red) set against the orbits of outer Solar System objects (Pluto’s orbit is purple).
perihelion at about 76 AU. This perihelion was 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][lower-alpha 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]
Pluto in 1930. In 2004, the discoverers placed an upper limit of 1,800 km on its diameter,[26] but by 2007
this was revised downward to less than 1,600 km after
observation by the Spitzer Space Telescope.[27] 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
visible light Sedna is one of the reddest objects in the
Solar System, nearly as red as Mars.[14] 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.[28] 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.[28] Sedna and
two other very distant objects – 2006 SQ372 and (87269)
When first discovered, Sedna was thought to have an
2000 OO67 – share their color with outer classical Kuiper
unusually long rotational period (20 to 50 days).
belt objects and the centaur 5145 Pholus, suggesting a
was initially speculated that Sedna’s rotation was slowed
similar region of origin.[29]
by the gravitational pull of a large binary companion,
similar to Pluto's moon Charon.[14] A search for such a Trujillo and colleagues have placed upper limits in
satellite by the Hubble Space Telescope in March 2004 Sedna’s surface composition of 60% for methane ice and
found nothing,[22][lower-alpha 5] and subsequent measure- 70% for water ice.[28] The presence of methane further
ments from the MMT telescope suggest a much shorter supports the existence of tholins on Sedna’s surface, berotation period of about 10 hours, rather typical for a cause they are produced by irradiation of methane.[30]
Barucci and colleagues compared Sedna’s spectrum with
body of its size.[24]
that of Triton and detected weak absorption bands belonging to methane and nitrogen ices. From these observations, they suggested the following model of the
3 Physical characteristics
surface: 24% Triton-type tholins, 7% amorphous carbon, 10% nitrogen, 26% methanol, and 33% methane.[31]
Sedna has a V-band absolute magnitude (H) of about The detection of methane and water ices was confirmed
1.8, and it is estimated to have an albedo of about in 2006 by the Spitzer Space Telescope mid-infrared
0.32, thus giving it a diameter of approximately 1,000 photometry.[30] The presence of nitrogen on the surface
km.[6] At the time of its discovery it was the intrinsi- suggests the possibility that, at least for a short time,
cally brightest object found in the Solar System since 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 or 0.000014 atm).[31]
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, which
happens on Triton and probably on Pluto.[30]
esis has also been advanced by both Alessandro Morbidelli and Scott Jay Kenyon.[39][40] 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 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.[39][41]
Models of internal heating via radioactive decay suggest
that Sedna might be capable of supporting a subsurface
ocean of liquid water.[32]
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.[33][34]
Comparison of Sedna with the other largest trans-Neptunian objects and with Earth (all to scale)
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.[38][42] 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.[43] 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
If Sedna formed in its current location, the Sun’s original protoplanetary disc must have extended as far as 75
AU into space.[35] 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.[36] 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 It has been suggested that Sedna’s orbit is the result of
influence by a large binary companion to the Sun, thouMike Brown and his team favored the hypothesis that sands of AU distant. One such hypothetical companion
Sedna was lifted into its current orbit by a star from the is Nemesis, a dim companion to the Sun that has been
Sun’s birth cluster, arguing that Sedna’s aphelion of about proposed to be responsible for the supposed periodicity
1,000 AU, which is relatively close compared to those of of mass extinctions on Earth from cometary impacts, the
long-period comets, is not distant enough to be affected lunar impact record, and the common orbital elements of
by passing stars at their current distances from the Sun. a number of long-period comets.[42][45] However, to date
They propose that Sedna’s orbit is best explained by the no direct evidence of Nemesis has been found, and many
Sun having formed in an open cluster of several stars that lines of evidence (such as crater counts), have thrown its
gradually disassociated over time.[13][37][38] That hypoth- existence into doubt.[46][47] 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.[48]
Morbidelli and 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☉).[39][40][49][50]
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.[51] 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).[51]
In 2014, astronomers announced the discovery of 2012
VP113,[19] an object half the size of Sedna in a 4200-year
Main article: Sednoid
orbit similar to Sedna’s and a perihelion within Sedna’s
Sedna’s highly elliptical orbit means that the probability
range of roughly 80 AU,[52] which led some to speculate
that it offered evidence of a trans-Neptunian planet.[53]
6 Classification
Artist’s conception of the surface of Sedna, with the Milky Way,
Antares, the Sun and Spica above
of its detection was roughly 1 in 80, which suggests that,
unless its discovery was a fluke, another 40–120 Sednasized 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.[39]
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.[44]
Sedna compared to some other very distant orbiting bodies
The Minor Planet Center, which officially catalogs the
objects in the Solar System, classifies Sedna as a scattered
object.[54] 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),[55] detached objects,[56] distant detached objects (DDO),[42] or scattered-extended in
the formal classification by the Deep Ecliptic Survey.[57]
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 orAcquiring a larger sample of such objects would help bit. Sedna has a Stern–Levison parameter estimated to be
in determining which scenario is most likely.[51] “I call much less than 1,[lower-alpha 6] and therefore cannot be conSedna a fossil record of the earliest Solar System”, said sidered 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,[59] and several astronomers
have called it one.[60][61][62][63][64]
[4] 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
Sedna will come to perihelion around 2075–
2076.[lower-alpha 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,[65] NASA is
not known to be considering any type of mission at this
time.[66] 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.[67]
See also
• List of Solar System objects most distant from the
Sun in 2015
[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 .[58](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.
[1] 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 11,800year 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] 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
10 References
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External links
• Media related to Sedna 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
Text and image sources, contributors, and licenses
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