Orcus

90482 Orcus

90482 Orcus, provisional designation 2004 DW, is a trans-Neptunian object with a large moon, Vanth. With a diameter of 910 km (570 mi), it is a possible dwarf planet. The surface of Orcus is relatively bright with albedo reaching 23 percent, neutral in color and rich in water ice. The ice is predominantly in crystalline form, which may be related to past cryovolcanic activity. Other compounds like methane or ammonia may also be present on its surface. It was discovered by American astronomers Michael Brown, Chad Trujillo, and David Rabinowitz on 17 February 2004.

Orcus is a plutino, a trans-Neptunian object that is locked in a 2:3 resonance with the ice giant Neptune, making two revolutions around the Sun to every three of Neptune's. This is much like Pluto, except that the phase of Orcus's orbit is opposite from Pluto's: Orcus is at aphelion when Pluto is at perihelion and vice versa. Moreover, the aphelion of Orcus's orbit points in nearly the opposite direction from Pluto's, although the eccentricities and inclinations are similar. Because of these similarities and contrasts, along with its large moon Vanth that recalls Pluto's large moon Charon, Orcus has been regarded as the anti-Pluto. This was a major consideration in selecting its name, as the deity Orcus was the Etruscan equivalent of the Roman Pluto, and later became an alternate name for Pluto.

Discovery

Orcus was discovered on 17 February 2004, by American astronomers Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. Precovery images taken by the Palomar Observatory as early as 8 November 1951 were later obtained from the Digitized Sky Survey.

Name

The minor planet Orcus was named after one of the Roman gods of the underworld, Orcus. While Pluto was the ruler of the underworld, Orcus was a punisher of the condemned. The approved naming citation was published by the Minor Planet Center on 26 November 2004 (M.P.C. 53177). Under the guidelines of the International Astronomical Union's (IAU) naming conventions, objects with a similar size and orbit to that of Pluto are named after underworld deities. Accordingly, the discoverers suggested naming the object after Orcus, the Etruscan god of the underworld and punisher of broken oaths. The name was also a private reference to the homonymous Orcas Island, where Brown's wife had lived as a child and that they visit frequently.

On 30 March 2005, Orcus's moon, Vanth, was named after a winged female demon of the Etruscan underworld. She could be present at the moment of death, and frequently acted as a psychopomp, a guide of the deceased to the underworld.

Physical Characteristic

The absolute magnitude of Orcus is approximately 2.3. The detection of Orcus by the Spitzer Space Telescope in the far infrared and by Herschel Space Telescope in submillimeter estimates its diameter at 958.4 km (595.5 mi), with an uncertainty of 22.9 km (14.2 mi). Orcus appears to have an albedo of about 21–25 percent, which may be typical of trans-Neptunian objects approaching the 1,000 km (620 mi) diameter range. The magnitude and size estimates were made under the assumption that Orcus is a singular object. The presence of a relatively large satellite, Vanth, may change them considerably. The absolute magnitude of Vanth is estimated at 4.88, which means that it is about 11 times fainter than Orcus itself. The ALMA submillimeter measurements taken in 2016 showed that Vanth has a relatively large size of 475 km (295 mi) with albedo of about 8 percent while Orcus's has a slightly smaller size of 910 km (570 mi). Using a stellar occultation by Vanth in 2017, Vanth's diameter has been determined to be 442.5 km (275.0 mi), with an uncertainty of 10.2 km (6.3 mi). Michael Brown's website lists Orcus as a dwarf planet with "near certainty", Tancredi concludes that it is one, and is massive enough to be considered one under the 2006 draft proposal of the IAU, but the IAU has not formally recognized it as such.

Mass and density

Orcus and Vanth are known to constitute a binary system. The mass of the system has been estimated to be (6.348±0.019)×1020 kg, approximately equal to that of the Saturnian moon Tethys (6.175×1020 kg). Compared to the most massive known dwarf planet, Eris, the mass of the Orcus system is about 3.8 percent that of Eris (1.66×1022 kg). How this mass is partitioned between Orcus and Vanth depends on their relative densities. While Orcus is known to have a density of about 1.53 g/cm3, the density of Vanth is uncertain, with estimates ranging from 1.53 g/cm3, to 0.8 g/cm3.

Spectre and surface

The first spectroscopic observations in 2004 showed that the visible spectrum of Orcus is flat (neutral in color) and featureless, whereas in the near-infrared there were moderately strong water absorption bands at 1.5 and 2.0 μm. The neutral visible spectrum and strong water absorption bands of Orcus showed that Orcus appeared different from other trans-Neptunian objects, which typically have a red visible spectrum and often featureless infrared spectra. Further infrared observations in 2004 by the European Southern Observatory and the Gemini telescope gave results consistent with mixtures of water ice and carbonaceous compounds, such as tholins. The water and methane ices can cover no more than 50 percent and 30 percent of the surface, respectively. This means the proportion of ice on the surface is less than on Charon, but similar to that on Triton.

Later in 2008–2010 new infrared spectroscopic observations with a higher signal-to-noise ratio revealed additional spectral features. Among them are a deep water ice absorption band at 1.65 μm, which is an evidence of the crystalline water ice on the surface of Orcus, and a new absorption band at 2.22 μm. The origin of the latter feature is not completely clear. It can be caused either by ammonia/ammonium dissolved in the water ice or by methane/ethane ices. The radiative transfer modeling showed that a mixture of water ice, tholins (as a darkening agent), ethane ice and ammonium ion (NH4+) provides the best match to the spectra, whereas a combination of water ice, tholins, methane ice and ammonia hydrate gives a slightly inferior result. On the other hand, a mixture of only ammonia hydrate, tholins and water ice failed to provide a sCrystalline water ice on the surfaces of trans-Neptunian objects should be completely amorphized by the galactic and Solar radiation in about 10 million years. Thus the presence of crystalline water ice, and possibly ammonia ice, may indicate that a renewal mechanism was active in the past on the surface of Orcus. Ammonia so far has not been detected on any trans-Neptunian object or icy satellite of the outer planets other than Miranda. The 1.65 μm band on Orcus is broad and deep (12%), as on Charon, Quaoar, Haumea, and icy satellites of giant planets. Some calculations indicate that cryovolcanism, which is considered one of the possible renewal mechanisms, may indeed be possible for trans-Neptunian objects larger than about 1,000 km (620 mi). Orcus may have experienced at least one such episode in the past, which turned the atisfactory match. So, as of 2010, the only reliably identified compounds on the surface of Orcus are crystalline water ice and, possibly, dark tholins. A firm identification of ammonia, methane and other hydrocarbons requires better infrared spectra.

Cryovolcanism

Crystalline water ice on the surfaces of trans-Neptunian objects should be completely amorphized by the galactic and Solar radiation in about 10 million years. Thus the presence of crystalline water ice, and possibly ammonia ice, may indicate that a renewal mechanism was active in the past on the surface of Orcus. Ammonia so far has not been detected on any trans-Neptunian object or icy satellite of the outer planets other than Miranda. The 1.65 μm band on Orcus is broad and deep (12%), as on Charon, Quaoar, Haumea, and icy satellites of giant planets. Some calculations indicate that cryovolcanism, which is considered one of the possible renewal mechanisms, may indeed be possible for trans-Neptunian objects larger than about 1,000 km (620 mi). Orcus may have experienced at least one such episode in the past, which turned the amorphous water ice on its surface into crystalline. The preferred type of volcanism may have been explosive aqueous volcanism driven by an explosive dissolution of methane from water–ammonia melts. Models of internal heating via radioactive decay suggest that Orcus may be capable of sustaining an internal ocean of liquid water.

Orbit and rotation

Orcus is in a 2:3 orbital resonance with Neptune, having an orbital period of 245 years, and is classified as a plutino. Its orbit is moderately inclined at 20.6 degrees to the ecliptic. Orcus's orbit is similar to Pluto's (both have perihelia above the ecliptic), but is oriented differently. Although at one point its orbit approaches that of Neptune, the resonance between the two bodies means that Orcus itself is always a great distance away from Neptune (there is always an angular separation of over 60 degrees between them). Over a 14,000-year period, Orcus stays more than 18 AU from Neptune. Because their mutual resonance with Neptune constrains Orcus and Pluto to remain in opposite phases of their otherwise very similar motions, Orcus is sometimes described as the "anti-Pluto". Orcus last reached its aphelion (farthest distance from the Sun) in 2019 and will approach perihelion (closest distance to the Sun) in about 2143. Simulations by the Deep Ecliptic Survey show that over the next 10 million years Orcus may acquire a perihelion distance (qmin) as small as 27.8 AU.

The rotation period of Orcus is uncertain, as different photometric surveys have produced different results. Some show low amplitude variations with periods ranging from 7 to 21 hours, whereas others show no variability. The rotational axis of Orcus probably coincides with the orbital axis of its moon, Vanth. This means that Orcus is currently viewed pole-on, which could explain the near absence of any rotational modulation of its brightness. Astronomer José Luis Ortiz and colleagues have derived a possible rotation period of about 10.5 hours, assuming that Orcus is not tidally locked with Vanth. If, however, the primary is tidally locked with the satellite, the rotational period would coincide with the 9.7-day orbital period of Vanth.

Satellites

Orcus has one known moon, Vanth (full designation (90482) Orcus I Vanth). It was discovered by Michael Brown and T.-A. Suer using discovery images taken by the Hubble Space Telescope on 13 November 2005. The discovery was announced in an IAU Circular notice published on 22 February 2007. A spatially resolved submillimeter imaging of Orcus–Vanth system in 2016 showed that Vanth has a relatively large size of 475 km (295 mi), with an uncertainty of 75 km (47 mi). That estimate for Vanth is in good agreement with the size of about 442.5 km (275.0 mi) derived from a stellar occultation in 2017. Like Charon compared to Pluto, Vanth is quite large compared to Orcus, and is one reason for characterizing Orcus as the 'anti-Pluto'. If Orcus is a dwarf planet, Vanth would be the third-largest known dwarf-planet moon, after Charon and Dysnomia. The ratio of masses of Orcus and Vanth is uncertain, possibly anywhere from 1:33 to 1:12.

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Black hole

Why does black hole spin? Only three numbers define a black hole: Mass, Charge and Angular Momentum. This is known popularly by the statement “Black Holes have no hair”. But actually, Black Holes aren’t that bald. They have three hairs! But that is a discussion for some other time.

Consider the mass of star that came before the Black Hole… It was formed from a solar nebula, gaining it’s rotation by averaging out the momentum of all the individual particles in the cloud.

As mutual gravity pulled the star together, through the conservation of angular momentum, it rotated more rapidly!

When a star becomes a Black Hole, it still harbors all that mass, but now compressed down into an infinitesimally smaller space. And to conserve that angular momentum, the Black Hole’s rate of rotation speeds up! A LOT!!!

The entire history of everything, the black hole ever consumed, averages down into a single number… Spin Rate!

If the black hole could shrink down to an infinitely small size, common man’s logic dictates that it’s spin rate would blow up to infinity!

But here’s where things get interesting!

Black holes have a Speed Limit!!!

It is sort of set by the fact that the faster a black hole spins, the smaller is it’s Event Horizon. I’ll get to that in a second…

There is this region called the Ergosphere that exists between the Event Horizon and another boundary outside of that. The Ergosphere is a region outside the event horizon, where gravitational forces start to influence objects movements. Objects here can no longer remain stationary in space. Depending on the distance between object and to the event horizon, the influence can be extremely strong or very weak.

General relativity theory predicts that any rotating mass drags surrounding space-time with it. This makes the Ergosphere not just a characteristic of black holes, but it is present with all regular cosmic objects of mass, including Earth, planets or the Sun.

The Ergosphere is worth a discussion because that’s where a variety of interesting effects can occur!

Imagine the Event Horizon as a sphere in space. And then surrounding this black hole, is the Ergosphere. The faster the black hole spins, the more this Ergosphere FLATTENS OUT!

The speed limit is set by the fact that the Event Horizon eventually gets smaller and smaller at a high enough spin and reaches the Singularity. And you can’t have a naked singularity sitting in free space, exposed to the rest of the Cosmos! It would mean that the Singularity, all by itself, could emmit energy or light and someone from the outside could actually see it! And we certainly know that can’t happen. That’s the limit to how fast a black hole can spin.

Just think about it… The Black hole spins so fast that it is just about to reveal itself! But that’s impossible! The laws of Physics, like a devil, won’t let it spin any faster!

It is even more fascinating to know that Astronomers have, actually detected super massive black holes spinning at the limits predicted by these theories!

One Black Hole, at the heart of Galaxy NGC 1365 is turning at 84% of the speed of light! It has reached the cosmic speed limit and can’t turn any faster without revealing it’s Singularity.

The Cosmos is a crazy place to be in!

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Nix ( Moon)

 Nix

Nix is a natural satellite of Pluto, with a diameter of 49.8 km (30.9 mi) across its longest dimension. It was discovered along with Pluto's outermost moon Hydra on 15 June 2005 by the Pluto Companion Search Team. [citation needed] It was named after Nyx, the Greek goddess of the night. Nix is the third moon of Pluto by distance, orbiting between the moons Styx and Kerberos.

Nix was imaged along with Pluto and its other moons by the New Horizons spacecraft as it flew by the Pluto system in July 2015. Images from the New Horizons spacecraft reveal a large reddish area on Nix that is likely an impact crater.

Discovery

Nix was discovered by researchers of the Pluto Companion Search Team, using the Hubble Space Telescope. The New Horizons team had suspected that Pluto and its moon Charon might be accompanied with other moons, hence they used the Hubble Space Telescope to search for faint moons around Pluto in 2005. Since Nix's brightness is about 5,000 times fainter than Pluto, long exposure images were taken in order to find it.

The discovery images were taken on 15 May 2005 and 18 May 2005. Nix and Hydra were independently discovered by Max J. Mutchler on 15 June 2005 and by Andrew J. Steffl on 15 August 2005. The discoveries were announced on 31 October 2005, after confirmation by precovering archival Hubble images of Pluto from 2002. The two newly announced moons of Pluto were subsequently provisionally designated S/2005 P 1 for Hydra and S/2005 P 2 for Nix. The moons were informally referred to as "P1" and "P2", respectively by the discovery team.

Naming

The name Nix was approved by the International Astronomical Union (IAU) and was announced on 21 June 2006 along with the naming of Hydra in the IAU Circular 8723. Nix was named after Nyx, the Greek goddess of darkness and night and mother of Charon, the ferryman of Hades in Greek mythology. The two newly named moons were intentionally named that the order of their initials N and H honors the New Horizons mission to Pluto, similarly to how the first two letters of Pluto's name honors Percival Lowell. The original proposal for the naming of Nix was to use the classical spelling Nyx, but to avoid confusion with the asteroid 3908 Nyx, the spelling was changed to Nix, the Egyptian spelling of the name. The adjectival form of the name is Nictian (cf. Russian Никта Nikta).

Origin

Pluto's smaller moons, including Nix, were thought to have formed from debris ejected from a massive collision between Pluto and another Kuiper belt object, similarly to how the Moon is believed to have formed from debris ejected by a large collision of Earth. The ejecta from the collision would then coalesce into the moons of Pluto. However, the collisional hypothesis cannot explain how Nix maintained its highly reflective surface.

Physical Cherectristic

Nix has an elongated shape, with its longest axis measured at 49.8 km (30.9 mi) across and its shortest axis 31.1 km (19.3 mi) across. This gives Nix the measured dimensions of 49.8 km × 33.2 km × 31.1 km (30.9 mi × 20.6 mi × 19.3 mi).

Early research appeared to show that the surface of Nix is reddish in color. Contrary to this, other studies show that Nix is spectrally neutral, similar to the small moons of Pluto. The neutral spectrum of Nix signifies that water ice is present on its surface. Nix also appeared to vary in brightness and albedo, or reflectivity.

Orbit

Nix orbits the Pluto-Charon barycenter at a distance of 48,694 km (30,257 mi), between the orbits of Styx and Kerberos. All of Pluto's moons including Nix have very circular orbits that are coplanar to Charon's orbit; the moons of Pluto have very low orbital inclinations to Pluto's equator. The nearly circular and coplanar orbits of Pluto's moons suggest that they may have gone through tidal evolutions since their formation. At the time of the formation of Pluto's smaller moons, Nix may have had a more eccentric orbit around the Pluto-Charon barycenter. The present circular orbit of Nix may have been caused by Charon's tidal damping of the eccentricity of Nix's orbit, through tidal interactions. The mutual tidal interactions of Charon on Nix's orbit would cause Nix to transfer its orbital eccentricity to Charon, thus causing the orbit of Nix to gradually become more circular over time. 

Rotation

Nix is not tidally locked and tumbles chaotically similarly to all smaller moons of Pluto; the moon's axial tilt and rotation period vary greatly over short timescales. Due to the chaotic rotation of Nix, it can occasionally flip its entire rotational axis. The varying gravitational influences of Pluto and Charon as they orbit their barycenter causes the chaotic tumbling of Pluto's small moons, including Nix. The chaotic tumbling of Nix is also strengthened by its elongated shape, which creates torques that act on the object. At the time of the New Horizons flyby, Nix was rotating with a period of 43.9 hours retrograde to Pluto's equator with an axial tilt of 132 degrees — it was rotating backwards in relation to its orbit around Pluto. The rotation rate of Nix had increased by 10 percent since Nix was discovered.

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Sedna

 Sedna

90377 Sedna, or simply Sedna, is a large planetoid in the outer reaches of the Solar System that was, as of 2020, at a distance of about 85 astronomical units (1.27×1010 km; 7.9×109 mi) from the Sun, about three times as far as Neptune. Spectroscopy has revealed that Sedna's surface composition is similar to those 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 a possible dwarf planet. Sedna is approximately tied with 2002 MS4 and 2002 AW197 as the largest planetoid not known to have a moon. For most of its orbit, it is even farther from the Sun than at present, with its aphelion estimated at 937 AU (31 times Neptune's distance, or about 1.5% of a light-year), making it one of the most distant-known objects in the Solar System other than long-period comets. 

History

Sedna (provisionally designated 2003 VB12) was discovered by Michael Brown (Caltech), Chad Trujillo (Gemini Observatory), and David Rabinowitz (Yale University) on 14 November 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 were made in November–December 2003 with the SMARTS telescope at Cerro Tololo Inter-American Observatory in Chile, the Tenagra IV telescope in Nogales, Arizona, and the Keck Observatory on Mauna Kea in Hawaii. Combining those with precovery observations taken at the Samuel Oschin telescope in August 2003, and from the Near-Earth Asteroid Tracking consortium in 2001–2002, allowed accurate determination of its orbit. The calculations showed that the object was moving along a distant highly eccentric orbit, at a distance of 90.3 AU from the Sun.

Naming

Brown initially nicknamed Sedna "The Flying Dutchman", or "Dutch", after a legendary ghost ship, because its slow movement had initially masked its presence from his team. For an official name for the object, Brown settled on "Sedna", a name from Inuit mythology, which Brown chose partly because the Inuit were the closest polar culture to his home in Pasadena, and partly because the name, unlike Quaoar, would be easily pronounceable. On his website, he wrote:

Our newly discovered object is the coldest, most distant place known in the Solar System, 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.

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. The team made the name "Sedna" public before the object had been officially numbered. 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. No objection was raised to the name, and no competing names were suggested. The IAU's Committee on Small Body Nomenclature accepted the name in September 2004, 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.

Rotation and orbit

Sedna has the second longest orbital period of any known object in the Solar System of comparable size or larger, calculated at around 11,400 years. Its orbit is extremely eccentric, with an aphelion estimated at 937 AU and a perihelion at about 76 AU. This perihelion was the largest of that of any known Solar System object until the discovery of 2012 VP113. At its aphelion, Sedna orbits the Sun at a mere 1.3% of Earth's orbital speed. When Sedna was discovered it was 89.6 AU 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 aphelion at 97 AU. 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, 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.

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

PHYSICAL CHERICTRISTIC

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. 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, but by 2007 this was revised downward to less than 1,600 km after observation by the Spitzer Space Telescope. 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. Because Sedna has no known moons, determining its mass is currently impossible without sending a space probe. Sedna is currently the largest trans-Neptunian Sun-orbiting object not known to have a satellite. Only a single attempt has been made to find a satellite, and it has been suggested that there is a chance of up to 25% that a satellite could have been missed. 

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 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. Because it is a great deal closer to the Sun than was expected for an Oort cloud object, and has an inclination roughly in line with the planets and the Kuiper belt, they described the planetoid as being an "inner Oort cloud object", situated in the disc reaching from the Kuiper belt to the spherical part of the cloud.

If Sedna formed in its current location, the Sun's original protoplanetary disc must have extended as far as 75 AU into space. 

Population

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. 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.

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 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.

Classification

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

The discovery of Sedna resurrected the question of which astronomical objects should be considered planets and which should not. On 15 March 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 24 August 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, and therefore cannot be considered to have cleared the neighborhood, even though no other objects have yet been discovered in its vicinity.

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