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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