(90377)Sedna
Sedna (symbol: .svg){kind=small}
) is the lower body of the solar system number 90377; specifically it is a transneptunian object. In 2012 it was approximately three times further from the Sun than Neptune. During most of its orbit it is even further away from the Sun, with its estimated 960 Ua astronomical units—32 times the distance of Neptune—so it is one of the most distant objects known of the solar system, which are not long-term comets. The exceptionally long and elongated orbit of Sedna, which takes about 11 400 years to complete, and its distant point of maximum approximation to the Sun, to 76 Ua, have led to much speculation as to its origin.
It was discovered on November 14, 2003 from the Mount Palomar Observatory. Sedna's name comes from the mythological Eskimo goddess of the sea and marine animals. Hostile to men and endowed with gigantic height, Sedna was doomed to live in the cold depths of the Arctic Ocean, near Canada.
Spectroscopy revealed that its surface composition is similar to that of other trans-Neptunian objects, being largely a mixture of ice and tholin with frozen methane and nitrogen. Its surface is one of the reddest in the solar system. Neither its mass nor its size is well known, and the International Astronomical Union has not formally recognized it as a dwarf planet, although several astronomers estimate it to be.
The Minor Planet Center places you in the scattered disk, a group of objects sent into greatly elongated orbits by Neptune's gravitational influence. However, this classification is disputed, as Sedna never gets close enough to Neptune to affect it, leading some astronomers to conclude that it is actually the first known member of the densest and deepest region of the great galaxy. oort cloud. Others speculate that it could have been pushed into its current orbit by a transiting star, perhaps from within the Sun's birth group, or even captured from another star system. Another hypothesis suggests that its orbit may be evidence of another planet beyond Neptune's orbit. Astronomer Michael E. Brown—co-discoverer of Sedna and the dwarf planets Eris, Haumea, and Makemake—believes that it is the most important trans-Neptunian object found to date, as studying its unusual orbit can provide valuable information about its origin and location. early evolution of the solar system.
Discovery and name
Mike Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University on November 14, 2003 discovered Sedna—provisionally designated 2003 VB12—as part of a study that It began in 2001 with the Samuel Oschin Telescope at the Palomar Observatory, using Yale's 160-megapixel Palomar Quest camera. They observed an object moving at 4.6 arc seconds in 3.1 hours relative to the stars, indicating that its distance was about 100 AU. Follow-up observations in November-December 2003 with the SMARTS telescope at the Cerro Tololo Observatory in Chile, as well as with the Tenagra IV telescope at the W. M. Keck Observatory in Hawaii, revealed that the object was moving along a distant orbit. highly eccentric. The object was later identified in earlier precoverage images taken by the Samuel Oschin Telescope as well as in images from the Near Earth Asteroid Tracking program. These earlier positions extended its known orbital arc and allowed for a more precise calculation of the orbit.
“Our newly discovered object is the coldest and furthest known place in the solar system," Mike Brown said on his website, "so I feel it's appropriate naming it after Sedna, the Inuit goddess of the sea, who was believed to live at the bottom of the frigid Arctic Ocean." Brown also suggested to the International Astronomical Union's Minor Planet Center that objects discovered in the Sedna's orbital region in the future should also be named after entities in Arctic mythology. The team released the name "Sedna" before the object had been officially numbered. Brian Marsden, director of the Minor Planet Center, said that such an action was a violation of protocol, and that some IAU members might vote against it. However, there were no objections to the name, and no others were proposed. The IAU Committee on the Nomenclature of Minor Bodies formally accepted the name in September 2004, and also considered that, in similar cases of extraordinary interest, it might in future allow names to be announced before they were officially numbered.
Orbit and rotation
Sedna has the longest orbital period of any known large object in the solar system, estimated to be about 11,400 years. Its orbit is extremely eccentric, with an aphelion estimated to be 937 AU and a perihelion of about 76 au, being the largest for any known solar system object. When discovered it was approximately 89.6 au from the Sun, and was the most distant object observed in the solar system. The same team of researchers later discovered Eris at 97 au. Although the orbits of some long-period comets extend beyond that of Sedna, they are too diffuse to be discovered except as they approach perihelion in the inner Solar System. Even if Sedna reaches perihelion around 2076, the Sun would appear simply as a very bright star in its sky, only a hundred times brighter than the full moon on Earth, and too far away to be visible as a disk with the naked eye.
Sedna was thought to have an unusually long rotational period—20 to 50 days—at the time it was discovered. It was initially speculated that its rotation was slowed by a binary companion, such as Pluto's moon Charon. In 2004 the Hubble Space Telescope searched for that satellite, but found nothing, and subsequent measurements by the telescope suggested much shorter rotation periods—approximately 10 h—quite typical for a body of its size.
Physical characteristics
Sedna has an absolute magnitude V band —H— of about 1.8 and is estimated to have an albedo of about 0.32, giving it a diameter of about 1000 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 estimated the maximum limit of its diameter at 1,800 km, but in 2007 this value was revised and reduced to less than 1,600 km later. from being observed by the Spitzer Space Telescope. In 2012, measurements by the Herschel Space Observatory suggested that Sedna's diameter is 995 ± 80 km, which would make it smaller than Charon. As Sedna has no known satellites, determining its mass is currently impossible without sending a space probe. However, if Pluto's density of 2.0 g/cm³ is taken as a reference in addition to the above calculations for its diameter, the estimated mass range is approximately 1 x 1021 kg.
Observations from the SMARTS telescopes show that in visible light Sedna is one of the reddest objects in the solar system, almost as red as Mars. It was suggested that Sedna's deep red color was due to a surface layer of hydrocarbon mud, or tholin, formed from simpler organic compounds after long exposure to ultraviolet radiation. Its surface is homogeneous in color and spectrum, which may be due to Sedna, unlike One of the closest objects to the Sun, it is rarely struck by other bodies, which would expose bright parts of freshly frozen material, as in Asbolo. Sedna and two other very distant objects—2000 OO67 and 2006 SQ372—share their color with the classical Kuiper belt objects and the centaur Folo, suggesting an origin in a similar region.
Upper limits to the composition of Sedna's surface have been set at 60% frozen methane and 70% ice. The presence of methane also supports the existence of tholins on Sedna's surface, as they are produced by methane irradiation. Sedna's spectrum was compared to that of Triton and weak absorption bands belonging to frozen methane and nitrogen were detected. From these observations, the following model of the surface was suggested: 24% Triton-like tholins, 7% amorphous carbon, 10% nitrogen, 26% methanol, and 33% methane. The detection of methane and Frozen water was confirmed in 2006 by mid-infrared photometry from the Spitzer Space Telescope. The presence of nitrogen on the surface suggests the possibility that, at least for a short time, Sedna may have possessed an atmosphere. Over a period of about two hundred years near perihelion Sedna's maximum temperature must have exceeded 35.6 K (-237.6 °C), the transition temperature between the alpha-solid phase of N2 and the beta phase seen on Triton. At 38 K, the vapor pressure of N2 would be 14 microbar (0.000 014 atmospheres). However, its deep red spectral tilt is indicative of a high concentration of organic matter in its surface, and its faint methane absorption bands indicate that the methane on Sedna's surface is old, rather than recently deposited. This means that Sedna is too cold for methane to evaporate from the surface and then fall back as snow, as it does on Triton and probably Pluto.
Models of internal heating through radioactive decay suggest that Sedna might be able to support a subterranean ocean of liquid water.
Origin
In the paper announcing the Sedna find, the discoverers described it as the first observed body belonging to the Oort cloud, a hypothetical cloud of comets believed to exist at a distance of about one light-year from the Sun. They observed that, unlike scattered disk objects such as Eris, Sedna's perihelion—76 au—is too far away to have been influenced by Neptune's gravity. Because it is much closer to the Sun than would be expected for an object of the Oort cloud and having a tilt roughly similar to that of the planets and Kuiper belt objects, they described it as an "inner Oort cloud object", located in the disk from the Kuiper belt to the spherical part of the cloud.
If Sedna formed at its present location, the original protoplanetary disk must have extended as far as 75 au from the Sun. Also, Sedna's initial orbit must have been circular because otherwise it could not have formed by accretion of bodies smaller, since the large relative velocities between the planetesimals would have been too damaging. Therefore, a gravitational interaction with another body must have caused its current eccentric orbit. In the initial paper, the discoverers suggested three possible candidates for the perturbing body: a planet hidden beyond the Kuiper belt, a star in transit or one of the young stars integrated with the Sun in the star cluster in which it formed. long period—is not far enough away to be affected by transiting stars at their current distances from the Sun. They proposed that Sedna's orbit is best explained if the Sun formed in an open star cluster that gradually dissociated with time. Other astronomers later advanced this hypothesis. Computer simulations show that multiple transits between stars The youngsters of such open clusters could cause many objects to orbit Sedna-like. first 100 million years of the existence of the solar system approximately.
Various astronomers advanced the trans-Neptunian planet hypothesis in various ways. One scenario involves perturbations of Sedna's orbit by a hypothetical planet-sized body within the Oort cloud. Recent simulations show that Sedna's orbital characteristics could be explained by perturbations from a Neptune-mass object at 2,000 au — or less — a Jupiter-mass at 5,000 au, or even an Earth-mass object at 1,000 au. Computer simulations suggested that Sedna's orbit may have been caused by an Earth-sized body ejected outward by Neptune early in the formation of the solar system and would today be in an elongated orbit between 80 and 170 AU from the Sun. Several surveys of the sky have been conducted without detecting Earth-sized objects at a distance of about 100 AU. However, such an object may have been ejected out of the solar system after the formation of the inner Oort cloud.
Some astronomers have suggested that Sedna's orbit is the result of the influence of a companion to the Sun thousands of astronomical units away. One such hypothetical star is Nemesis, a dark companion to the Sun proposed to be responsible for the alleged periodicity of mass extinctions on Earth from cometary impacts, the lunar impact record, and the orbital commonalities of a long-period comet series. However, there is to date no direct evidence for the existence of Nemesis and many lines of investigation, for example those involving the rate of cratering, have called into question its existence. Sol could cause an object to go into an orbit like Sedna's.
Other hypotheses suggest that Sedna did not originate in our solar system, but was captured by the Sun from a transiting extrasolar planetary system, specifically that of a brown dwarf with a mass about twenty times less than the Sun.
Population
Sedna's highly elliptical orbit indicates that the chance of detecting it was about 1 in 80, suggesting that, unless its discovery was a fluke, between forty and one hundred and twenty objects the size of Sedna. This suggests that Sedna could be the first of a series of icys located between the Kuiper belt and the Oort cloud called the "Sedna population". Another object, (148209) 2000 CR105, has a similar orbit, but less extreme: it has a perihelion of 44.3 AU, an aphelion of 394 AU, and an orbital period of 3,240 years. It could be affected by the same processes as Sedna.
Each of the proposed mechanisms for Sedna's extreme orbit would leave a distinctive imprint of the structure and dynamics of a broader population. If a trans-Neptunian planet was responsible, all those objects would share roughly the same perihelion (≈ 80 au). If Sedna was captured from another planetary system rotating in the same direction as the solar system, then all members of the Sedna population would have relatively low inclinations and possess semi-major axes ranging from 100 to 500 au. If it rotated in the opposite direction, two populations would be formed, one with a low inclination and one with a high one. The gravity of the disturbing stars would produce a wide variety of perihelions and inclinations, depending on the number and angle of such encounters.
Getting a larger sample of those objects could help determine the most likely scenario. "I would say that Sedna is an early solar system fossil record," Brown said in 2006. "Over time, when find other fossil records, Sedna will help us understand how the Sun formed and the number of stars close to the Sun when it formed." sedna. Although the study was sensitive to motions at 1000 au and discovered the dwarf planet candidate 2007 OR10, no new bodies in orbits like Sedna's were detected. Later simulations incorporating the new data suggest that in this region they probably exist around of forty Sedna-sized objects. Another study in 2011 found eighteen outer solar system objects, fourteen of which were unknown trans-Neptunian objects. Several of these objects could be in hydrostatic equilibrium and therefore be dwarf planets. The study concluded that, compared to the main Kuiper belt population and for larger objects (H < 4.5 mag), the scattered disc population appears to have few times as many objects, while the Sedna population may be several times greater.
Classification
The Minor Planet Center, which officially catalogs objects in the solar system, classifies Sedna as a scattered object. However, this group is strongly disputed and many astronomers have suggested that, along with some other objects—for example, (148209) 2000 CR105—, will be placed in a new category of distant objects that could be called “extended scattered disk objects” —E-SDO—, “detached objects”, “objects dispersed at a distance" —DDO— or "dispersed-extended" in the official classification of the Deep Ecliptic Survey.
The discovery of Sedna raised again the question of which astronomical objects should be considered planets and which should not, something that was already raised with the Quaoar discovery. On March 15, 2004, several news agencies reported that "the tenth planet had been discovered." However, on August 24, 2006 the International Astronomical Union in Prague redefined what should be understood by a planet, demanding that it should have cleared the neighborhood around its orbit —orbital dominance. Sedna is estimated to have a Stern–Levison parameter much less than 1, and therefore cannot be considered to have cleared its environment, although no other objects were discovered in its vicinity. In order to qualify as a dwarf planet, Sedna had to show hydrostatic equilibrium, that is, be essentially spherical. It is bright enough, and therefore large enough, that this is expected to be the case.
Exploration
Sedna will reach perihelion around 2075-2076. This approach to the Sun offers a study opportunity that will not occur again for 12,000 years. Although Sedna is listed on NASA's Solar System Exploration website, the agency is not considering any type of mission to Sedna in 2012.