90377 Sedna

{{Infobox Planet | | bgcolour=#FFFFC0 | name=90377 Sedna | image= | caption= Discovery image of Sedna (identified by the yellow arrow) | discovery=yes | discovery_ref= | discoverer=Michael E. Brown,C. Trujillo,D. Rabinowitz | discovered=November 14, 2003 | mp_name=90377 Sedna | pronounce = | alt_names= | named_after = Sedna | mp_category=Trans-Neptunian objectdetached object | orbit_ref = | spectral_type= (red) B-V=1.24; V-R=0.78 | magnitude = 21.1

Discovery and naming

Sedna (provisionally designated ) was discovered by Mike Brown (Caltech), Chad Trujillo (Gemini 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 W. M. Keck Observatory in Hawaii revealed that the object was moving along a distant highly eccentric orbit. Later the object was identified on older precovery 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. Brian Marsden, the head of the Minor Planet Center, complained that such an action was a violation of protocol, and that some members of the IAU might vote against it. However, no objection was raised as to the name itself, and no competing names were suggested. The IAU's Committee on Small Body Nomenclature formally 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. At its discovery it was approaching perihelion at 89.6 AU from the Sun, and was the most distant object in the Solar System yet observed. Eris was later detected by the same survey at 97 AU. Although 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 a bright star in its sky, too small to resolve with the human eye, it would be only 100 times brighter than a full Moon on Earth.

When first discovered, Sedna was thought to have an unusually long rotational period (20 to 50 days). and subsequent measurements from the MMT telescope suggest a much shorter rotation period, only about 10 hours, rather typical for bodies of its size.

Physical characteristics

Sedna has an absolute magnitude (H) of 1.6, and it is estimated to have an albedo of 0.16 to 0.30, thus giving it a diameter between 1,200 and 1,600 km. 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. As Sedna has no known moons, determining its mass is very difficult. However, if the above estimates for its diameter are coupled with Pluto's density of 2.0 g/cm³, the resultant estimated mass range is 1.8–4.3 x 10²¹ kg.

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. 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. 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.

Trujillo and colleagues have placed upper limits in Sedna's surface composition of 60% for methane ice and 70% for water ice. The detection of methane and water ices was confirmed in 2006 by Spitzer Space Telescope mid-infrared photometry.

Models of internal heating via radioactive decay suggest that Sedna might be capable of supporting a subsurface ocean of liquid water.

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 11 billion km into space. Also, Sedna's initial orbit must have been circular, otherwise its formation by the accretion of smaller bodies into a whole would not have been possible, as 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. 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 formed. That hypothesis has also been advanced by both Alessandro Morbidelli and Scott J. Kenyon. 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.

The trans-Neptunian planet hypothesis has been advanced in several forms by a number of astronomers, including 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 at 5,000 AU, or even an Earth-mass object at 1,000 AU. 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. 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 cloud.

It has been suggested that Sedna's orbit is the result of influence by a large binary companion to the Sun, thousands of AU distant. One such hypothetical companion is Nemesis, a dim companion to the Sun which has been proposed to be responsible for the supposed periodicity of mass extinctions on Earth from cometary impacts, the lunar impact record, and the common orbital elements of a number of long period comets. However, to date, no direct evidence of Nemesis has been found. 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 five times the mass of Jupiter lying at roughly 7850 AU from the Sun could produce a body in Sedna's orbit.

Morbidelli and Kenyon have also suggested that Sedna did not originate in our Solar System, but was captured by the Sun from a passing extrasolar planetary system, specifically that of a brown dwarf about 20 times less massive than the Sun. Subsequent simulations incorporating the new data suggested about 40 Sedna-sized objects probably exist in this region. However, this grouping is heavily questioned, and many astronomers have suggested that it, together with a few other objects (e.g. ), be placed in a new category of distant objects named extended scattered disc objects (E-SDO), detached objects, distant detached objects (DDO)

The discovery of Sedna resurrected the question of which astronomical objects should be considered planets and which should not. On March 15, 2004, articles 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 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. To qualify as a dwarf planet, Sedna must be shown to be in hydrostatic equilibrium. It is not bright enough to conclusively prove this by the absolute magnitude threshold of +1 specified by the IAU naming guidelines, so other evidence will have to be acquired. However, it remains bright enough that it is expected to be a dwarf planet.

Exploration

Sedna's perihelion will be reached around 2075–2076. This close approach to the Sun provides an opportunity of study which will not occur again for 12,000 years. Though Sedna is listed on NASA's solar system exploration website, NASA is not considering any type of mission at this time.

Notes

Taking Brown's estimates for the diameter of 1,200–1,600 km and assuming Pluto's density of 2.0 (<0.26 Eris) , Sedna is 87.2 AU from the Sun; Eris, the most massive known dwarf planet, is currently farther from the Sun than Sedna at 96.6 AU. Eris is near its aphelion (furthest distance from the Sun), while Sedna is nearing its 2076 perihelion (closest approach to the Sun).^{(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 7 kg, Sedna's Λ is (7/1.9891)² / 519^3/2 × 1.7 = 1.8. This is much less than 1, so Sedna is not a planet by this criterion. The HST search found no satellite candidates to a limit of about 500 times fainter than Sedna (Brown and Suer 2007). Using JPL Horizons, the barycentric orbital period is approximately 11,400 years.

References

External links

NASA's Sedna page (Discovery Photos)

Mike Brown's Sedna page

Category:Astronomical objects discovered in 2003 Category:Discoveries by Chad Trujillo Category:Discoveries by David L. Rabinowitz Category:Discoveries by Michael E. Brown Category:Dwarf planet candidates Category:Scattered disc and detached objects