(90377) Sedna - 6th Largest TNO?

90377 Sedna is a very large trans-Neptunian object. Sedna is the 6th largest TNO currently known. Neither its mass nor its size are well known, and the IAU has not formally designated it as a dwarf planet, although it is considered to be one by several astronomers.

Discovery

Sedna 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.

At the time of its discovery it was the largest object found in the Solar System since Pluto in 1930.

Naming

Sedna was provisionally designated 2003 VB12.

As the newly discovered object is at the coldest most distant place known in the Solar System, Mike Brown the discoverer felt 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.

In Inuit mythology, Sedna is the goddess of the sea and marine animals such as seals. A creation myth, the story of Sedna shows how she came to rule over Adlivun, the Inuit underworld.

Stats

Diameter: 1400 km (1200 - 1600 km)
Aphelion: 937 AU
Perihelion: 76.36 AU
Semi-major axis: 518 AU
Orbital Period: 11,400 years
Rotation period: 10 hrs
Date discovered: 2003.11.14
Satellite: 0
Classification: TNO, Detached Object

Orbit

Sedna has the longest orbital period of any known large object in the Solar System, other than long-period comets, calculated at around 11,400 years. Sedna's orbit is extremely eccentric, with an aphelion estimated at 937 AU and a perihelion at about 76 AU, the most distant perihelion ever observed for any Solar System object.

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).

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.

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.

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.

The detection of methane and water ices was confirmed in 2006 by Spitzer Space Telescope mid-infrared photometry. The presence of nitrogen on the surface suggests the possibility that, at least for a short time, Sedna may possess an 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.

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, as happens on Triton and probably on Pluto.

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

Origin

Sedna's exceptionally long and elongated orbit, taking approximately 11,400 years to complete, and distant point of closest approach to the Sun, at 76 AU, have led to much speculation as to 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, as 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, or even that it was captured by the Sun from a passing extrasolar planetary system.

A hypothesis suggests 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, and many lines of evidence (such as crater counts), have thrown its existence into doubt.

Another hypothesis suggests that its orbit may be evidence for an as-yet-undiscovered large planet beyond the orbit of Neptune.

How big is it?

Sedna is so far away in the outer solar system that we don't know for sure how large it is. Because all we see is a dot of light, which is sunlight reflected off the surface of the TNO. But we don't know if the object is bright because it is large or if it is bright because it is highly reflective or both.