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Planetary News: Trans-Neptunian Objects (2004)

Beyond the Kuiper Belt: Astronomers Announce Discovery of Farthest Object Orbiting the Sun

By Amir Alexander
15 March 2004

The most distant Sun-orbiting object ever observed from Earth was detected last November by a team of researchers surveying the outer reaches of the solar system. The object may be as large as three quarters the size of Pluto, and even on its closest approach to the Sun it is still twice as far away as that farthest of planets. Officially designated 2003 VB12 by the International Astronomical Union, the new object’s discoverers propose to permanently name it “Sedna” after the Inuit goddess of the deep.

ASedna was discovered by a veteran team that specializes in detecting objects at the outer reaches of the solar system. Michael Brown of Caltech, David Rabinowitz of Yale, and Chad Trujillo of the Gemini Observatory in Hawaii, have previously detected giant objects in the Kuiper Belt, the mass of rocky debris orbiting the Sun in the region of Pluto. Using the sixty year old 48 inch Samuel Oschin Telescope at Mount Palomar Observatory near San Diego, in conjunction with a state of the art QUEST CCD camera, the team has been conducting a systematic sky survey in search of distant space rocks orbiting the Sun. Since the start of the survey in the summer of 1991, they have already detected three massive Kuiper Belt Objects (KBO’s) designated 2002AW197, Quaoar, and 2004DW, each estimated to be between half and three quarters the size of Pluto.

Like the other objects in the survey, Sedna was detected by pointing the Samuel Oschin telescope to a specific location in the sky, and recording three separate images, 90 minutes apart. When those images are compared, the fixed stars appear in precisely in the same location on all three takes. An object orbiting the Sun, however, will have moved ever so slightly - enough to be detected and identified. Since 2001 the survey has been proceeding systematically, taking repeated images of each tiny patch of the skies. By 2005, according to Trujillo, he and his colleagues hope to completely cover the entire band of sky visible from Mount Palomar.

Once Sedna was detected in this manner in November of 2003 Brown and his team have been able to track its orbital history back to 2001 by looking at older records. Sedna is, however, different from any of the other objects found by the survey, and according to Brown may not be a KBO at all. Even at its closest, Sedna comes no nearer to the Sun than 76 astronomical units (AU), each AU equaling the average distance of the Earth from the Sun. In contrast, says Brown, the Kuiper Belt has a well defined outer edge at about 55 AU. Furthermore Sedna’s elongated orbit takes it far beyond any known KBO, to a distance of 900 AU from the Sun. It takes Sedna over ten thousand(!) years to complete each orbit, as against two to three hundred years for Pluto and its Kuiper Belt companions.

David Jewett of the International Astronomical Union agreed that Sedna's orbit is unique among the objects discovered so far at the outer edge of the solar system. While its elongated orbit takes it as far as 900 Astronomical Units (AU) from the Sun, are other known objects that range even further. An object designated 2000 0067 for example travels as far as 1010 AU before turning back towards the Sun. Sedna, however, is unique because it was discovered at a greater distance from the Earth than any solar system object ever was. In addition, coming no closer to the Sun than 76 AU, its minimum distance from the Sun (or perihelion) is greater than that of any known object.

Brown prefers to think about the Sedna not as the outer frontier of the Kuiper Belt, but rather as the inner edge of the Oort Cloud - that thick envelope of billions of orbiting space rocks that surrounds the solar system. The Oort cloud is much farther away than the Kuiper Belt, and is generally estimated to occupy the region from 5000 AU to 100,000 AU from the Sun – about half way to the nearest star. Although it has never been observed from Earth, its presence is deduced from the orbits of long-period comets that originate in the distant region.

Although Sedna never approaches the Oort Cloud as commonly delineated, Brown hypothesizes that the newly discovered object is part of a previously unknown “Inner Oort Cloud.” Just as the Oort Cloud itself was probably formed by passing stars sending rocky ejecta from the solar system back into Solar orbit, so the supposed “Inner Oort Cloud” was formed by a transient star, though this one passing much closer to the Sun than the others. If Brown proves correct, Sedna will be the first Oort Cloud Object ever observed from the Earth.

Jewett, an expert on KBO's, expressed a more guarded view of the significance of the discovery. "What is special about Sedna, he said, is that unlike almost all Kuiper Belt Objects it is too far away to be affected by the gravitational pull of Neptune." Nevertheless, he pointed out, even in this Sedna is not the first: a KBO designated 2000 CR105 discovered three years ago also remains too distant to be affected by Neptune. At the same time both Sedna and 2000 CR105 are too close to the Sun to be influenced by passing Stars and galactic tides, as is the case in the Oort Cloud. "Sedna and 2000 CR105 represent two instances of a new class of objects that is neither classical KBO nor classical Oort Cloud Object" said Jewett. There may be many more."

Regarding how Sedna arrived at its current orbit, Jewett said it is too early to tell. It may be that a passing star did affect its orbit as Brown suggests. It is also possible that other large mass objects that were present in the early solar system but are gone today may have pushed Sedna out to its present location. "Scientifically," said Jewett, "the significant thing about this discovery is that it could teach us something important about the dynamics of the early solar system." "It is a truly great object," said Jewett " and I wish I would have discovered it myself."

While Sedna is undoubtedly large, determining its exact size has proved difficult. One method Brown and his colleagues tried was to estimate the object’s size based on its heat radiation. For any given temperature, at a known distance, a large object would radiate more heat than a small one. Since Sedna’s distance is accurately known, and since the temperature in that region is –240 degrees Celsius (-400 degrees Fahrenheit), the object’s size could theoretically be deduced from the amount of heat it radiates. Unfortunately neither the IRAM 30 meter telescope in Germany, rigged to detect infrared (heat) radiation, nor the Spitzer Space Telescope, could detect any heat radiating from Sedna. This means that the object is no more than 1800 kilometers in diameter, since had it been larger its heat radiation would have been detectable.

Another method for estimating the size of distant objects is based on their brightness. If the distance of an object and its reflectivity (or albedo) are known, then its brightness can provide a good measure of its size. But while Sedna’s distance is well known, its reflectivity has proved elusive since astronomers cannot determine what it is made of. Unlike the large KBO’s Sedna does not have large amounts of water and methane ice. Although it is relatively bright, its light has a distinct red tinge, second only to Mars in its redness. For now, Brown and his team have not yet determined what to make from this assortment of clues. Sedna’s composition, and hence its reflectivity, remain unknown for the moment.

Nevertheless, despite the inconclusive evidence from reflectivity and heat measurements, Brown is convinced that Sedna is not much smaller than its maximum size of 1800 kilometers in diameter would allow. “We are virtually certain that the size is larger than the 1250 kilometer size of Quaoar” he stated, but added cautiously that “this object has shown many unexpected characteristics, so we can’t rule out a smaller size.”

When Sedna last passed unnoticed near the Sun 10,000 years ago, our planet was just emerging from the throes of its latest ice age. Not a city had been built, and not a word written down by humans, who were just beginning their long march towards civilization. What the world or humanity will be like when Sedna makes its next visit 10,000 years hence, no one can imagine. But right now, during its present approach to the Sun, Sedna has been observed, recorded, and granted a unique distinction: it is the most distant object in the solar system ever observed by humans from the Earth.