90377 Sedna

FIELDS OF STUDY: Sub-planetary Astronomy; Cosmology

ABSTRACT: 90377 Sedna is a large, icy trans-Neptunian object (TNO) that orbits the sun. It is located in the inner Oort cloud far beyond Neptune. Sedna is a rounded object likely about 1,600 kilometers (about 1,000 miles) in diameter. Therefore, it is probably a dwarf planet. However, its distant location makes it difficult for scientists to officially classify it as such. Sedna’s location and unique orbit raise questions about the origin and nature of objects beyond the planets of the solar system.

PRINCIPAL TERMS

• aphelion: the point farthest from the sun along an object’s orbital path.
• dwarf planet: a celestial body with enough mass to attain a nearly round shape but lacking the gravity to keep its neighborhood free of other space objects. Dwarf planets orbit the sun and do not act as satellites to other space objects.
• Oort cloud: a vast, spherical area of space far beyond Neptune that is filled with trillions of icy objects and comets.
• perihelion: the point closest to the sun along an object’s orbital path.
• trans-Neptunian object (TNO): a space object that orbits beyond Neptune at a distance more than 30 astronomical units (AU)—about 4.5 billion kilometers, or 2.8 billion miles—from the sun.

First Look into the Oort Cloud

Far beyond the orbit of Neptune lies a vast, spherical region of space that surrounds the solar system. This area, called the Oort cloud, is located at the very edge of the solar system. Most experts place the Oort cloud at a distance about 50,000 to 100,000 AU from the sun. The Oort cloud is more distant in space than even the Kuiper Belt, a disk-shaped region of space just beyond the orbit of Neptune. The Oort cloud is named for famed Dutch astronomer Jan Hendrik Oort (1900–92), who first hypothesized its existence.

Scientists believe the Oort cloud is filled with trillions of icy objects and comets. These are celestial remains from the formation of the solar system about 4.6 billion years ago. Because these objects are located at such an extreme distance, the sun’s gravitational influence on them is significantly less than on closer objects. In fact, astronomers believe the sun’s influence is quite weak in the Oort cloud. Thus, objects there are influenced by gravity from passing space objects.

Moving objects such as stars, star clusters, and even yet-to-be-discovered planets are believed to create gravitational disturbances that can change the orbits of Oort cloud objects. Some are ejected into interstellar space. Others are pushed into the inner solar system as long-period comets (comets that take at least two hundred years to orbit the sun).

Although astronomers had speculated about the Oort cloud since the early 1950s, its existence was not confirmed until 2003. At that time, astronomers at Palomar Observatory near San Diego, California, discovered 90377 Sedna.

The Palomar team was conducting an ambitious five-year project to systematically image the night sky using the forty-eight-inch Samuel Oschin Schmidt Telescope and the Palomar QUEST large-area CCD camera. The team was searching for previously undiscovered trans-Neptunian objects (TNOs). These are objects other than comets that orbit the sun at a greater average distance than the planet Neptune.

In November 2004, the equipment produced images of a large, rounded object orbiting at a distance nearly ninety times greater than that between Earth and the sun. The team studied the images carefully and referred to archived images from prior years. After performing a variety of calculations, they determined that the object was about 1,800 kilometers (1,118 miles) in diameter. They also found that it was situated at a distance greater than 100 AU (15 billion kilometers or 9.3 billion miles) from the sun.

Because this distance is far beyond the planetary region of the solar system, which ends at about 50 AU from the sun, the team realized they had identified the most distant object yet observed in the solar system. It was also the first observable "planetoid," or minor planet, in the inner Oort cloud.

The team named the icy object Sedna, after the Inuit goddess who rules over the frigid Arctic Ocean. Later, the International Astronomical Union (IAU) gave the object the official designation 2003 VB12.

Characteristics of Sedna

In the years since Sedna’s discovery, astronomers have been able to improve estimates of Sedna’s size and characteristics. Some of this information has been derived from observations by the Herschel Space Observatory, a space telescope developed and managed by the European Space Agency (ESA). Herschel uses infrared and other invisible radiation to learn about distant stars and galaxies.

Information from Herschel and other research revealed that Sedna is smaller than originally estimated, with a diameter no larger than about 1,000 kilometers (620 miles). Thus, it is smaller than Pluto. Sedna’s surface area is bright and quite red. In fact, it is nearly as red in color as the planet Mars. This suggests it has long been exposed to ultraviolet radiation. Its consistent coloration also suggests that it has not experienced impacts by other space bodies, which typically causes pits and craters that reveal more about the inner composition of a space object.

Sedna’s surface temperatures are believed to be extremely icy. Its estimated average temperature is −240 degrees Celsius (−400 degrees Fahrenheit). Scientists believe its surface is composed of both methane ice and water ice.

Unique Orbital Path

One of the most interesting characteristics of Sedna is its eccentric orbit. Sedna travels completely in the outer solar system. It orbits the sun along a long, oval path that takes about 11,400 years to complete. Its aphelion, or farthest point from the sun, is 937 AU, while its perihelion, or closest point, is 76.36 AU. One unresolved question about Sedna is if it classifies as a dwarf planet. The IAU created the dwarf planet class of space objects in 2006 to differentiate small, rounded planetary bodies from the eight regular planets of the solar system. IAU Resolution B5 defined dwarf planets as space objects that orbit the sun, have enough mass and gravity to attain a mostly round shape, and lack the gravitational power to clear other space objects out of their orbit. Additionally, dwarf planets are not satellites (moons) to other space objects.

Dwarf planets include Pluto, Eris, and Ceres. Although some researchers state with "near certainty" that Sedna should be classified as a dwarf planet, its extreme distance makes it difficult to officially classify it as such.

Origins and Evolution of Oort Cloud Objects

Sedna’s extreme distance and unique path of travel raise questions about its origin and evolution. Experts believe that Oort cloud objects are remnants from the formation of the solar system 4.6 billion years ago. Asteroids traveling in the asteroid belt are known to collide with each other and break into smaller pieces over time. However, objects in the Oort cloud seem relatively untouched and intact since their formation. This offers astronomers a unique look at the forces that shaped the early solar system. For example, because Sedna and similar objects are located so far from Neptune, most researchers do not believe that they were pushed into space by the gravity around Neptune. One theory is that an as-yet-to-be-identified planet exerted gravitational influence on Sedna and other objects in the Oort cloud. Other researchers have theorized that passing stars or star clusters may have played a role. Regardless of its origins, Sedna raises many questions about the origin and history of the early solar system.

In 2014, the announcement of the discovery of another TNO, known as 2012 VP113, gave scientists further proof of the existence of a greater number of such objects in the Oort cloud. Some also argued that the existence of 2012 VP113, in combination with Sedna, gave greater credence to the theory that a "Super Earth" could exist further out whose gravity is affecting their more unique orbits.

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