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The discoverers of Sedna describe it as an inner Oort Cloud object, because it never enters the Kuiper Belt. Sedna never comes closer to the Sun than 76 AU. Sedna is quite an oddity: nobody expected to find an object like it in the largely empty space between the Kuiper Belt and the Oort Cloud. Possibly the Oort Cloud extends much farther in toward the Sun than previously thought, or perhaps Sedna is yet another type of object from the very early solar system, trapped between the Kuiper Belt and the Oort Cloud. Other notable features of Sedna include its size and reddish color; it is the second reddest object in the solar system, after Mars. At an estimated size of three-fourths the size of Pluto, it is likely the largest object found in the solar system since Pluto was discovered in 1930. Sedna lies extremely far from the Sun, in the coldest known region of our solar system, where the temperature never rises above minus 240 degrees Celsius (minus 400 Fahrenheit). 

The KBO is usually even colder because it approaches the Sun this closely only briefly during its 10,500 year orbit around the Sun. At its most distant, "Sedna" is 130 billion kilometers (84 billion miles) from the Sun. That is 900 times Earth's distance from the Sun.

Scientists used the fact that even the Spitzer telescope was unable to detect the heat of the extremely distant, cold object to determine that it must be no more than 1,700 kilometers (about 1,000 miles) in diameter, smaller than Pluto. By combining all available data, Brown estimates the size at about halfway between that of Pluto and Quaoar, the planetoid discovered by the same team in 2002. Until "Sedna" was detected, Quaoar was the largest known body beyond Pluto.

Because KBOs are so distant, their sizes are difficult to measure. The given diameter of a KBO depends on assumptions about how its brightness relates to its size. To estimate size based on brightness, one assumes what percentage of sunlight the object's surface reflects; this percentage is known as the albedo. Thinking that the albedo of an average KBO is similar to that of comets, astronomers calculated the sizes of KBOs based on the reflectivity of comets, which is about 4 percent. An efficient way to calculate an object's albedo is to measure the heat it radiates in the infrared. In 2004, astronomers using the Spitzer Space Telescope did a survey of KBOs at infrared wavelengths and found that they averaged about 12 percent; thus, KBOs might be smaller objects than astronomers originally thought. However, new discoveries may alter this perception.