Sedna is one of the most distant known objects in the solar system, discovered in 2003 orbiting far beyond the main belt of the Kuiper Belt. It is officially designated as a Trans-Neptunian Object (TNO) but is frequently referred to by astronomers as a candidate dwarf planet or a planetoid. The question of its true classification highlights the complex nature of defining solar system bodies at the farthest reaches of the Sun’s gravitational influence. The official designation of Sedna remains a major debate among planetary scientists, who must rely on indirect observations to determine its physical properties. Its ambiguous status stems from formal rules that require specific physical characteristics which are challenging to confirm from Earth.
Defining Sedna: The Extreme TNO
Sedna is an icy, reddish body that represents a unique class of objects known as Sednoids, which are detached from the gravitational influence of Neptune. This object possesses one of the most eccentric and elongated orbits in the solar system. At its closest approach to the Sun, or perihelion, Sedna comes no closer than about 76 astronomical units (AU), which is well past the orbit of Neptune.
The orbit stretches outward to an aphelion, its farthest point from the Sun, reaching approximately 936 to 1,007 AU. Sedna is so distant that it takes around 11,400 to 12,600 Earth years to complete a single trip around the Sun. This massive distance means Sedna currently resides in the inner region of what is theorized to be the Oort Cloud, though it is sometimes categorized as part of the Scattered Disc.
Observations suggest Sedna is composed of a mixture of solid ices, including water, methane, and nitrogen, which contributes to its reddish hue. Its diameter is estimated to be somewhere between 1,000 and 1,700 kilometers. However, the lack of a natural satellite makes precise mass and density measurements impossible, meaning many of its characteristics must be inferred rather than directly measured.
Establishing the Dwarf Planet Criteria
The International Astronomical Union (IAU) established a formal definition for a dwarf planet in 2006 to categorize bodies like Pluto and other large Trans-Neptunian Objects. For a celestial body to be officially classified as a dwarf planet, it must meet three specific criteria.
The first criterion is that the object must be in orbit around the Sun, confirming its status as a member of our solar system. The second requirement dictates that the object must have sufficient mass for its own gravity to pull it into a shape of hydrostatic equilibrium, which is observed as a nearly round or spherical form. This shape distinction separates dwarf planets from irregularly shaped asteroids and smaller minor bodies.
The third and most distinguishing criterion is that the body must not have “cleared the neighborhood” around its orbit. To clear its neighborhood means that the object must be gravitationally dominant enough to either absorb or eject other significant objects from its orbital path. Planets, by definition, satisfy this third criterion, while dwarf planets do not.
This third rule is the reason that objects like Pluto, which orbit in the crowded Kuiper Belt alongside other large bodies, were reclassified as dwarf planets. The criteria thus define a dwarf planet as a body that is large enough to be round but not massive enough to be the sole dominant gravitational influence in its orbital zone.
Sedna’s Classification: Candidate Status and Ambiguity
When Sedna is measured against the IAU’s three criteria, it easily satisfies two of them. It unquestionably meets the first rule, as it orbits the Sun, even if that orbit is enormously long and highly eccentric. Sedna also meets the third criterion, as it shares its orbital zone with numerous other Trans-Neptunian Objects and has not cleared its neighborhood of other celestial bodies.
The uncertainty surrounding Sedna’s status rests entirely on the second criterion: the requirement for hydrostatic equilibrium, or being nearly round. Since Sedna is far too distant for telescopes to resolve its exact shape, astronomers must estimate its roundness based on its size and composition.
Planetary scientists have calculated that an icy body like Sedna must reach a diameter of at least 400 kilometers to generate enough internal gravity to overcome rigid forces and become spherical. Sedna’s estimated diameter, ranging from 1,000 to 1,700 kilometers, is well above this threshold, making it highly likely that the object is indeed round.
Because its shape is inferred and not directly confirmed, the IAU has not officially recognized Sedna as a dwarf planet, maintaining its formal classification as a Trans-Neptunian Object. However, most astronomers consider it a strong dwarf planet candidate due to the high probability that its size has forced it into a spherical shape. Until a dedicated mission or powerful new observation confirms its mass and shape, Sedna will remain in this state of formal ambiguity.