A Detached and Lonely Path
On November 14, 2003, astronomers Michael Brown, Chad Trujillo, and David Rabinowitz spotted a faint object using the Samuel Oschin telescope at Palomar Observatory. This object, later named 90377 Sedna, presented a deep orbital puzzle. It travels on a wildly elongated path that takes approximately 11,400 years to complete. At its closest point, or perihelion, it is 76 astronomical units (AU) from the Sun—twice the distance of Neptune. At its farthest, or aphelion, it recedes to a remote 937 AU.
The main anomaly is its perihelion. Most objects in this region with eccentric orbits are part of the "scattered disk," their paths shaped by gravitational kicks from Neptune. Sedna, however, never gets close enough to Neptune for this to be possible. Its orbit is detached from the influence of the known giant planets, suggesting an external force put it there. This makes Sedna the first observed body of a new class of objects, possibly belonging to the hypothetical inner Oort Cloud.
Sedna itself is about 1,000 kilometers in diameter and has one of the reddest surfaces in the Solar System, second only to Mars. This coloration is likely due to complex organic compounds called tholins, formed as cosmic rays and solar ultraviolet radiation alter simple ices like methane and nitrogen on its surface. Temperatures on Sedna never rise above -240° Celsius (-400° F).
Sculpting an Impossible Orbit
Several hypotheses attempt to explain how Sedna achieved its bizarre trajectory. One leading idea involves a passing star. Early in the Solar System's history, when the Sun was still part of its birth star cluster, a close stellar flyby could have gravitationally perturbed Sedna, lifting its perihelion to its current distant location.
Another prominent theory involves a yet-undiscovered planet far beyond Neptune, often called Planet Nine. Researchers at Caltech proposed that a planet perhaps six times the mass of Earth, in a distant and eccentric orbit, could gravitationally shepherd objects like Sedna into their detached orbits. The alignment of the orbits of several such extreme trans-Neptunian objects lends support to this model.
Other possibilities exist. The collective gravity of many smaller trans-Neptunian objects could have jostled Sedna into its current path over billions of years. Less likely scenarios include Sedna being a captured object from another star system or being perturbed by a rogue planet that was later ejected from our own Solar System. Understanding Sedna's origin provides information about the early environment of our solar system.