Sedna Is So Distant, We're Not Even Sure It's a Dwarf Planet

This enigmatic celestial body was first discovered November 14, 2003, with the Samuel Oschin Telescope at the Palomar Observatory by a research team consisting of Michael E. Brown from Caltech, Chad Trujillo of the Gemini Observatory and David Rabinowitz of Yale University.

In line with the protocol of the International Astronomical Union (IAU) for naming trans-Neptunian objects after creation deities, the discoverers chose the name "Sedna" after the Inuit goddess of the deep sea and sea creatures. This nomenclature was deemed apt for a body dwelling in the frigid, outermost reaches of our solar system.

Sedna is considerably smaller than Earth. It sports a reddish hue, possibly due to the presence of hydrocarbon or tholin. Sedna is about 1,118 miles (1,800 kilometers) in diameter, which is, diameter-wise, about halfway between Pluto and the largest known Kuiper belt object, Quaoar.

Orbiting the sun at an average distance that surpasses even that of Neptune by a factor of 10, Sedna follows a highly elliptical path. It's currently about 86 astronomical units (au) from the sun, but swings out to an incredible 936 au at its farthest point. This distance is so vast that it leads to an extremely distant region of the solar system known as the inner Oort cloud, thought to contain billions of icy objects. An au is roughly the distance from Earth to the sun and equals 92,955,807 miles (149,597,871 kilometers), which explains why one full orbit, or Sedna-year, takes a staggering 11,400 Earth years.

Sedna's unusual orbit has posed a compelling puzzle for astronomers. Traditional models of solar system formation struggle to explain an orbit of such eccentricity and distance.

Given its extremely elliptical orbit and the fact that Sedna remains far beyond the gravitational influence of Neptune (which shapes the orbits of many other distant objects), some astronomers have proposed the presence of an undiscovered planet or a passing star in our solar system's distant past. The latter could have gravitationally disturbed Sedna into its current orbit. Sedna's highly eccentric and distant orbit is thus of critical importance to the understanding of the solar system’s history and potentially its future, providing us with a unique perspective on the mysterious and distant regions beyond Neptune.

While the IAU hasn't officially categorized Sedna as one of the dwarf planets, it ticks all the necessary boxes. According to the IAU, a celestial body is considered a dwarf planet if it orbits the sun, has enough mass for gravity to mold it into a round shape, but has not cleared the neighborhood around its orbit of debris. Sedna, spherical in shape and with a distinct orbit, fits this description and is generally considered by astronomers to be a dwarf planet candidate, yet it shares its orbital zone with numerous other trans-Neptunian objects, disqualifying it from full planet status.

Although Sedna's extremely elongated and distant orbit makes precise observations a daunting task, astronomers are still seeking evidence of a potential moon. Variations in Sedna's brightness and the slow speed of its rotation were once thought to hint at a satellite. However, subsequent study suggests that these fluctuations likely stem from a faster rotation than originally thought and an uneven surface.

Current data does not conclusively prove the presence of a moon around Sedna. Nonetheless, future observations and missions targeting the outer solar system may yet reveal further secrets about this mysterious dwarf planet, including a potential satellite.

Sedna, like other celestial bodies populating the depths of the solar system, offers invaluable clues to understanding the evolution of our cosmic neighborhood. Its unique orbit and other characteristics challenge conventional theories about the solar system's development, driving astronomers to explore alternative hypotheses such as the existence of a yet undiscovered massive planet, or the impact of a passing star early in the solar system's history.

This article was created in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.