The dwarf planet Eris is the farthest known large object orbiting the Sun, existing deep within the solar system’s outer reaches. Its discovery initiated a debate that redefined what a planet is, solidifying Eris’s role as a significant trans-Neptunian object. Understanding the physical characteristics of this distant world, especially the length of its day, helps astronomers piece together the complex history of our solar system’s formation. This rotation speed provides a fundamental piece of information about the dynamics of this icy body.
The Eris Day Length
Determining Eris’s rotation period has been challenging due to the difficulty of observation. The prevailing and long-cited rotational period for Eris was approximately 25.9 hours, which is remarkably similar to an Earth day. However, the current scientific consensus, supported by combined data, indicates that Eris is tidally locked to its moon, Dysnomia. This tidal locking means Eris’s rotation period matches Dysnomia’s orbital period, resulting in a much slower rate. The precise figure for this synchronous rotation is approximately 15.8 days, meaning an Eris “day” is about 379 Earth hours long.
Defining Eris and its Solar System Environment
Eris is classified as a dwarf planet, a member of the icy bodies orbiting the Sun far beyond Neptune. It resides primarily in the scattered disc, a distant region of space that overlaps with the Kuiper Belt, characterized by objects with highly inclined and eccentric orbits. Eris is massive, exceeding the mass of Pluto by about 28%, though Pluto is slightly larger by volume. Its diameter is estimated to be around 2,326 kilometers, making it roughly the size of Pluto and smaller than Earth’s Moon.
Eris orbits the Sun at an average distance of about 68 astronomical units (AU), or 6.3 billion miles. This extreme distance makes it an extremely cold and dark world, receiving very little solar energy. Sunlight takes over nine hours to travel from the Sun to Eris’s surface. The dwarf planet’s highly reflective, bright surface is thought to be covered in methane ice, a consequence of its frigid environment.
Measuring Rotation in the Distant Solar System
Determining the rotation period of a faint, distant object like Eris requires specialized astronomical techniques, primarily relying on photometry. Since Eris is too far away for direct imaging to reveal surface features, scientists use light curve analysis to infer its spin rate. This method involves continuously measuring the brightness of the dwarf planet over an extended period.
If a celestial body is not perfectly spherical or if it has patches of varying reflectivity (albedo) on its surface, the amount of sunlight it reflects toward Earth will change as it rotates. Plotting these fluctuations in brightness over time creates a light curve, which is a graph revealing the object’s rotational cycle. The time interval between two identical brightness peaks indicates the rotation period.
For Eris, the brightness variation is very small, which initially led to conflicting rotation estimates. The low variability suggests that Eris is nearly spherical and that its surface is uniformly bright. The definitive conclusion of a 15.8-day rotation was reached by analyzing long-term data from multiple sources, confirming the subtle, repeating brightness cycle.
Comparing Eris’s Day to its Year
The two time scales that define Eris’s motion are its day, which is its rotation period, and its year, which is its orbital period around the Sun. Eris’s day is approximately 15.8 Earth days long, a slow spin rate caused by tidal forces with its satellite, Dysnomia. This slow rotation contrasts sharply with its immense orbital period.
Eris takes an extraordinary amount of time to complete a single orbit around the Sun, a duration that defines its year. The orbital period is approximately 557 Earth years. This difference means that for every one of Eris’s years, the dwarf planet will have rotated on its axis only about 12,900 times. The extreme length of Eris’s year is a direct consequence of its highly distant orbit, which averages 68 AU from the Sun.