The Mystery of Sedna: The Solar System's Icy Oddball World

The Mystery of Sedna: The Solar System's Icy Oddball World
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In 2004, astronomers announced the discovery of a red, frigid planet-like body at the outskirts of our solar system. Michael E. Brown, the Caltech astronomer who spotted the object (and who would later "kill" Pluto) dubbed it "Sedna", after the Inuit goddess who rules the seas from the bottom of the chilly Arctic Ocean. Sedna quickly spurred imaginations, gracing magazine covers and prompting media speculation of a tenth planet (Pluto was still considered a planet back then.) Brown quelled the planet hype, and Sedna's public limelight gradually faded, but more than decade later, the object remains exceedingly popular with astronomers.

Sedna is a fascinating place. Spherical and close to 1,000 kilometers in diameter, astronomers aren't exactly sure what to make of it. It could be a comet or a dwarf planet. A possible sludge-like coating of hydrocarbons renders it nearly as red as Mars, yet at the same time, it is one of the intrinsically brightest objects in the solar system owing to its slick, reflective composition of methane and water ice.

Sedna's psychedelic look isn't its most intriguing aspect, however. Astronomers are more interested in its orbit. At perihelion -- the point where an orbiting body is closest to the Sun -- Sedna is 76 astronomical units (11.37 billion kilometers) away from the Sun, placing it firmly beyond the orbit of Pluto. But at its farthest distance, it is an astounding 937 astronomical units away! That means that Sedna's orbit, which takes 11,400 years to complete, looks something like this (the purple line is Pluto):

To astronomers, Sedna's whacky orbit begged an obvious question: How did it get out there? Southwest Research Institute Planetary scientist Harold F. Levison explained scientists' general state of bemusement to Emily Lakdawalla of the Planetary Society.

"Sedna is on an eccentric orbit, and it can't form on that orbit, it's got to be on a circular orbit in order to accrete. So it got scattered by something, but it's too far away from the planets for them to do it, because the perihelion is too large, and it's too close to the Sun for the galaxy to do it. So Sedna was a surprise because it's in a "no man's land" because we don't know of anything that could put it there." (Emphasis mine.)

Until recently, that is. Astronomers are now abuzz with the possibility of a large "Planet Nine". Brown and fellow Caltech astronomer, Konstantin Batygin put forth the notion a year ago, and it was Sedna that initially clued them in to its existence.

Other objects like Sedna have since been discovered. 2014 UZ224 orbits ever 1,136 years. 2012 VP113 orbits every 4,175. V774104, currently the farthest-observed object from the Sun, could take even longer.

In his recent book, Facts From Space!, astronomer Dean Regas hails Sedna as a "Final Frontier Forerunner."

"Could Sedna be the forerunner of another class of objects that spend the majority of their orbits at the edge of the solar system? If so, what do you call them? Planets? Dwarf planets? Other? That's why Sedna is so intriguing. It opens up so many possibilities."

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