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Shrinking Iceball

October 19, 2012

Back in 2003, astronomers spotted what was then the farthest object ever seen orbiting the Sun — more than eight billion miles away, which is more than twice as remote as distant Pluto is. Since the newfound world lies far from the warmth of the Sun, it must be very cold, so the discoverers named it Sedna, after an Inuit sea goddess who dwelled at the bottom of the Arctic Ocean.

At the time of its discovery, Sedna was thought to be roughly two-thirds the diameter of Pluto. But more recent observations have cut Sedna down to size.

Because Sedna is so cold, it should emit most of what little heat it does produce at far-infrared wavelengths. Those wavelengths are much longer than the human eye can see, but they’re well within the range of several telescopes in space.

One of those, Spitzer Space Telescope, failed to see Sedna at infrared wavelengths. Recently, however, the European Herschel Space Observatory succeeded where Spitzer failed — it picked up distant Sedna’s feeble heat radiation.

Its observations indicate that Sedna is just 620 miles across — less than half the diameter of Pluto. The same observations indicate that Sedna reflects about a third of the sunlight that strikes it.

Herschel also measured Sedna’s temperature. And as you might imagine, it’s pretty darn low: minus 424 degrees Fahrenheit. So while astronomers initially overestimated Sedna’s size, they did pick a perfect name for this frigid and remote little world.


Script by Ken Croswell, Copyright 2012

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