the remote object known as Sedna is even
smaller than previously believed.
When astronomers discovered Sedna in 2003, it
was the most distant known object in the solar
system, orbiting more than twice as far from the
sun as Pluto. (Shortly afterward, astronomers
announced the discovery of an even more distant
body, now called Eris.)
Initial estimates based on optical brightness
suggested Sedna was roughly two-thirds Pluto's
size.
But new infrared observations have slashed
Sedna's estimated size, finding the body to be
just 43 percent of Pluto's width.
Sedna's Unexpected Reflections
For the new study, András Pál, of Konkoly
Observatory in Hungary, and his colleagues
used the European Space Agency's Herschel
Space Observatory to detect the meager heat
that Sedna emits at far-infrared wavelengths.
"It's very cold," Pál said of the object. At more
than 8 billion miles (13 billion kilometers) from
the sun, Sedna is a frigid 20 Kelvin (-424
degrees Fahrenheit, or -253 degrees Celsius).
In fact, the body is so chilly and small that years
ago NASA's Spitzer Space Telescope tried and
failed to detect Sedna via infrared light.
The new Herschel observations indicate that
Sedna reflects a third of the sunlight striking it
—much more than expected. That means the
object must be very small, since it reflects so
much light yet looks so faint.
"We expected something darker and larger," Pál
said.
Based on the new data, Pál and colleagues think
Sedna is just 620 miles (995 kilometers) across
—smaller than Pluto's largest moon, Charon.
"Good, Solid Detection"
The Herschel data represent "a good, solid
detection," said Mike Brown, an astronomer at
the California Institute of Technology who co-
discovered Sedna.
"We have never had a good measurement
before," so scientists were largely uncertain of
Sedna's size, added Brown, who wasn't part of
the new study.
Pluto itself once suffered similar downsizing.
Fifty years ago Pluto was thought to be larger
than Mercury but smaller than Mars.
Now Pluto is known to be only about half
Mercury's size—and Sedna, it seems, is less than
half of Pluto's.
The new paper about Sedna's size was published
online this month at arXiv.org and will appear in
an upcoming issue of the journal Astronomy and
Astrophysics Letters.
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