ar
beyond Pluto, out where the Sun is only a pinpoint of pale
light, a frozen world has been found on the dark fringes of the
solar system. Astronomers say it is by far the most distant
object known to orbit the Sun and the largest one to be detected
since the discovery of Pluto in 1930.
With one discovery, it seems, the solar system has gotten
much bigger, glimpses of its outer reaches bringing a sense of
reality to what had been a remote frontier of hypothesis. And
perhaps it has gotten stranger, too.
"There's absolutely nothing else like it known in the solar
system," Dr. Michael Brown, an astronomer at the California
Institute of Technology who led the discovery team, said of the
newfound object.
But in a telephone news conference yesterday from Pasadena,
Dr. Brown added, "Our prediction is that there will be many,
many more of these objects discovered in the next five years,
and some of them will probably be more massive."
The researchers, whose observations were supported by the
National Aeronautics and Space Administration, said the object,
referred to as a planetoid, is extremely frigid (minus 400
degrees Fahrenheit) and peculiarly red, probably more so than
any other body in the solar system except Mars. They are not
sure why, and also have few ideas of the object's composition.
It could be a primordial mix of rock and ice.
Dr. Brown's group has proposed naming the object Sedna, after
the Inuit goddess who created the sea creatures of the Arctic.
For the time being, it is designated as 2003 VB12. The first
sighting was made last November at the Palomar Observatory,
operated by Caltech.
Sedna's remoteness has inspired scientists to conjecture over
how much the discovery could be telling them about the far
reaches of the solar system. The planetoid is more than three
times as far from the Sun as the current distance of Pluto,
normally considered the edge of the planetary system. But it
travels a widely eccentric orbit, taking 10,500 years to revolve
around the Sun.
Calculations by the researchers show that Sedna, now a
relatively close 8 billion miles from Earth, wanders out as far
as 84 billion miles, in a region presumably populated with icy
bodies too small to be observed by telescopes.
Dr. Brown said Sedna "is so far away from everything that it
must be the first observed member of the long-hypothesized Oort
Cloud, a sphere of comets out to halfway the distance to the
nearest star."
In 1950, a Dutch astronomer, Jan Oort, predicted the
existence of a swarm of icy bodies stretched somewhere beyond
the orbit of Pluto. The cloud is thought to surround the Sun and
extend outward halfway to the next nearest star, Proxima
Centauri.
The Oort Cloud is considered a repository of the comets that
get pulled in toward the Sun. Dr. Brown said that though the
danger is nil, "if this object were to come into the inner solar
system, it would be the most spectacular comet ever seen."
Other astronomers, however, questioned whether Sedna should
be considered a part of the Oort Cloud. In theory, the cloud's
innermost boundary is charted to be well beyond the farthest
point in Sedna's orbit. Some scientists suggested that Sedna
could instead have been part of a closer region of cometary
material, the Kuiper Belt, which stretches from Neptune to just
beyond Pluto, and that it had somehow been dislodged and sent
off on a more distant orbit.
Dr. Brown agreed that Sedna was much closer than expected for
the Oort Cloud. But in a statement, he suggested that Sedna
could reside at least part of the time in an inner sector of the
cloud. The sector, he said, could have been separated from the
greater cloud by the gravitational pull of a rogue star that
came close to the Sun early in its existence.
Dr. Brian G. Marsden of the Harvard-Smithsonian Center for
Astrophysics said the discovery was puzzling and exciting. "You
wonder where indeed did the object come from," Dr. Marsden said.
"Is it connected to an inner Oort Cloud? What does that mean and
how did it get there?"
He said that it was likely that there were similar icy bodies
beyond Pluto, and that some may be larger than Sedna.
Two years ago, Dr. Brown and his colleagues, Dr. Chad
Trujillo of the Gemini Observatory in Hawaii and Dr. David
Rabinowitz of Yale, found a smaller object, called Quaoar (KWAH-o-ar),
which until now was the largest known object beyond Pluto. It is
estimated to be about 40 percent as big as Pluto.
Even Pluto itself is thought by some astronomers to be less a
planet and more like a Kuiper denizen.
The astronomers got their first look at Sedna with a 48-inch
telescope at Palomar, in southern California. Within days, other
telescopes in Chile, Spain, Arizona and Hawaii made
observations. When NASA's new Spitzer Space Telescope took a
look with its infrared detectors, astronomers were able to make
rough estimates of the planetoid's size.
To the other telescopes, the object was no more than a point
of light. But infrared measurements of heat radiating from the
object led the researchers to estimate its diameter at no more
than 1,100 miles. Pluto's is 1,400.
Dr. Trujillo said that the nature of Sedna's surface was a
mystery and that its ruddy color was "nothing like what we would
have predicted or what we can currently explain."
But Dr. Marsden said the redness was not necessarily a
surprise. "Comets, many of them, tend to be reddish," he said.
It was hard enough finding Sedna in the first place. In
looking for small solar system travelers, astronomers need a
sequence of pictures to reveal that an object has moved in
relation to background stars. But an object like Sedna, with its
10,500-year orbit, seems hardly to move at all. Only by using
archival pictures from two years ago were the astronomers able
to detect and clock its millennial pace.
As the scientists calculated it, Sedna should reach its
nearest point in 72 years and then begin heading back out to the
far frontier of the solar system.
"The last time Sedna was this close to the Sun," Dr. Brown
said, "Earth was just coming out of the last ice age."