New Objects Beyond Pluto/Charon
In 2003 and 2004, astronomers, using earth-based telescopes, began looking
deeper into the night sky than ever before in a search for potential planet-like
bodies out beyond the orbit of Pluto and its "sister" planet/moon
Charon. Well, these astronomers began to find things out in the dark realms
of our Sun's influence. The Kuiper Belt and Oort Cloud are the regions where
millions, billions, and perhaps even trillions of comets are believed to exist.
As astronomers began to find occasional bodies, a few turned out to be much
bigger than the typical comets that occasionally visit our Earth and vent beautiful
trails. This page will feature two objects that are smaller than Pluto's moon,
Charon named Quaoar and Sedna.
Hubble Spots an Icy World Far Beyond Pluto - Quaoar
Below is the full press release text that accompanied the discovery of Quaoar:
NASA's Hubble Space Telescope has measured the largest object discovered in
the solar system since the discovery of Pluto 72 years ago.
Approximately half the size of Pluto, the icy world 2002 LM60, dubbed "Quaoar"
(pronounced kwa-whar) by its discoverers, is the farthest object in the solar
system ever to be resolved by a telescope. It was initially detected by a ground-based
telescope, as simply a dot of light, until astronomers aimed the powerful Hubble
telescope at it.
Quaoar is about 4 billion miles away from Earth, well over a billion miles
farther away than Pluto. Unlike Pluto, its orbit around the Sun is very circular,
even more so than most of the planetary-class bodies in the solar system.
Although smaller than Pluto, Quaoar is greater in volume than all the asteroids
combined (though probably only one-third the mass of the asteroid belt, because
it's icy rather than rocky). Quaoar's composition is theorized to be largely
ices mixed with rock, not unlike that of a comet, though 100 million times greater
This finding yields important new insights into the origin and dynamics of
the planets, and the mysterious population of bodies dwelling in the solar system's
final frontier: the elusive, icy Kuiper belt beyond Neptune.
Michael Brown and Chadwick Trujillo of Caltech are reporting the findings
today at the 34th annual meeting of the Division for Planetary Sciences of the
American Astronomical Society in Birmingham, Ala.
Earlier this year, Trujillo and Brown used the Palomar Oschin Schmidt telescope
to discover Quaoar as an 18.5-magnitude object creeping across the summer constellation
Ophiuchus (it's less than 1/10,000th the brightness of the faintest star seen
by the human eye). Brown had to do follow-up observations using Hubble's new
Advanced Camera for Surveys on July 5 and August 1, 2002, to measure the object's
true angular size of 40 milliarcseconds, corresponding to a diameter of about
800 miles (1300 kilometers). Only Hubble has the sharpness needed to actually
resolve the disk of the distant world, leading to the first-ever direct measurement
of the true size of a Kuiper belt object (KBO).
Like Pluto, Quaoar dwells in the Kuiper belt, an icy debris field of comet-like
bodies extending 7 billion miles beyond Neptune's orbit. Over the past decade
more than 500 icy worlds have been found in the Kuiper belt. With a few exceptions
all have been significantly smaller than Pluto.
Previous record holders are a KBO called Varuna, and an object called 2002
AW197, each approximately 540 miles across (900 kilometers). Unlike Hubble's
direct observations, these diameters are deduced from measuring the objects'
temperatures and calculating a size based on assumptions about the KBOs' reflectivity,
so the uncertainty in true size is much greater.
This latest large KBO is too new to have been officially named by the International
Astronomical Union. Trujillo and Brown have proposed naming it after a creation
god of the Tongva native American tribe, the original inhabitants of the Los
Angeles basin. According to legend, Quaoar, "came down from heaven; and,
after reducing chaos to order, laid out the world on the back of seven giants.
He then created the lower animals, and then mankind."
Quaoar's "icy dwarf" cousin, Pluto, was discovered in 1930 in the
course of a 15-year search for trans-Neptunian planets. It wasn't realized until
much later that Pluto actually was the largest of the known Kuiper belt objects.
The Kuiper belt wasn't theorized until 1950, after comet orbits provided telltale
evidence of a vast nesting ground for comets just beyond Neptune. The first
recognized Kuiper belt objects were not discovered until the early 1990s. This
new object is by far the "biggest fish" astronomers have snagged in
KBO surveys. Brown predicts that within a few years even larger KBOs will be
found, and Hubble will be invaluable for follow-up observations to pin down
Hubble Spots Another Icy World Beyond Pluto - Sedna
An image of the newly discovered planetoid Sedna made by the Hubble Space
Telescope (news - web sites) released Wednesday, April 14, 2004. Too small to
qualify as a planet at 800 to 1,000 miles in diameter, Sedna is about three
quarters the size of Pluto, its closest neighbor, its discoverer, Mike Brown
of the California Institute of Technology, said. Deemed the coldest most distant
place known in the solar system, it was named in honor of Sedna, the Inuit goddess
of the sea, who is thought to live at the bottom of the frigid arctic ocean.
(AP Photo/California Institute of Technology). I have included some of the information,
but great details about Sedna and its discovery can be found at the Cal
Tech/Sedna site or by reviewing the actual
research article written by Mike Brown.
Below is the press release that accompanied the discovery:
Artist impression of Sedna and its potential "moon."
Pictures of the newly discovered planetoid Sedna show it moonless,
spinning alone some 8 billion miles from Earth. Sedna, though, still might have
a moon that was hiding somewhere or too dark to be photographed by the Hubble
Space Telescope (news - web sites), said astronomer Mike Brown, its discoverer.
Given the planetoid's slow rotation, the seeming lack of a moon surprises Brown.
"I still am convinced there is one there, and it's just darker than we
expected and we haven't seen it yet," Brown, of the California Institute
of Technology, said Wednesday.
At 800 to 1,000 miles in diameter, Sedna is too small to qualify as a planet.
It is only about three quarters the size of Pluto, its closest neighbor. Objects
that size should complete one rotation in a matter of hours, but observations
so far show it takes 20 to 40 days, possibly due to the drag of a moon, Brown
The Hubble Space Telescope took 35 visible light images in March, after Brown
announced the discovery of the frozen, crimson world the most distant
object in our solar system. The images show Sedna with a faint, distant star
in the background. There is a small chance a moon could have been behind or
in front of and indistinguishable from the planetoid, Brown said.
While the first set of images concentrated on blue light to more accurately
determine Sedna's size, that also raised the possibility of overlooking a faintly
lit moon. Brown said he is hoping to use the Hubble again and concentrate more
on the red end of the visible light spectrum in hopes of detecting a companion.
If there is a moon, Brown said he expects it should be about 400 miles across.
Other possibilities include the destruction or the loss of the moon after it
slowed Sedna's rotation, he said. The pictures were released by the Baltimore-based
Space Telescope Science Institute, which operates the Hubble.
The image above depicts the view of our solar system from the
surface of Sedna.
A HIDDEN MOON?
Brown and his colleagues announced the find on March 15, and immediately began
searching the area with the orbiting Hubble telescope, expecting to see Sedna's
moon. Hubble didn't find one, but it could still be there, Brown said. Sedna's
moon might be hiding directly behind it, or the satellite might be fainter than
astronomers expected, making it hard for even Hubble to detect. Other possibilities
include the notion that Sedna once had a moon, which slowed the planetoid's
rotation, but the moon was destroyed by an impact with another cosmic body or
was pulled away by a close encounter with another planetoid.
Moon or no moon, though, Sedna's status as a planetoid remains unchanged,
Brown said. "Whether or not an object has a moon in no way influences what
we think of as a planet," he said. "Mercury and Venus have no moons
and there's no doubt that they're planets. There are many asteroids that have
moons and other objects out beyond Neptune ... which are clearly not planets.
so unfortunately this doesn't help to answer the situation." Brown and
other researchers believe Sedna is just one of many such planetoids at the solar
For now, though, Sedna is a cosmic oddball: one of the reddest objects in
our planetary system after Mars, it takes a highly elliptical path around the
sun, reaching 84 billion miles at its farthest point. The complete orbit takes
10,500 years to complete. Even at its closest to the sun, the temperature on
Sedna never gets above minus 400 degrees Fahrenheit. The planetoid, believed
to be half rock and half ice, is named after the Inuit goddess said to have
created the sea creatures of the Arctic.
When astronomers announced the discovery last month of Sedna, the most distant
known object in the solar system, they were nearly certain it had an unseen
satellite. New observations by the Hubble Space Telescope (news - web sites)
find no moon, however, deepening the mystery surrounding this already strange
object. Sedna is about three-fourths the size of Pluto. It is so far away that
it takes 10,000 years to orbit the Sun. Its discovery has astronomers arguing
over whether to call it a planet or a planetoid, and whether to count it as
one of many objects in the Kuiper Belt, where Pluto roams, or the first known
example of an expected halo of more distant objects called the Oort Cloud.
Caltech astronomer Mike Brown, who led the discovery, said Sedna's slow rotation
rate had him convinced there was an unseen satellite exerting a gravitational
tug. Here's why: Most objects in the solar system that don't have companions
complete a rotation, or day, in a matter of hours. There are many examples of
fast-spinning asteroids and similarly whirling large, round Kuiper Belt objects.
Pluto, on the other hand, has had its rotational period slowed to six Earth-days
by its companion, Charon.
Sedna spins on its axis once every 20 Earth-days, or perhaps even more slowly,
making the presence of a moon practically inevitable, Brown had thought. So
shortly after the discovery, Hubble was pointed at Sedna. "Much to our
surprise, there's no satellite," he told reporters today. "I'm completely
baffled at the absence of a moon," Brown said. "This is outside the
realm of expectation and makes Sedna even more interesting. But I simply don't
know what it means." The rotation of Sedna, officially named 2003 VB12,
was determined by noting changes in brightness from its surface during repeated
ground-based observations over about 3 months. Brown and his colleagues assumed
differences in surface composition accounted for the changes. At first they
determined the rotation period was 40 days. They later refined that to learn
that it's at least 20 days, and possibly still as much as 40.
After the Hubble observations, the researchers considered that maybe they
had not properly analyzed the data. But a "careful reanalysis" leaves
them convinced they've determined the possible range of the rotation period
accurately. "I'm completely lost for an explanation as to why the object
rotates so slowly," Brown said. There remain some possibilities for a rational
explanation. There's a small chance that during the Hubble observations, the
suspected satellite of Sedna was hidden, lurking either directly behind or directly
in front of Sedna. Or a satellite might have throttled Sedna's rotation long
ago, then been destroyed in a collision or lost in a gravitational interaction
with a planet. Or, Sedna might rotate every 25 hours instead of 24 days, a setup
that could fool astronomers into drawing their present conclusion. This latter
possibility can be confirmed or ruled out with more observations.
All these scenarios are seen as unlikely. More likely, Brown figures, is that
the satellite is darker than expected and simply didn't show up. "Even
though it's quite large, it could be quite dark," he said. "We still
very strongly believe there is or was a satellite."Brown said the moon
would be about 400 miles wide, or 2.5 times smaller than Sedna and six times
fainter. Further attempts to locate the satellite might be made with other telescopes,
with fresh expectations for how to look.
Brown's colleague, Yale University researcher David Rabinowitz, has a slightly
different opinion of the results. The Hubble observations "rule out a large
moon," Rabinowitz said. If there is something there, he said, it is 10
times smaller than Sedna and it's not clear it could have slowed Sedna's rotation
Brown said the observations of Sedna are equal to spotting a soccer ball at
900 miles away. Hubble was unable to resolve Sedna as a disk, so an exact size
can't be determined. But that shortcoming sets an upper limit of about 1,000
miles on its diameter, based on assumptions of how much light it reflects. If
Sedna has a brighter surface than assumed, it could be smaller. Pluto is 1,413
miles (2,274 kilometers) wide. Given Hubble's unsurpassed ability, Brown does
not know how Sedna's diameter will be firmly determined until a new generation
of telescopes come online in the next decade.
Meanwhile, astronomers still are not sure where or how Sedna formed or how
it came to travel on such a long, looping orbit. While Pluto's orbit is, on
average, 39 times the Earth-Sun distance, Sedna roams from 76 to 1,000 times
the Earth-Sun distance. Answering questions about the strange object's origins,
orbit and rotation -- along with similar discoveries of other distant objects
-- will help researchers clarify the now-fuzzy picture of the outer solar system.
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