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 in volume.

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 sizes.

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 said.

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.


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 system fringe.

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 rate.

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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