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 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.
Refining the diameter
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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