Astronomers are looking for a missing moon. They think it is orbiting the tiny, icy, red planetoid named Sedna they discovered last month, far beyond Pluto, until then the most distant planet in the solar system. A search with the most powerful ground and space telescopes has failed to turn a moon up.
Moons don't just get lost, or do they? That's one of the theories California Institute of Technology astronomer Michael Brown is considering to explain why he cannot locate one he believes is circling Sedna at the edge of our system.
With a telescope in California, he discovered the faint Sedna in March, 13 billion kilometers away from Earth, three times farther from us than Pluto. Its highly elliptical orbit takes it 10 times farther away from us than that, thereby greatly expanding the known size of the solar system.
Soon after the discovery, Mr. Brown used the orbiting Hubble Space Telescope to search for the moon he expected to accompany Sedna, but the Hubble, which has seen further toward the edge of the universe than any other instrument, could not detect it. "The interesting thing that we were quite surprised to find is that no satellite whatsoever was visible in our observations with the Hubble Space Telescope," he said.
Why does Mr. Brown believe Sedna has a moon? Because it rotates very slowly, a sign that something nearby is pulling on it to slow it down. To measure the rotation, he observed Sedna's brightness cycle, the time it takes to go from dimmer to brighter. It is 20 days, meaning a day on Sedna is 20 times longer than an Earth day. "We were so convinced that there had to be a satellite because there is no other good scientific explanation for why something will rotate as slowly as 20 days," he added. "Most objects in the solar system rotate in either a few hours or a day like the Earth, but 20 days or longer is very unusual and there has to be a good reason for it, and this was the most likely one."
Mr. Brown is so sure a moon orbits Sedna that he has devised several explanations for why he could not find it. It might have been too close when he looked, or behind Sedna. It might have been in front of Sedna and blended in with the planetoid. It could be a dark moon that reflects little light. Or it might have been lost, but how do you lose a moon? "It could have been destroyed by an impact with another object," he suggested. "It could have been lost by a close encounter with another planet that ripped the two apart. We think this probability is not very high, but at this time we can't rule this out."
Another explanation for the missing moon is that there never was one, but if there is no nearby gravitational pull, why would Sedna spin so slowly? Mr. Brown suggests that this may be an illusion. Maybe the tiny planet is really rotating every 25 hours. If you look at it once a day at the same time, you might see it in a slightly different position from the day before. That would make it seem like it has advanced very little, suggesting a very long, slow turn, but Mr. Brown thinks that this, too, is an unlikely explanation.
Disappointed but not defeated by Hubble's failure, he plans to spend time with the orbiting Spitzer infrared telescope, setting it to look specifically for a dim moon, but if the Spitzer cannot find Sedna's moon, what can? "That's a very difficult question and something that keeps me awake at night," he said.
If he is unable to find the moon with the equipment available today, Mr. Brown said that he might have to wait for a new generation of more powerful ground telescopes, about a decade away.