Throw another log on the fiery debate as to what makes a planet: Six Saturn-sized objects that neither shine like stars nor orbit like planets were recently discovered by the Hubble Space Telescope.
Scientists at the Space Telescope Science Institute unintentionally discovered the enigmas while trying to measure the size of stars in the M22 globular cluster, about 8,500 light years from Earth. The cluster lies a third of the way between Earth and the center of the Milky Way and is home to some the oldest stars in the universe.
The measurements the team came away with, published in this week's issue of the journal Nature, revealed dark objects with masses at least a quarter the size of Jupiter. The objects did not appear to be orbiting with any stars though.
That means the objects are not planets, at least according to recently released guidelines drafted by the International Astronomical Union.
To Kailash Sahu and the other scientists that made the observations, they have found objects that by all definitions are planets, albeit lost and wandering.
"We explored every other possible explanation," said Sahu. "We asked, 'Maybe its a cosmic ray? Something wrong with the data processing, or the data itself?' But this may be the only possible explanation."
Alan Boss of the Carnegie Institution in Washington, D.C., said there are other possible explanations for objects like these, albeit speculative.
"These objects might well be the equivalent of the free-floating planetary-mass objects recently discovered in the star-forming Sigma Orionis region," he said. "Those objects apparently formed in the same way in which stars form, but ended up with less mass than brown dwarf stars, and so I have suggested that they be called "sub-brown dwarfs" (stars).
"In the case of M22's objects, the mass seems to be at least another factor of 10 lower than the Sigma Orionis sub-brown dwarfs, which might be explained by the much lower metallicity in stars in a globular cluster. The earliest stars are metal-poor, as they were some of the first stars to form," he said.
The STScI scientists and Boss agree that the latest observations are intriguing enough to warrant further investigation. The data was recorded every three days for four months starting in February 2001.
"We are going to make more observations that last at least a week," said Nino Palagia of STScI. "That way it is highly probable that the planetary bodies will be perfectly characterized."
Mark McCaughrean of Germany's Potsdam Astrophysical Institute, though, is more skeptical of the possibility of making accurate measurements with Hubble again, as the planet-esque bodies will have moved on.
"A key thing is to make much more frequent measurements [They] say they're going to do this with the Hubble for seven days continuously, but it's not clear that's enough," he said. "After all, they only saw six events in four months of sporadic monitoring, and so the chances of seeing one in seven days are low."
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