New Scientists News Service, 3 June:

Kicking Pluto out of the planet club was nothing compared to this. An astronomer is calling for demoting two entire arms of our galaxy, after they failed to turn up in a sensitive new map of the Milky Way's stars.

Astronomers have long believed that our galaxy possesses four spiral arms, since radio observations show concentrations of gas that trace such a spiral structure.

But now, two of the Milky Way's arms have failed to turn up in a sensitive new survey that used the Spitzer Space Telescope to map the distribution of millions of stars. Spitzer is well-suited to mapping the galaxy's stars because its infrared vision can pierce through the dust that obscures stars at optical wavelengths of light.

Astronomer Robert Benjamin of the University of Wisconsin in Whitewater, US, says these two arms, called Sagittarius and Norma, may be mostly concentrations of gas, perhaps sprinkled with pockets of young stars.

By contrast, the other two arms, called Scutum-Centaurus and Perseus, appear rich not only in gas, but in stars both young and old. "These major arms . . . could be the things that would really stand out if you were looking at the Milky Way galaxy from Andromeda [a nearby galaxy]," Benjamin says.