But to be fair to "Earth II" enthusiasts, there is some reason for optimism about the discovery. Gliese 581c is only five times heavier than Earth which suggests that it could be rocky like our planet. This might also mean it's similar to Jupiter's largest moon Ganymede; that is, a planet composed of silicate rock and water ice with an ice crust floating over a warmer ice mantle that might contain an ocean of water.
The prevailing idea about Gliese 581c is that it has a diameter about 1.5 times bigger than Earth. This means 581c would have an atmosphere, but what's in that atmosphere remains a mystery. The European research team that discovered the planet believes the average surface temperature on 581c is between a comfortable 32 and 104 degrees Fahrenheit. However, most skeptical astronomers are cautioning that it's just too early to tell whether there is water on 581c.
"You need more work to say it's got water or it doesn't have water," said ex-NASA astronomer Steve Maran, press officer for the American Astronomical Society. "(Even if we had the spacecraft technology available today) you wouldn't send a crew there assuming that when you get there, they'll have enough water to get back."
What's in the Sky: Under fine viewing conditions, deep-sky object M5 (5.6 mag.) can be glimpsed with your naked eye on Sept. 12. Look in the southwest, above Virgo, in the early evening (see map). M5, located 24,000 light years away, is a globular cluster best seen with at least a 3-inch telescope.
Lou Varricchio, M.Sc., is a former NASA senior science writer. He lives in Middlebury, Vt. He is involved with the NASA/JPL Solar System Ambassador program in Vermont.