[This story is based on material from CNN, Reuters and Newsday.]
Mysterious gassy planets spotted outside the Earth's solar system may not be hospitable to life as we know it, but rocky moons orbiting them might be, U.S. scientists said on January 16
Darren Williams and colleagues at Pennsylvania State University said they had identified two distant star systems that might have planets with such moons.
"Such a moon would... need to be large enough to retain a substantial and long-lived atmosphere, and would also need to possess a strong magnetic field in order to prevent its atmosphere from being sputtered away by the constant bombardment of energetic ions from the planet's magnetosphere," they wrote in a letter to the science journal Nature.
There are precedents. Jupiter's moons Io and Europa have water and Earth-like atmospheres, and Ganymede has a magnetosphere. Saturn's moon Titan also has a measurable atmosphere.
"All this suggests that the systems belonging to 47 Ursae Majoris and 16 Cygni B should be considered as possible abodes for extraterrestrial life," they wrote. These are two of the systems shown to have large planets since the first such discovery was made in late 1995.
So far, nine probable planets have been spotted outside our solar system, but they are all as large or larger than Jupiter and considered unlikely to support life as we know it.
But new photos of Jupiter's moon Europa, taken by the Galileo spacecraft on December 19 and announced by NASA on January 17, show what appears to be an ice-covered ocean, a discovery that could greatly improve the odds of life existing elsewhere, scientists say.
The new images indicate that Europa has a thin, fragile layer of blue ice that covers a thick layer of what could be a liquid water, or semi-frozen slush. Complex markings in the ice suggest the ice often cracks and shifts, allowing what is below to ooze to the surface.
If the findings pan out, Europa would be the first body outside of Earth where liquid water is abundant. So the photos are touching off speculation about the presence of life on Europa, in part because life thrives beneath the sea ice at Antarctica.
"I think Europa is an excellent laboratory for the existence of possible life," said Jim Brown, a planetary scientist at Brown University in Rhode Island. "It's got the ingredients."
One thing that needs to be explained is how Europa, which should be frozen solid because it is so far from the sun, might keep water warm enough so it remains in a liquid state. The likely source of energy is tidal friction, a constant torquing and twisting of the moon caused by
interactions with the gravity of Jupiter and the other three Galilean moons.
This suggests that a whole new category of planet-like moons could be suitable for unusual forms of life. These moons would not rely on their sun, but rather on the gravity of their parent planet, to produce life-sustaining warmth.
It is already known that if water and sufficient warmth are present, solar radiance is not necessary for life. On earth, exotic lifeforms have been found surrounding thermal vents in ocean trenches so deep that no sunlight can touch them.
The new color pictures from Europa show a smooth, uncratered surface. The absence of craters indicates that Europa's existing surface is very young. Any craters formed by bombarding meteorites seem to be quickly erased by whatever is "re-paving" the moon's cold landscape.
The new images of Europa offer such big surprises that scientists are already trying to persuade NASA to allow Galileo to explore the remarkable moon in greater detail. Galileo will make its closest scheduled pass at Europa on February 20, when it comes within 364 miles of the moon's surface.
But more can and should be done, scientists say, beyond the scheduled end of Galileo's mission later this year. The only obstacles to extending Galileo's explorations will be its onboard supply of maneuvering gas, and NASA's limited supply of money.
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