Scientists have gotten their best look ever at the composition of planets outside our solar system, but were more surprised by what they didn't see than what they did see.
Three teams of scientists described on Wednesday data collected by NASA's orbiting Spitzer Space Telescope on two Jupiter-like gas planets hundreds of trillions of miles away--one in the constellation Pegasus and the other in the constellation Vulpecula.
The scientists had been confident of finding water, in the form of vapor, in the atmospheres of the two planets. They were wrong.
And one of the planets had evidence of small sand-like particles, called silicates, in the atmosphere, suggesting it is wrapped in high, dusty clouds unlike any planets in our solar system.
Spitzer observed for the first time enough light to figure out signatures of molecules in the atmospheres of planets from outside our little corner of the universe.
"It's actually rather momentous," said Carl Grillmair, of NASA's Spitzer Science Center at the California Institute of Technology in Pasadena. "It's a step in a long chain of events that hopefully will eventually lead us to discovering life on some other planet."
These two planets--terrifically hot gas giants whizzing around their stars in alarmingly close orbits--are unlikely to harbor life as we know it, Grillmair said.
But scientists hope to use similar techniques to scour smaller, rocky planets more like Earth for indications of life, perhaps in the form of oxygen or possibly chlorophyll.
Teams led by Grillmair, Jeremy Richardson of NASA's Goddard Space Flight Center and Mark Swain of NASA's Jet Propulsion Laboratory studied the spectrum emitted by each planet. The findings appear in the journal Nature and the Astrophysical Journal Letters.
A spectrum is produced when radiation is split into its component wavelengths, like when light passes through a prism and is split into the colors of the rainbow. In this research, the scientists split up the light in the infrared region.
"The main finding is that we don't see evidence for water in the two planets," Richardson said. "The theorists will tell you that water, in the form of vapor or steam, must be present in the planets' atmospheres. This is because they are made mostly of hydrogen, with some oxygen present."
Water may in fact be there, Richardson said, hidden by a thick layer of clouds.
Grillmair said scientists had also expected to see methane and carbon dioxide but saw none.
The two planets are known as "hot Jupiters." They are a bit bigger than the solar system's largest planet, but 100 times closer to its star than Jupiter is to the Sun and 10 times closer than our innermost planet, Mercury, with planetary temperatures at a scorching 1,500 degrees Fahrenheit.
Planet HD 189733b, about 62 light-years away in the constellation Vulpecula, orbits its star once every 2.2 Earth days. Planet HD 209458b, about 153 light-years away in the constellation Pegasus, orbits its star once every 3.5 days.
Both planets, from the vantage point of Earth, pass behind their stars during orbit, temporarily disappearing from view. Spitzer's equipment measured the decline in infrared light when the planet disappeared to determine what light was coming exclusively from the planet.
Scientists have identified about 200 planets outside our solar system.