U.S. astronomers have made the first measurements of the atmospheres of planets far outside our solar system by using the light they emit. They were searching for the spectral signature of water, but found none. Yet, as VOA's David McAlary reports from Washington, the scientists say the technique is promising as a way to hunt for water in the quest for life elsewhere in the universe.

Dusty eye of the Helix nebula

Three teams of astronomers aimed the U.S. space agency's at planets around stars in the constellations Pegasus and Vulpecula. They are huge giant gas planets like Jupiter, but even so, they are too far away to be seen directly, so their presence is detected when the stars' light dims as the planets pass in front.

By subtracting each star's light from the combination of light from each star and its planet together, the researchers have become the first to determine the spectrum of feeble light the planets emit alone.

"By measuring the spectrum of a planet, we can learn more about the structure of a planet's atmosphere and find some clues as to what the planets might be made of," said Jeremy Richardson.

Jeremy Richardson is the leader of one team, of the U.S. space agency NASA. He looked at a planet in one of the constellations, while another team led by Carl Grillmair of the California Institute of Technology looked at the other.

Grillmair says they were certain they would detect the light signatures of water vapor because the two suns they orbit have water's building blocks, hydrogen and oxygen, both very stable elements.

"That was really what we expected to see before we made these observations and, of course, what surprised us when we did not find it," said Carl Grillmair.

In fact, one planet's atmosphere exhibited signs of the dust of silica, the material found in rocks on Earth. Jeremy Richardson says the dust might be blocking the light from water vapor he is sure is there. His findings are published in the journal Nature, while the others are in Astrophysical Journal Letters.

Even though they did not find water, the scientists say the pioneering spectral technique has much promise for the future. The leader of the third research team, NASA astronomer Mark Swain, says it can be used with ever more powerful telescopes of the future to seek water on planets in other solar systems.

"These results are a very important stepping stone for our ultimate goal of characterizing other planets around other stars where life could exist," said Mark Swain. "So the results you are seeing today are really a dress rehearsal for what we are looking forward to in the future."

With or without a finding of water, Jeremy Richardson is excited by the new astronomy technique.

"It has always been really exciting to me that we can learn so much detailed information about these objects that are so dim and so far away," he said.