nets -- 246 by latest count -- revolving around suns beyond our own, the discovery of water on one of them strikes a hopeful note.

Using NASA's $720 million Spitzer Space Telescope, astrophysicist Giovanna Tinetti and her colleagues at the European Space Agency in Paris detected water molecules on a planet 370 trillion miles away, they reported recently in Nature. This world, known only as HD 189733b, orbits a cool, dim star in the constellation Vulpecula, a stellar landmark so remote that its light takes about 64 years to reach us.

In the atmosphere of this giant planet, water may exist only as steam in boiling skies. Slightly larger than Jupiter, the planet is just 1/30th the distance to its star than Earth is to the Sun, the scientists said. Consequently, its normal temperature is 1,700° Fahrenheit, hot enough to melt silver.

"Our discovery shows that water might be more common out there than previously thought," said Dr. Tinetti. "I hope we can find water on planets less hostile."

Water, crucial for the origin of life on Earth, has animated NASA's search for life-forms on other worlds for decades. Probes have detected oceans of slush on Europa, a moon of Jupiter, and active geysers on Enceladus, a moon of Saturn. Tomorrow morning, the U.S. space agency expects to launch a $386 million probe to once again seek water on Mars.

Astronomers hoped that water also would be plentiful on so-called extrasolar planets circling other stars, but researchers tried and failed to find it until now.

These extrasolar explorations tax the ingenuity of astronomers. "You can't actually see the planet," said Harvard University astronomer Heather Knutson, who published the first temperature map of an extrasolar planet in Nature in May. "You can only measure the light from the system."

To detect water 64 light years away, Dr. Tinetti and her team used the Spitzer Telescope to measure tiny variations in wavelengths of infrared light as the planet orbited in front of its star and eclipsed it. "You stare at the planet, watch the dip in light and measure it," said Sean Casey at Caltech's Spitzer Science Center in Pasadena, Calif. They saw that for each wavelength, a different amount of light was absorbed by the planet. The pattern matches that of water molecules.

All told, 68 ground observatories and 18 satellites are scanning tens of thousands of stars for signs of alien worlds, according to the online Extrasolar Planets Encyclopedia. The newest -- Europe's $413 million Corot space telescope -- discovered its first planet in May.

Astronomers have catalogued a menagerie of planetary curiosities, including a Goliath 17 times the size of the planet Jupiter and a collection of gas giants so close to their stars that they orbit them in a day, compared with the 88 days it takes Mercury, our system's fastest planet, to circle the Sun.

In all, there may be six billion such hot Jupiter-like planets, researchers working with the Hubble Space Telescope estimated last year. It is unlikely any of them harbor life, Dr. Tinetti said. In 2009, NASA plans to launch a $550 million space telescope, called Kepler, capable of finding smaller Earth-size planets.

Our knowledge of the universe we call home -- and the search for water worlds hospitable to life -- is expanding almost as quickly as the cosmos itself.

Look up on any midsummer midnight and consider the high dome of stars. It still reverberates from the Big Bang of its creation 13.7 billion years ago. By current scientific understanding, it mostly contains not the stuff we normally can see and touch, but a dark matter that no one so far can measure directly.

Black holes inhale entire galaxies. Gamma-ray bursts release more energy in a blink than our Sun can produce in a billion years. Super-nova explosions scatter elements, like pollen, on the stellar wind.

In the face of so much strangeness, it may be only human nature to seek out the familiar. Those searching for extraterrestrial life, however, ought to abandon the assumption that alien organisms would utilize the same biochemical architecture as life on Earth, a research panel recently urged.

"It is clear that nothing would be more tragic in the American exploration of space than to encounter alien life without recognizing it," said University of Washington oceanographer John Baross. He is chairman of a National Research Council panel that recently issued a report called "The Limits of Organic Life in Planetary Systems."

Life, the panel speculated, might arise as readily in seas of liquid methane, in streams of ammonia, or in caverns of nitrogen ice.

Still, we stare longingly into the mirror of the cosmos -- lonely hearts yearning for a glimpse of our own reflection.