NASA scientists using the Hubble Space Telescope have discovered what they believe are 16 new planets deep in the Milky Way, leading them to conclude there are probably billions of planets spread throughout the galaxy.

Over the past 15 years, astronomers have identified more than 200 planets outside our solar system, but the new ones identified by the Hubble are at least 10 times as far from Earth.

That planets can be found at the center of the galaxy, as well as near our solar system, has given NASA researchers confidence that they are likely to be everywhere. If that is the case, then the likelihood of other Earth-like planets becomes greater.

"We all are dreamers, and part of that dream is to find life somewhere," said Mario Livio, head of the science program at the Space Telescope Science Institute in Baltimore, which oversees Hubble operations. "We're finding that the galaxy is full of planets, and the chances are, somewhere out there, we will find one with the conditions necessary to be habitable."

The new planets were introduced yesterday as mostly "candidates," since only two could be definitively described as planets. But Livio and team leader Kailash Sahu said the chances are good that some, or even all, of the 16 will ultimately meet all the criteria to be called planets. Based on the number of planets identified and the number of stars in the Milky Way, the scientists estimated that as many as 6 billion Jupiter-size planets exist in the galaxy.

"Our discovery . . . gives very strong evidence that planets are as abundant in other parts of the galaxy as they are in our solar neighborhood," Sahu said. One of the biggest surprises of their work, Sahu said, was that five of the likely planets orbit so close to their suns that they make it around in less than one Earth day.

These close-in, Jupiter-size planets are not necessarily the most prevalent, he said, but rather are the ones most easily identified using the techniques available for peering deep into the galaxy. The planet closest to its star has an inhospitable estimated surface temperature of 3,000 degrees Fahrenheit.

The search for what are called "extrasolar" planets is done through indirection, since they cannot be seen by even the strongest telescopes. Instead, astronomers identify them by the way they briefly block some of the light from distant stars, an event called a "transit." The planet would have to be about the size of Jupiter to block enough starlight to be detected by Hubble.

The Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS) used the Hubble's deep-field telescope for seven days in early 2004. The telescope monitored 180,000 stars in the crowded central bulge of the Milky Way for the periodic dimming caused by planet transits. The area examined by the orbiting telescope is about 26,000 light-years away.

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