Three planets have been discovered in other solar systems and are the closest ever found to Earth in size, marking an important step in the search for planets that could support life elsewhere in the universe, scientists have announced.

The planets are significantly smaller than the many dozens found so far and might even be rocky, an essential platform for life to evolve. The scientists who discovered the three planets said they were probably too hot to support life themselves, although one has a lukewarm zone that could conceivably support biological organisms.

While the existence of habitable planets elsewhere in the universe has long been hypothesized, the discoveries bring scientists much closer to finding another planet like Earth in another solar system. Given the number of stars in the universe, such planets might well be plentiful.

Many conditions would need to be met for any other planets to support life. Candidates need to be at an optimal distance from a star, and neither too hot nor too cold. They would probably need liquid water and would not trap harmful radiation, as Venus does. Once such a planet has been found, scientists could answer the final question: Was life on Earth a miraculous quirk or the inevitable result of physics, chemistry and celestial geography?

"The ingredients of life are abundant in the universe," said Geoffrey Marcy, a planet hunter at the University of California at Berkeley, who helped make the discoveries. Referring to rocky planets as "petri dishes" in which the ingredients of life could come together, Marcy added, "in the next five, 10, 20 years, maybe we will learn if there are microbes, furry creatures, even intelligent life on other planets."

Two of the planets were announced yesterday by teams of U.S. scientists, and the third was announced earlier in the week by a team of European scientists. While the Americans and Europeans squabbled over whose discovery came first and which planet was the smallest, all agreed that an important frontier in planet-hunting had been crossed.

The two planets discovered by the Americans were found orbiting around stars 33 and 41 light-years from Earth in the direction of the constellations Leo and Cancer, respectively. A light-year refers to the distance light travels in a year, and both these constellations are relatively nearby in astronomical terms. The planet found by the Europeans is about 50 light-years away, according to Didier Queloz, an astronomer at the University of Geneva who helped discover that planet.

The Americans referred to the group of new planets as Neptune-size -- Neptune has about 17 times the mass of Earth. The Europeans described the planet they had discovered as Uranus-size -- which is about 14 times the mass of Earth. The smaller the planet, the greater the odds that it is similar to Earth.

All scientists hunting for planets are hamstrung by an inherent problem: No planet outside our solar system is visible to the naked eye or even through the most powerful telescopes. Because planets do not emit light -- they only reflect it from the star they orbit -- they are essentially invisible.

To detect them, scientists use a sophisticated technique that measures changes in the light emitted by a star as a planet revolves around it. The technique tends to spot large, Jupiter-like planets orbiting close to stars. Not surprisingly, most of the planets found to date have been so-called gas giants with short orbits -- remarkable objects unlike anything in our own solar system, and, because they are gaseous, unlikely platforms for life.

By finding smaller planets, scientists say, there is at least a chance that they are rocky planets.

Guessing the composition of the new planets is speculative, but Queloz said they could be rocky. He argued that gaseous planets often form at great distances from a star and then migrate toward it, collecting more mass in the process -- this is the leading theory to explain the many Jupiter-like planets with short orbits found so far. By contrast, a smaller planet found close to a star probably originated much closer to the star and was therefore probably rocky.

Paul Butler, a planet hunter at the Carnegie Institution in Washington who helped make the American discoveries, said that right now, the data offered no clue as to whether the planets were rocky or gaseous.

One planet found by an American team is at least 21 times Earth's mass and zips around its star every 2.64 days, according to the National Science Foundation, which helped support the scientists, along with NASA. The star being orbited is an M dwarf -- such objects are especially plentiful but difficult to study because they are relatively dim. As to whether the planet could support life, Marcy said it was probably too close to the star.

"One side is blowtorched, and the other side is dark and cold," he said. But in between, he added, "there is a ring where the temperatures are lukewarm."

The other planet is about 18 times Earth's mass, and revolves around its star every 2.8 days. This planet is part of a solar system where three other planets were discovered previously, making that the solar system that most closely resembles our own.

Barbara McArthur of the University of Texas at Austin, who helped discover that planet, said scientists "don't know what it is composed of, but it could be rocky." The planet is too close to the star and too hot to support life, she said.

Marcy said there are about 20 billion planetary systems in the Milky Way galaxy alone -- meaning that scientists are at the very start of their search for planets similar to Earth.

The planet discovered by the European team had an orbit of about 9.5 days, Queloz said, and is also too hot to support life.

While the Europeans announced their discovery to the public ahead of the Americans, the Americans had beaten their competitors in submitting findings for scientific publication several weeks ago, said Alan Boss, an astronomer at the Carnegie Institution of Washington who offered comments on the new findings at a briefing at NASA headquarters yesterday. The Europeans, he said, submitted a paper for publication only last week.

"I would award them the bronze [medal] this time," Boss said.

In an e-mail, Stephane Udry and Michel Mayor of the European team said they took strong objection to the Americans' describing the European finding as "preliminary." And they contested the claim that the two planets found by the Americans were the "smallest extra-solar planets yet."

"This is obviously wrong," the European scientists wrote, arguing that they had found the smallest planet to date.

At the NASA briefing yesterday, Butler said all the planets were about the same size. He dismissed the idea that there was a difference between Uranus-sized planets and Neptune-sized planets, saying, "It's ridiculous."

Paul Butler, with from left, fellow scientists Geoffrey Marcy, Anne Kinney, Barbara McArthur and Alan Boss announces the discovery of planets outside the solar system.