WASHINGTON – Three new 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 new planets are significantly smaller than the many dozens found so far and might even be rocky, as opposed to gaseous, an essential platform for life to evolve.

The scientists who discovered the three planets said the bodies 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 new discoveries bring scientists much closer to actually 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 have to be met before anyone can ask whether other planets could support life.

Candidates need to be at an optimal distance from a star, neither too hot nor too cold. They 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 new discoveries.

Referring to rocky planets as “petri dishes” where 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 Tuesday 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 stars 33 and 41 light-years from Earth in the direction of the constellations Leo and Cancer. A light-year refers to the distance light travels in a year, and both these stars 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-sized – Neptune has about 17 times the mass of Earth. The Europeans described the planet they had discovered as “Uranus-sized” – 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.

In order 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 unlikely platforms for life.

By finding smaller planets, scientists say, there is at least a chance that they are rocky planets, as opposed to gas giants.

Guessing the composition of the new planets is speculative, but Queloz said they could be rocky.