Fournewfound planets orbiting two nearby stars add weight to the promise ofdetecting habitable worlds within the next few years, researchers said today.

Two of theextrasolar planets are considered super-Earths,more massive than Earth but less massive than Uranus and Neptune. Spotting trueEarth-sized planetsis challenging with current technology, but the presence of super-Earthssuggests finding a world like ours is just a matter of time, researchers say.

"Thesedetections indicate that low-mass planets are quite common around nearby stars,"said study team member Steven Vogt, a professor of astronomy and astrophysicsat the University of California, Santa Cruz. "The discovery of potentiallyhabitable nearby worlds may be just a few years away."

Theastronomers are not sure if the super-Earths are rocky like our own world or ifthey have some other composition.

The teamfound the new planet systems by combining data gathered at the W. M. KeckObservatory in Hawaii and the Anglo-Australian Telescope (AAT) in New South Wales, Australia. They inferred the existence of the planets by noting theworlds' gravitational effects on the parent star's orbit. This method is calledthe radial velocity, or wobble, technique.

The objectshave not been photographed.

Three ofthe exoplanets orbit the star 61 Virginis, which is virtually a twin of the sunand lies 28 light-years away in the constellation of Virgo. (At this time ofyear, Virgo can be seen rising a few hours before the sun.)

Theresearchers estimated the minimum mass of each planet as: 5.1 Earth masses for 61Vir b, 18 Earth masses for 61 Vir c, and 23 Earth masses for 61 Vir d, according study team member Chris Tinneyof the University of New South Wales.

"Sothe smallest one is in the super-Earth mass range, and is the first planet likethis to be found around a sun-like star," Tinney told SPACE.com.

Othersuper-Earths have been found aroundstars that are cooler and redder than the sun, he said.

Tinneyadded, "This is exciting, because it demonstrates the ability of our teamto find planets at these interesting, small masses around solar-mass stars. Ifwe want to one day find habitable planets that are really like the Earth insystems that are really like ours, then those are the sorts of stars we need tobe able to find low-mass planets around."

The secondnew system found by the team features a 7.5-Earth-mass planet orbiting HD 1461,another near-perfect twin of the sun located 76 light-years away in theconstellation Cetus. The scientists say at least one and possibly twoadditional planets also orbit the star.

HD 1461 canbe seen with the naked eye in the early evening under good dark-sky conditions.

Theresearchers say they aren't sure whether the planet, now called HD 1461b, is ascaled-up version of Earth, composed largely of rock and iron, or whether, likeUranus and Neptune, it is composed mostly of water.