Astronomers have found as many as six new planets orbiting nearby stars that are very similar to our sun.

Two of the planets are "super-Earths," rocky planets with masses five and 7.5 times the mass of Earth. They are the first super-Earths to be found around a sun-like star, the researchers say.

"These detections indicate that low-mass planets are quite common around nearby stars. The discovery of potentially habitable nearby worlds may be just a few years away," said Steven Vogt of the University of California, Santa Cruz.

Australian, American and British astronomers found the planets using the Anglo-Australian Telescope in New South Wales, Australia, and the Keck Telescope in Hawaii.

This graphic shows the orbits of the three newly found planets orbiting 61 Virginis superimposed on the orbits of planets in our solar system. (Chris Tinney) Three of the new planets orbit the star 61 Virginis, a star about 28 light years away, visible in the constellation Virgo. The star is the most similar to our sun of our closest stellar neighbours.

The smallest of the three planets is 5.3 Earth masses and orbits its star once every 4.2 days. The astronomers say the planet is locked into synchronous rotation, like the moon's orbit around the Earth, so one side always faces the star.

The largest of the three is 24.9 Earth masses. The orbits of all three planets would fit inside the orbit of Venus.

"These planets are particularly exciting," said team member Chris Tinney of the University of New South Wales. "Neptune in our solar system has a mass 17 times that of the Earth. It looks like there may be many sun-like stars nearby with planets of that mass or less. They point the way to even smaller planets that could be rocky and suitable for life."

The other super-Earth found orbits a star called HD 1461, another "twin" of the sun, located 76 light years away and visible in the constellation Cetus.

The planet is 7.5 times the mass of the Earth, although the researchers aren't sure whether it is a large rocky planet or a planet consisting of gas and ice, like Uranus and Neptune.

A Jupiter-sized planet was also found orbiting a sun-like star, 23 Librae, found 84 light years away in the constellation Libra. The planet has a 14-year orbit, very similar to Jupiter's 12-year-orbit.

None of the planets were observed directly, but were detected by observing the telltale wobble of their stars produced by the planets' gravity.

The research appears in this week's issue of the Astrophysical Journal.