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An artist's impression of the planetary system, which is about 41 light-years away. ((Courtesy of ESO))

A Neptune-sized planet has been discovered orbiting the habitable zone of a distant star,along with two other planets of similar size.

Astronomers have found more than 170 planets outside our solar system, but most are gas giants like Jupiter that are too hot or too cold for vital liquid water to exist.

For two years, an international team of scientists used a 3.6-metre telescope at La Silla, Chile, to observethe unique planetary system consisting of three planets, one of which may includean atmosphere needed to support life.

The two innermost planets are thought to be extremely hot and rocky on the inside. One of the planets takes only eight days to orbit the star.

"The planet closest to the star is probably rocky and the farthest is the first exoplanet of that mass that is in the habitable zone of its star, meaning, where water could be found in liquid form," the observatory said in a news release.

The planets orbit a star called HD 69830 about 41 light-years away, the team reports in Thursday's issue of the journal Nature.

The star itself is barely visible with the naked eye in the southern constellation of Puppis.

Christophe Lovis of the University of Geneva and his team inferred the presence of the planets based on how their gravitational pull affected light from the star.

If a star has a planet orbiting it then the starlight appears to "wobble." The time it takes to wobble is the planet's year. Scientists are also able to infer the size of the planet and its density.

To observe small, potentially habitable planets directly, NASA is building a Terrestrial Planet Finder telescope.