This artist's impression shows Gliese 667 C, one of the new exoplanets found by HARPS. The planet, six times the mass of Earth, belongs to a triple star system. Two of the stars can be seen in the distance. (ESO/L. Calcada)

European astronomers have found at least 32 new planets, bringing the number of known planets outside our solar system to more than 400.

Six of the new planets discovered by the European Southern Observatory are less than 20 times the mass of Earth. The discovery increased the number of known super-Earths by 30 per cent.

At the extremes, two of the new planets are about five times the size of Earth, and one is up to five times larger than Jupiter. None of the new planets is in the habitable zone of a star.

Astronomer Stéphane Udry of the University of Geneva announced the discovery at a conference Monday in Porto, Portugal. He said it supports the idea that planets are a common feature in the universe.

"I'm pretty confident that there are Earth-like planets everywhere," Udry said in a web-based news conference. "Nature doesn't like a vacuum. If there is space to put a planet there, there will be a planet there."

The discoveries came from an instrument known as HARPS (High Accuracy Radial Velocity Planet Searcher), a spectrograph for the ESO 3.6-metre telescope in La Silla, Chile. The instrument detects planets indirectly by looking for tiny wobbles in a star's movements.

HARPS has found more than 75 planets outside the solar system, or exoplanets, including 24 of the 28 known super-Earths.

Most of the super-Earths were found in systems with multiple planets, up to five planets per system.

As well, the survey found gas giant planets orbiting low-mass stars called M dwarfs, presenting a challenge to current theories of planet formation.

"These observations have given astronomers a great insight into the diversity of planetary systems and help us understand how they can form," said astronomer Nuno Santos, who contributed to the discovery.

With files from The Associated Press