An artist's concept of an extrasolar planet. Photo: Greg Bacon, STScI

An international team of astronomers announced Thursday the discovery of 11 new planets orbiting other stars, including a pair locked in an orbital resonance and another with a very Earth-like orbit.

The team, led by veteran planet hunter Michel Mayor of Geneva University in Switzerland, used telescopes in Chile, France, and Israel to detect the planets, all orbiting Sun-like stars 28 to 65 parsecs from the Earth.

Among the new 11 planets were a number of interesting discoveries. Two new solar systems ­ defined as more than one planet orbiting the same star ­ were discovered. One, HD 74156, features one planet with a minimum mass 1.5 times that of Jupiter orbiting 40 million kilometers from the star, and a second 5 times as massive orbiting 725 million kilometers away, almost the same distance Jupiter is from the Sun.

Another new planet orbits the star HD 82943, where another planet was discovered last year. The new planet, just under 90% of the planet Jupiter, orbits 110 million kilometers from the star and completes an orbit every 222 days, exactly half the orbital period of the other planet. Astronomers attribute this to an orbital resonance, when the gravity of the two planets lock them into marchstep, and is seen as additional evidence that the planets truly exist. It's the second such resonant system discovered: last year astronomers reported finding two planets locked into a similar "2:1" resonance around the star Gliese 876.

All the planets discovered by Mayor's group are gas giants, up to ten times the mass of Jupiter, and most lie in orbits that are either very close to their parent stars or very distant. However, one, orbiting HD 28185, is about the same distance from its star as the Earth is from the Sun, and has a relatively circular orbit, making it one of the most promising planets for life yet discovered. While it is unlikely the planet itself could support life -- it is at least 5.6 times as heavy as Jupiter -- any large moons that orbit the planet would at least have the right amount of solar radiation to support Earth-like conditions.

One planet unlikely to support life in any way is the one found orbiting HD 80606. It has the most eccentric orbit of any extrasolar planet yet discovered, going between 5 and 127 kilometers from its parent star.

These new planets were detected using the radial velocity technique, the same method used to find most of the extrasolar planets discovered to date. The technique looks for a wobble in the parent star caused by the gravity of the orbiting planet. That wobble is manifested in the form a periodic Doppler shift of the spectra of the star, which is measured by high-resolution spectrographs mounted on telescopes.

A drawback of the technique is that it can only provide minimum masses of the planets, since astronomers don't know if they are viewing the planet's orbit edge-on, which would maximize the wobble and thus minimize the mass, or nearly face-on, which would minimize the wobble and thus require a far more massive planet. Astronomers plan to use new interferometer systems on telescopes like the Very Large Telescope in Chile in an attempt to measure the wobbles in the positions of the stars themselves, and thus better determine the masses of the planets.