Using the ESO's (European Southern Observatory) HARPS instrument, a team of astronomers have discovered a group of at least five planets orbiting a "Sun-like" star known as HD 10180. With strong evidence that two other planets may be present (one of which would have the lowest mass ever found) - this becomes the largest planetary system, outside of our own, ever discovered.

The team of astronomers used the HARPS (High Accuracy Radial velocity Planet Searcher) spectrograph, attached to ESO’s 3.6-metre telescope at La Silla, Chile, for a six-year-long study of the Sun-like star HD 10180, located 127 light-years away in the southern constellation of Hydrus (the Male Water Snake). HARPS is an instrument with unrivalled measurement stability and great precision and is the world’s most successful exoplanet hunter.

Using 190 individual HARPS measurements, the astronomers detected the tiny back and forth motions of the star caused by the complex gravitational attractions from five or more planets. The five strongest signals correspond to planets with Neptune-like masses — between 13 and 25 Earth masses — which orbit the star with periods ranging from about 6 to 600 days. These planets are located between 0.06 and 1.4 times the Earth–Sun distance from their central star.

“We have found what is most likely the system with the most planets yet discovered,” says Christophe Lovis, lead author of the paper reporting the result. “This remarkable discovery also highlights the fact that we are now entering a new era in exoplanet research: the study of complex planetary systems and not just of individual planets. Studies of planetary motions in the new system reveal complex gravitational interactions between the planets and give us insights into the long-term evolution of the system.”

The newly discovered system of planets around HD 10180 is unique in several respects. First of all, with at least five Neptune-like planets lying within a distance equivalent to the orbit of Mars, this system is more populated than our Solar System in its inner region, and has many more massive planets there. Furthermore, the system probably has no Jupiter-like gas giant. In addition, all the planets seem to have almost circular orbits.

So far, astronomers know of fifteen systems with at least three planets. The last record-holder was 55 Cancri, which contains five planets, two of them being giant planets. “Systems of low-mass planets like the one around HD 10180 appear to be quite common, but their formation history remains a puzzle,” says Lovis.

For more information, visit the ESO release.