Using ESO's sensitive HARP instrument, astronomers have discovered a solar system containing at least five planets, with indications that two more, including a hot, rocky world, might also be present.

The planets orbit a Sun-like star called HD 10180, and their orbital distances follow a regular pattern similar to that in our own Solar System, with each planet roughly twice as far away from the Sun as the previous object. The planets also appear to track around their star on nearly circular orbits.

“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 astronomers made the discovery using the HARPS spectrograph attached to ESO's 3.6 metre telescope at La Silla in Chile, and made 190 individual measurements over six years. This enabled the team to detect the tiny motions of the star caused by the gravitational tug of the orbiting planets. Five strong signals indicate Neptune-sized planets, which orbit their Sun with "years" ranging from six to six hundred Earth days.

“We also have good reasons to believe that two other planets are present,” reveals Lovis. While one of the suspects is likely a similar size to Saturn, orbiting the star in 2,200 days, the other could be a hot, rocky planet just 1.4 times the mass of the Earth. However, this planetary candidate orbits very close to the star in just 1.18 days, and its influence on the parent star is difficult to measure.

“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. To date, astronomers know of fifteen solar systems with at least three planets, but HD 10180 is unique in having its brood of (at least) five planets orbiting within the innermost region of its system – all within a distance equivalent to that of Mars in our own Solar System.

As more and more exoplanet systems are identified – especially with the bounty of planets anticipated from the Kepler mission – scientists will become better equipped to understand how multiple planet systems form.