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Richest planetary system discovered

Scientists believe there are as many as seven planets orbiting this Sun-like star.
Provided by ESO, Garching, Germany
Star HD10180
This artist's impression shows the remarkable planetary system around the Sun-like star HD 10180.
ESO/L. Calçada
August 24, 2010
Astronomers using the European Southern Observatory's (ESO) High Accuracy Radial velocity Planet Searcher (HARPS) instrument have discovered a planetary system containing at least five planets, orbiting the Sun-like star HD 10180. The researchers also have tantalizing evidence that two other planets may be present, one of which would have the lowest mass ever found. This would make the system similar to our solar system in terms of the number of planets (seven as compared to the solar system's eight planets). Furthermore, the team also found evidence that the distances of the planets from their star follow a regular pattern, as also seen in our solar system.

"We have found what is most likely the system with the most planets yet discovered," said Christophe Lovis from the University of Geneva, Switzerland. "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 team of astronomers used the HARPS spectrograph, attached to ESO's 3.6-meter telescope at La Silla, Chile, for a 6-year-long study of the Sun-like star HD 10180, located 127 light-years away in the southern constellation Hydrus the Water Snake. HARPS is an instrument with unrivalled measurement stability and great precision, and it's the world's most successful exoplanet hunter.

Thanks to the 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 also have good reasons to believe that two other planets are present," said Lovis. One would be a Saturn-like planet, with a minimum mass of 65 Earth masses, orbiting in 2,200 days. The other would be the least massive exoplanet ever discovered, with a mass of about 1.4 times that of Earth. It is close to its host star, at just 2 percent of the Earth-Sun distance. One year on this planet would last only 1.18 Earth days.

"This object causes a wobble of its star of only about 2 mph (3 km/h) — slower than walking speed — and this motion is very hard to measure," said Damien Segransan from the University of Geneva. If confirmed, this object would be another example of a hot, rocky planet similar to Corot-7b.

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. 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 15 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," said Lovis.

Using the new discovery as well as data for other planetary systems, the astronomers found an equivalent of the Titius-Bode law that exists in our solar system — the distances of the planets from their star seem to follow a regular pattern. "This could be a signature of the formation process of these planetary systems," said Michel Mayor of the University of Geneva.

Another important result found by the astronomers while studying these systems is that there is a relationship between the mass of a planetary system and the mass and chemical content of its host star. All massive planetary systems are found around massive and metal-rich stars, while the four lowest-mass systems are found around lower-mass and metal-poor stars. Such properties confirm current theoretical models.
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