Lots of background on mu Arae’s (second) discovered planet in this press release from the European Southern Observatory. The discovery was made possible by the HARPS (High Accuracy Radial Velocity Planet Searcher) spectrograph attached to ESO’s 3.6-meter La Silla telescope in Chile. The planet previously discovered at mu Arae is roughly Jupiter-size and orbits the star every 650 days. The new one, much closer in, has the mass of Uranus, and thus straddles the line between rocky and gas giant worlds. To quote the press release (text in bold as given by ESO):

The mass of this planet places it at the boundary between the very large earth-like (rocky) planets and giant planets. As current planetary formation models are still far from being able to account for all the amazing diversity observed amongst the extrasolar planets discovered, astronomers can only speculate on the true nature of the present object. In the current paradigm of giant planet formation, a core is formed first through the accretion of solid “planetesimals”. Once this core reaches a critical mass, gas accumulates in a “runaway” fashion and the mass of the planet increases rapidly. In the present case, this later phase is unlikely to have happened for otherwise the planet would have become much more massive. Furthermore, recent models having shown that migration shortens the formation time, it is unlikely that the present object has migrated over large distances and remained of such small mass.

This object is therefore likely to be a planet with a rocky (not an icy) core surrounded by a small (of the order of a tenth of the total mass) gaseous envelope and would therefore qualify as a “super-Earth”.

Well, ‘super-Earth’ is an unfortunate term, given the temperatures that must prevail on the surface here, but finding rocky planets is big news no matter how close they orbit their primary. The star mu Arae is located in the southern constellation Ara (the Altar) and at 5th magnitude is bright enough to see with the naked eye.