The planet was discovered around a star called HD 149026, which is rich in elements heavier than hydrogen and helium. Such "metal-rich" stars are three times more likely than their Sun-like cousins to harbour detectable planets.
Astronomers using Japan's powerful Subaru Telescope in Hawaii first detected the planet, called HD 149026b, in July 2004 by the gravitational tugs it exerted on its host star. Subsequent measurements taken by the Keck Observatory in Hawaii confirmed the star's resultant wobble, revealing the planet orbits its star every 2.87 days.
But in May 2005, astronomers using a relatively small robotic telescope in Arizona noticed a 0.3% dip in the star's brightness every 2.87 days. That dimming showed the planet was among an elite group of eight which are known to pass directly between their host stars and Earth during their orbits. Such transits can reveal the planet's exact physical size and mass, from which its density can be calculated.
The results were surprising. The planet's diameter was about three-quarters that of Jupiter. But its density suggested that half to two-thirds of its mass was locked in heavy elements - mostly in a solid core.
That differs strikingly from other transiting extrasolar planets which, like Jupiter, are mostly hydrogen and helium gas with no more than a quarter of their mass taken up by their cores."This is a very weird object," says team member Greg Laughlin, an astronomer at the University of California in Santa Cruz, US.
The Extrasolar Planets Encyclopedia has an article with basic data on HD 149026 b and its star here, and links to the PDF preprint of the upcoming Astrophysical Journal article "A Transiting Hot Saturn Around HD 149026 With a Large Dense Core." This page provides much the same information and has a artist's recreation to boot.
I note that HD 149026, roughly 200 light-years away from us, is a yellow sub-giant star, located off of the main sequence of stars. Would the more astronomically knowledgeable be able to tell me if this status has anything to do with this star's unusual planet?