Artistic view of the system HD209458, shown to scale (a rare occurence
in astronomy!). The star, of type F8V to G0V, is slightly hotter
and larger than our Sun. The planet, about 30% less massive than
Jupiter, orbits around the star in 3.5 days, only 7 to 8 stellar radii
away! Its orbit is seen edge on from Earth, and the slight dimming of
the stellar light thus enables a determination of its radius:
approximatively 1.4 times that of Jupiter. This confirms that the
planet is a hydrogen-rich gas giant. It is similar to an inflated
Jupiter or Saturn, but whose atmosphere would be heated above 1200K.