The glow of 'hot Jupiter' planets has been detected with ground-based telescopes for the first time.The detections are just a first step, astronomers say. But the advance could help settle a mystery over why some of the massive, close-in planets are scorchingly hot on one half and fairly cool on the other, while others seem to spread their heat more evenly.More than 300 planets have been found orbiting other stars. Most are gas giants that circle their host stars so closely that they cannot be imaged directly, since the little starlight they do reflect gets lost in the glare of their host stars.But the heat from some of these planets has previously been measured by NASA's Spitzer Space Telescope.Spitzer used its heat-sensing infrared instruments to image a star with a planet at its side and then again with the planet hidden behind it. Then astronomers subtracted one from the other to distinguish the planet's heat from that of its star
EDIT: Ok, I've just learned helium is in liquid state for cooling the infrared sensors. So running out of helium is indeed not good... well, at least Herschel will take its place