Artist conception of the KELT-9 system. The host star is rapidly rotating A-type star that is about 2.5 times more massive and almost twice as hot as our sun
NASA/JPL-Caltech/R. Hurt (IPAC)

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This exoplanet is hotter than most stars

While the search continues for hospitable, Earth-like planets around other stars, one team has just found what may be the most hostile world so far discovered. KELT-9b (imagined above) is so close to its star—a hot “A-type” named KELT-9—that its star-facing side reaches temperatures almost as hot as our own sun. Only six other planets are known to orbit A-type stars, which have surface temperatures of up to 10,000 K. Such hot stars are bad for planet hunting because their spectra have few landmarks for astronomers to monitor to detect the back-and-forth tug of orbiting worlds; they also tend to spin fast, further blurring the picture. But a team using the Kilodegree Extremely Little Telescope (KELT), an array with instruments in Arizona and South Africa, got lucky when it spied KELT-9, with a surface temperature of 10,200 K. Scientists saw the shadow of a planet, KELT-9b, pass in front of it every 1.5 days. From the shape of the brightness dip, the team estimated that KELT-9b is roughly Jupiter-sized. But to find out how hot it is, they took another measure: the brightness of the planet, which they calculated by measuring how much the brightness of the system dropped when KELT-9b passed behind its star. That number—0.1%—helped them peg the planet’s dayside temperature at about 4600 K, hotter than most stars, they report today in Nature. No molecules are likely to survive KELT-9b’s atmosphere, which the team guesses is made up of atomic metals. The atmosphere is probably also eroding under the barrage of extreme ultraviolet radiation from the hot star. Though KELT-9b will teach scientists a lot about planetary systems very different from our own, don’t expect it to deliver any insights on alien life.