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KELT-9b, Hottest Planet Found So Far, Almost Surpasses Our Sun
KELT-9 is an A-type star named for the survey that discovered it, the Kilodegree Extremely Little Telescope (KELT). Still in its main sequence, its surface temperature is a mind-boggling 10,000º C (our Sun, for context, is also in its main sequence and is a modest 5,000º C on its surface). In fact, KELT-9 is so hot that astronomers think it fits snugly in the transitional zone between A-type and the even-hotter B-type stars.
The hot star has been found to have an orbiting planet, named KELT-9b, with a surface temperature of of stunning 4,327º C. The planet itself is hotter than an M-type star – making it hotter than most stars we know. It is estimated to be twice the size, and approximately equal to the mass, of Jupiter. The reason this discovery is even more remarkable is that the newly discovered gas giant orbits extremely quick around its host star, completing its journey around the orb in 36 hours.
The hotter a star is, the more rapidly it rotates. And as it whizzes around, it also throws out incredible amounts of ultraviolet radiation. Because KELT-9b is so close, the planet ought to be tidally locked to the star such that the same side faces it at all times. This results in the planet’s dayside being bathed with a glow that can split molecules (if hopes of alien life persisted this far, it is finally time to leave). In fact, astronomers believe that elements like gases and metals found on and around KELT-9b can only exist as atoms, not as molecules. The stellar radiation is also quite likely assiduously stripping away the planet’s atmosphere. We have never directly observed a planet this large and this hot, and so its atmosphere makes for very compelling studies.