A newly discovered planet has set a string of records. It is easily the hottest ever discovered and orbits the brightest and highest mass star known to have a planetary companion. If, as its orbit suggests, one face is permanently turned towards its parent start, temperatures on that side would reach 4,600 kelvin (7,800º F), hotter than red dwarf stars.
Although planets have been discovered in recent years at a dizzying pace, we are yet to find any around B-type stars, those with masses 2-16 times that of the Sun and temperatures between 10,000 and 30,000 kelvin. Only six have been found around A-type stars, which have temperatures 7,300–10,000 kelvin, and all have been at the cooler end of that range. That is until now, when the star KELT-9, whose characteristics include a temperature of 10,170 kelvin, putting it at the A-B border, was found to have a planet, known as KELT-9b.
The combination of a close orbit and KELT-9's strong output means that KELT-9b breaks the record for the hottest star-side temperature of any known by a margin of more than 1,000 degrees. In fact, even our Sun, hardly a faint star, is closer in temperature to the planet in the KELT-9 system than to the star itself.
Horrendous things happen at such temperatures. Although KELT-9b has 2.8 times Jupiter's mass, it is much larger, because the radiation from the star has caused it to puff up, and is probably blasting the atmosphere into space, producing a gaseous tail. Even the toughest molecules in the atmosphere could not survive at those temperatures, being rapidly ripped apart, although some could exist on the night side. “It’s a planet by any of the typical definitions based on mass, but its atmosphere is almost certainly unlike any other planet we’ve ever seen just because of the temperature of its day side,” said
“It’s a planet by any of the typical definitions based on mass, but its atmosphere is almost certainly unlike any other planet we’ve ever seen just because of the temperature of its day side,” said Professor Scott Gaudi of the University of Ohio in a statement.
KELT-9b was discovered by Gaudi and his co-authors because, as seen from Earth, it transits across the face of its star, temporarily blocking out some of its light, they report in Nature. This allows us to collect more information than we could if it could only be detected using the doppler-wobble technique used to identify the first planets beyond the Solar System. Its name comes from the Kilodegree Extremely Little Telescopes (KELTs) used to find it.