spaceSpace and Physics

Astronomers Discover Planet That Is Hotter Than Most Stars


Stephen Luntz

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer


Artist's impression of KELT-9, a star more than twice the mass and almost twice as hot as the Sun and a planet that has got stupidly close, orbiting in just 36 hours. NASA/JPL-Caltech/R. Hurt (IPAC)

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.


The first detection of changes in KELT-9's brightness consistent with a transiting planet was made at the KELT-North telescope in Arizona, but with an orbit of just 1.5 days, observations were needed in many time zones. A string of modest-sized telescopes, most costing a tiny fraction of the giant instruments usually responsible for big scientific discoveries, were pressed into service, confirming the discovery. Eventually, it is hoped time will be found on space telescopes to allow us to learn about KELT-9b in a depth the smaller scopes could never manage.

KELT-9 is 650 light-years away from us, not close enough for the sort of observations we might achieve for transiting planets around stars one-fifteenth of that distance, but still closer than many of the planets found by the Kepler Space Telescope. 

The extreme conditions are affecting more than KELT-9b's atmosphere. Even its orbit is changing faster than that of most planets, such that in 150 years it is expected to no longer transit, as seen from Earth, and won't come back into view for 3,500 years.

The shortage of planets around very bright stars has largely been a product of the challenges in studying these stars. For example, they tend to rotate more quickly than lower mass stars, which makes it harder to distinguish planetary effects from changes intrinsic to the star.


Moreover, there is little chance of life around these powerhouses. That's obviously true for KELT-9b, but even at a safer distance from such a bright object, life would have little time to evolve since large stars have much shorter lifespans before turning into red giants – something KELT-9 will do in around 200 million years. That's made these sorts of stars unattractive study objects for planet hunters, but the authors of the Nature paper still think we have plenty to learn from objects like KELT-9b, even if you definitely wouldn't want to visit.

spaceSpace and Physics
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  • hot jupiter,

  • exoplanet,

  • B-type star,

  • A-type star,

  • hottest known