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Horror in space: Hot alien giant boiled alive by nasty radiation-belching star

Colossus suffers cosmic tanning session from hell

Artist's impression of KELT-9b around KELT-9, its host star (Image credit: Robert Hurt, NASA/JPL-Caltech)

Astronomers have discovered KELT‑9b – the hottest giant exoplanet yet seen. It is twice the size of Jupiter, has a dayside temperature of 4,600 Kelvin, and is being stripped by ultraviolet radiation from its star, KELT‑9.

Thousands of exoplanets – alien worlds beyond our own solar system – have been spotted in the depths of space. But stumbling on one that orbits a hot A‑type star is rare. None have ever been found around blistering O‑type stars, the hottest class of stars.

"KELT‑9 is only the seventh A‑type star known to host a transiting companion, and is by a significant margin the hottest, most massive and most luminous known transiting giant-planet host," according to a letter published in Nature.

It was first noticed in 2014, when astronomers at US universities Ohio State and Vanderbilt spotted the brightness of KELT‑9 dipped by about 1 per cent every 1.5 days.

KELT‑9's surface is estimated to be a whopping 10,170 Kelvin (9,896.85°C or 17,846.33°F). To put that in perspective, our Sun's surface temperature is 5,778 Kelvin. Its passenger, KELT-9b, is almost three times more massive than Jupiter, but half as dense because the scorching heat has made its atmosphere expand and inflate.

KELT-9b's short orbital period places it very close to its parent star. It's also tidally locked to KELT‑9, like our Moon is to Earth, so only one side of the planet ever faces the star and is constantly bathed in radiation.

The strong ultraviolet rays prevent molecules such as water, carbon dioxide and methane from forming. Keivan Stassun, co-author of the paper and a professor of physics and astronomy at Vanderbilt University, said: "KELT‑9 radiates so much ultraviolet radiation that it may completely evaporate the planet. Or, if gas giant planets like KELT‑9b possess solid rocky cores as some theories suggest, the planet may be boiled down to a barren rock, like Mercury."

There's a chance that the star might swallow the exoplanet first, as it reaches its red giant phase, swelling up in about a billion years. "The long-term prospects for life, or real estate for that matter, on KELT‑9b are not looking good," Stassun added. ®

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