KELT-9b: Newly-Discovered ‘Hot Jupiter’ a Planet Hotter Than Most Stars

Artist’s illustration of the star KELT-9 (left) and its planet KELT-9b (right). The planet takes only 1.5 days to complete a revolution around its host star. Image credit: NASA / JPL-Caltech / Robert Hurt.

There is a newly-discovered exoplanet called KELT-9b, a Jupiter-like planet which is only 927 degrees Celsius cooler than our own sun (2,000 degrees Fahrenheit, or 1,200 degrees Kelvin) stretching the definition of the word "planet." KELT-9b is a gas giant 2.8 times more massive than Jupiter, however it is only half as dense.

The day-side temperature reaches 4,327 degrees Celsius (7,820 degrees Fahrenheit, or 4,600 degrees Kelvin), this world is hotter than most stars.

This hot Jupiter orbits close to KELT-9, the hottest, most massive and brightest star yet found to host a transiting giant planet, with a temperature of 9,900 degrees Celsius (17,850 degrees Fahrenheit, or 10,170 degrees Kelvin), the host star is half way between stars of type A and B. The star is also known as HD 195689, at a distance of 650 light-years in the constellation Cygnus.

Tidal lock example. Wikipedia.

KELT-9b is tidally locked to KELT-9, just like the Moon is to Earth, but apart from being incredibly close to its star, the planet also orbits the poles of KELT-9 rather than its equator. “It’s unclear how KELT-9b obtained its peculiar orbit. In addition to being ridiculously close, the planet orbits about the poles of its parent star rather than its equator. One can only imagine how it got there,” said Justin Crepp, Freimann assistant professor of physics at the University of Notre Dame.

Thanks to this tidal lock the day side of the planet is perpetually bombarded by stellar radiation, and as a result is so hot that molecules such as water, carbon dioxide and methane can’t form there. The properties of the night side are still mysterious — molecules may be able to form there, but probably only temporarily.

Though KELT-9b is larger than Jupiter, it is not as dense. Prof. Crepp and his colleagues believe low surface gravity combined with its extremely high temperature contributes to an inflated and gaseous atmosphere.

UV radiation from KELT-9 is so brutal that the planet may be literally evaporating away under the intense glare, producing a glowing gas tail.

“KELT-9 radiates so much UV radiation that it may completely evaporate the planet,” said Keivan Stassun, professor of physics and astronomy at Vanderbilt University.

“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.”

“That is, if the star doesn’t grow to engulf it first. KELT-9 will swell to become a red giant star in about a billion years,” Prof. Stassun said. “The long-term prospects for life, or real estate for that matter, on KELT-9b are not looking good.”

In an article in this week's issue of Nature, an international research team led by astronomers at Ohio State University and Vanderbilt University describes a planet with some very unusual features. The article is titled "A giant planet undergoing extreme ultraviolet irradiation by its hot massive-star host." 

Sources: sci-news, Wikipedia, Lehigh University, Vanderbilt University,