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Planet KELT-9b is Hotter than Stars

Newly-discovered, Jupiter-like planet KELT-9b is hotter than most stars. The findings are published in the journal Nature.

A planet hotter than most stars. Photo credits: Robert Hurt, NASA/JPL-Caltech.

Meet the hottest planet, hotter than most stars, planet KELT-9b! It has a day-side temperature of around 4326 degrees Celcius, which makes it only 926ºC cooler than our own star, the sun. Its temperature is just one of its many unusual characteristics, explains the international team of astronomers behind the discovery (scientists from The Ohio State University and Vanderbilt University).

If planet KELT-9b is staggeringly different from other planets, why are we calling it a planet at all? According to the researchers, it is defined as a planet based on the typical features thereof, like mass. But, that’s where the similarities with planets appear to stop. Its atmosphere, for instance, is (most probably) unlike any other known planet, points out study lead author, Scott Gaudi.

Like Jupiter, KELT-9b is a gas giant. It orbits a star to which it is tidally locked (like the Earth-moon system), an interaction that exposes its day side to constant stellar radiation, accounting for the super high temperature. The extreme radiation from the host star causes its atmosphere to inflate like a balloon; as a result, in spite of being 2.8 times bigger than Jupiter, its density is only half as much as the latter’s. Also, the high temperature makes it impossible for compounds like water, carbon dioxide, and methane to form. Its night side, on the other hand, remains a mystery; it might be having molecules, at least on a temporary basis, but nothing is known yet.

The planet’s star, named KELT-9, is two times bigger and hotter than our sun. KELT-9 is predicted to swell into a red giant star millions of years from now, according to the astronomers. It radiates high amounts of ultraviolet radiation that might fully evaporate the planet, says study author Keivan Stassun. Some theories suggest that the planet might even be boiled down to the extent of turning into a barren rock like Mercury (this would apply if it bears a solid rocky core).

These conditions imply that life on KELT-9b is not that good an idea. Why would scientists look into this type of planets, then? According to Gaudi and Stassun, studying planets with such extreme conditions complements endeavours focused on finding Earth-like planets (where conditions are not harsh) orbiting cooler stars—scientists can, thus, acquire insight into the other side of the coin. For instance, KELT-9b’s host star is more massive, with a higher temperature than our sun, and its exploration constitutes “a kind of touchstone for understanding how planetary systems form around hot, massive stars”, says Gaudi.

Furthermore, the authors highlight the importance of this type of studies, mentioning that understanding the conditions that lead to the destruction of planets is just as pertinent as learning about planet formation.

Now, the plan is to study KELT-9b using other telescopes like Spitzer, the Hubble Space Telescope (HST), and James Webb Space Telescope to gather more information about this new world.


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