KELT-9-b, the planet where temperatures exceed 4000 degrees Kelvin

It is very rare to observe planets in orbit with very hot and shining stars such as type A and type B stars. These stars are characterized by intense ultraviolet radiation and a strong stellar wind that sweeps away and evaporates very quickly any gas residual from star formation. It is thus practically impossible to find discs of gas and dust around these stars and it is from these discs that planets form.

However, some exoplanets have been discovered around this type of star. For example, KELT-9-b, discovered in 2014, is one of the very few known exoplanets orbiting a type B star.

The exoplanet orbits a star similar to Sirius, which has a mass of two solar masses and a surface temperature of nearly 10,000 Kelvin. It is located 670 light-years from Earth, in the direction of the constellation of the Swan.

The planet lies on a narrow orbit, characterized by a semimajor axis of only 0.034 AU and a period of 1.5 days. So we’re talking about a planet that’s 30 times closer to its star than the Earth is to the Sun, but orbiting a star that’s 50 times brighter than the Sun.

So it doesn’t take much to imagine that infernal conditions lie on this strange world. Being so close the planet is in synchronous rotation and therefore always turns the same hemisphere towards the star. According to the observations of the Spitzer Telescope on this hemisphere, temperatures above 4000 Kelvin are reached, comparable to those of an orange dwarf star. In the nocturnal hemisphere, without facing the star, temperatures are lower, but always above 2000 Kelvin.

KELT-9-b, which is a gas giant with a mass of 2.2 Jovian masses, will have a short life. The radiation and stellar wind emitted by the star are in fact rapidly sweeping away the planet’s atmosphere: it is estimated that since the beginning of its life KELT-9-b has already lost a Jovian mass.

As if that were not enough, being so close to its star, when it enters the final states of its evolution it will become a red giant enormously increasing its size and encompassing the planet, destroying it.

Credit: NASA, JPL.

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