Planets can be as hot as stars! Details


Discovering and studying exoplanets is not always aimed at discovering habitable worlds: it can also be a way for science to understand the evolution of our own solar system and how planets form. For this reason, the study of KELT-9B keeps astrophysicists busy: it is hotter than 80% of all stars in the Universe (those of class K and M – oranges and reds) and, with their temperatures that can reach more of four thousand degrees, holds the title of the hottest known exoplanet.

“In essence, hot giants offer an incredible opportunity to study physics in environmental conditions that are almost impossible to reproduce on Earth, helping to improve models of formation and planetary evolution,” said astronomer Billy Edwards of University College London (UCL) ) in an article for The Conversation.


He and his colleague, astrophysicist Quentin Changeat, try to explain the process of forming KELT-9b in an article for the Astrophysical Journal Letters. Discovered in 2016, the exoplanet is a gas giant (planets like it are called “super hot Jupiter”) orbiting a 300 million-year-old young blue star that, in itself, is a separate study.

It is twice the size and temperature of our Sun, and rotates so quickly that, in addition to looking like an egg, it has hotter poles than its equator. And it is over the poles that the orbit of KELT-9b passes, completing a cycle every 36 hours.


The distance between him and his star is 1/10 that between Mercury and the Sun. A phenomenon known as “tidal coupling” means that the same face is always facing its star, 620 light years from Earth, in the constellation of the Swan. Because of the proximity, extreme heat and high levels of ultraviolet radiation, the planet is probably evaporating.

The combination of the three factors caused the atmosphere of the KELT-9b to swell, making it reach 1.8 times the size of Jupiter and producing a tail of evaporated planetary material, like a comet.

It has always been thought that the heat of KELT-9b would not allow molecules to stabilize – this would only be possible when atoms flowed to the night side of the planet, where temperatures are low enough for chemical bonds to take place. To see if this is what happens in KELT-9b, the two astronomers analyzed the different wavelengths emitted by the elements present in the planet’s atmosphere.

KELT-9b was found in 2016 through the so-called transit method (when the brightness captured from a star decreased when a planet intervenes between it and the telescope), also used to determine which elements are vaporized: when passing in front of the star , the light is filtered through the atmosphere, showing its chemical composition.


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