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Discovery of a planet so dark it absorbs 99% of its light

Credits: LoganArt / Pixabay

A team of astronomers has announced the discovery of an exoplanet so dark that it absorbs almost all of the light that crosses its path, via a dense layer of mist. It is one of the darkest planets ever discovered. 

The planet in question, called WASP-104b, is a type of planet known as a “hot Jupiter”. These extrasolar worlds have similar characteristics to the largest planet in our solar system, Jupiter, but with orbital periods of less than 10 days. These planets thus have extremely high surface temperatures, because the orbit close to their parent stars. They are also relatively dark. The majority of them reflect only 40% of their star’s light.

But as always, there are exceptions. It can sometimes happen that astronomers discover ones that are darker than others. This is particularly the case with WASP-12b, discovered last year, which absorbs at least 94% of light. WASP-104b, for its part, is darker again. According to researchers from Keele University in the United Kingdom, this world absorbs more than 97 to 99% light. Such darkness may be explained by the planet’s proximity to its star, a yellow dwarf that can be found around 466 light years from the Earth, in the Lion constellation.

Like the majority of hot Jupiters, WASP-104b presents only one face to its host star (like the moon with the Earth). The planet thus has one side that is permanently “day” and one side which is permanently “night”. It orbits so close to its star (at a distance of around 4.3 million kilometers) that it only takes 1.75 days for a full orbit. This means that its “day” side is so hot that clouds cannot form; clouds are typically highly reflective. It is also too hot to have ice -for example, the type of surface that makes Encelade, the moon of Saturn, so bright.

Instead, WASP-104b presents with a thick, misty atmosphere, probably containing atomic sodium and potassium, which absorb light in the visible spectrum, making the planet very dark even on its “day” side. On the “night” side, far from the light of the stars, clouds can form -but this side will never see the light of day, thus there is no light to reflect.

Even if they are darker than normal, hot Jupiters are no more difficult to detect than regular planets. Astronomers don’t detect them directly, as they are much too far away. Instead, they are detected by observing a regular and periodic dimming of the star’s light levels while the planet moves between its host and our planet (the transit method).

The details of this study are available on the e-printing site Arxiv.org.

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