466 light years distant from Earth is a hot coal-colored Jupiter; one of the darkest worlds known. This gaseous, giant planet, the size of our Jupiter, orbits its star once every 1.75 days. Being so close to its sun, the intense radiation emitted has completely struck down its entire atmosphere and clouds, creating a fiery surface (more than 1,200 ° C) and full of mist that envelops this dark planet.
Hot Jupiters – like this one – are pretty strange planets. They are gas giants with masses similar to that of our Jupiter, but are extremely close to their stars, generally orbiting in a period of less than 10 days. Due to this proximity, these planets are also extremely hot. They are not a rarity, but they do have a set of characteristics that make them enigmatic.
One of them is that hot Jupiters are relatively dark. Most of them reflect about 40% of the starlight that hits them. However, a few, like WASP-12b discovered last year, go further. WASP-12b absorbed at least 94% of the light. Well, WASP-104b is even darker. According to researchers from Keele University (UK) it absorbs 97 to 99% of light.
The reason for this darkness probably has to do with the planet’s proximity to its star, a yellow dwarf about 466 light years from us, in the constellation Leo.
Like most hot Jupiters, WASP-104b is tidal-locked, meaning that one side of the world is always facing its host star . This implies that the planet has a permanent daytime side and is so close to the star, at a distance of about 4.3 million kilometers, that it takes just 1.75 days to complete a full orbit.
Being so hot, it makes it impossible for clouds to form. It’s also too hot for ice, the kind of surface that makes the moon Enceladus so bright. In return, WASP-104b has a thick, hazy atmosphere, probably containing atomic sodium and potassium, which absorb light in the visible spectrum, making the planet very very dark on the day side.
On the night side, away from your sun’s light, clouds can form, but that side never sees daylight, so there is no light nearby to reflect.
Although they are darker than normal, hot Jupiters are no more difficult to detect than normal planets. To detect them, a regular and periodic dimming of the host star’s normal light levels is observed as the planet moves in front of them. This is called the transit method, and that’s how Kepler, NASA’s planet hunter, works. A method that is quite effective.
Is it really that black?
Hot Jupiters are not really matte black, like coal, or Vantablack (the darkest material in the world); that comparison is to reflect, not to emit light. Due to their extreme heat, they should glow brightly. In this case, the closest thing would be a deep purple color rather than a dull black.
The darkest hot Jupiter we know of to date is a planet called TrES-2b, which reflects just 0.1% of the light that hits it.
The study is pending peer review.
Reference: WASP-104b is Darker than Charcoal. T. Močnik, C. Hellier, J. Southworth arXiv: 1804.05334 [astro-ph.EP]
