Astronomers have discovered one of the darkest planets ever seen. It’s so dark, they’re comparing it to charcoal.
WASP-104b is a fast gas giant. It is a Jupiter-mass planet that orbits its star once every 1.75 days. The planet circles close to its star, so it receives enough radiation that its clouds are swept away, letting the potassium and sodium come out to play. Those elements – found in the haze layer – absorb more than 97 to 99 per cent of the light that hits the planet’s surface, putting this world in a rare class of “dark” planets that reflect nearly no light at all.
“From all the dark planets I could find in the literature, this is top five-ish,” says at Keele University in Newcastle, UK, who led the research. “I think top three.”
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Those top three planets – which include TrES-2b and HAT-P-7b – all seem to absorb about the same amount of light. So, the competition for the darkest planet ever found could come down to a few fractions of a percent or a slim margin of error in the observations.
Finding the darkness
If WASP-104b reflects so little light, how did we find it? By its shadow, essentially. When a planet crosses in front of its star – making a “transit” – it blots out some of the star’s light.
The planet was originally discovered as part of the Wide Angle Search for Planets project in 2014, but it seemed to be a fairly typical hot gas giant. At the time, it was noted for its quick orbit compared to other swift giants in the class of so-called hot Jupiters, massive gas giants circling their stars in a few days. But we couldn’t identify what the planet looked like.
Later, the Kepler Space Telescope identified it as part of a rarer subgroup of hot Jupiters that absorb far more light than they reflect, and found that it might be the darkest ever seen. Kepler repeatedly recorded the period before and after its transit, which can yield clues about the planet itself, including how much light is reflecting off of it. The answer was … almost none.
Getting the picture
So what would it look like? Maybe a little bruised.
“When they say it’s darker than charcoal, it’s being a little misleading,” says of Princeton University, who co-authored a paper in 2000 predicting dark planets like these. “It’s probably going to be purple or something.”
He added that when these black planets orbit brown dwarfs, they can end up with a magenta hue. This darkening can also happen to any close-in world with an intact atmosphere, where cloud or haze material has been driven deeper down, leaving potassium and sodium to prosper at top layers of the atmosphere.
The James Webb Space Telescope and other observatories should be able to see what these dark planets truly look like, and provide insight into why some hot-Jupiters still shine while others hoard light. Looking both at the spectra and the light curves of the planets in infrared should reveal what these worlds are truly like—and if they’re as dark as they seem.
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