A lack of reflective clouds keeps WASP-12b in darkness, making the enormous planet appear almost completely black.
The Hubble Space Telescope has revealed that an extremely large exoplanet is so odd that it could be the centrepiece of its own Star Trek episode. WASP-12b, about twice the mass of Jupiter and three times the radius, is completely black. The bizarre world has the seemingly unique ability to trap at least 94 per cent of the visible starlight falling into its atmosphere.
WASP-12b, the hot Jupiter
WASP-12b known as a “hot Jupiter”. Like Jupiter, these exoplanets are gigantic and gaseous. They are very close to their host stars. The planet is only 3,2 million kilometres away from its star. Because of this, WASP-12b is very hot. So hot that most molecules cannot form in the 2 500-degree-Celsius day side of the planet. This leaves the atmosphere full of pure atoms. It is one of the hottest exoplanets known (although not the hottest).
Because the heat prevents molecules from forming on WASP-12b, researchers believe there are no clouds to reflect light into space. As a result, light from the sun penetrates the planet’s atmosphere. Deep inside the atmosphere, that light is absorbed by hydrogen atoms and converted to heat energy.
“We did not expect to find such a dark exoplanet,” says lead researcher Taylor Bell of McGill University in a Hubble press release. “Most hot Jupiters reflect about 40 per cent of starlight”. The night side of WASP-12b is about 1 000 degrees cooler, which allows for water vapor and clouds to form. This discrepancy shows how much we still have to learn about hot Jupiters.
“This new Hubble research further demonstrates the vast diversity among the strange population of hot Jupiters,” Bell says. “You can have planets like WASP-12b that are 2 500 degrees Celsius and some that are 1 200 degrees Celsius, and they’re both called hot Jupiters. Past observations of hot Jupiters indicate that the temperature difference between the day and night sides of the planet increases with hotter day sides. This previous research suggests that more heat is being pumped into the day side of the planet, but the processes, such as winds, that carry the heat to the night side of the planet don’t keep up the pace.”
Planets outside our solar system are full of surprises. As we continue to learn, most of them are nothing like the ones in our little neighbourhood.