exoplanet around star
September 20, 2017

Astronomers discover extremely hot, pitch-black exoplanet

Using the Hubble Space Telescope, astronomers have discovered a most unusual exoplanet that absorbs 94% of the visible light given off by its host star, making it seem as if it is pitch-black in color, according to research published last week in the Astrophysical Journal Letters.

This unusual world, officially known as WASP-12b, is a “hot Jupiter” – a giant gas planet which orbits very closely to its sun and which is heated to extreme temperatures – NASA explained in a statement. In this case, its day side reaches temperatures of up to 4,600 degrees Fahrenheit.

The reason that WASP-12b can reach such extreme temperatures, Popular Science explained, is because it is just two million miles from its host star, meaning that it can complete a trip around its sun in 1.1 Earth days. Conditions are so hot on the exoplanet that reflective clouds are unable to form, meaning that incoming light penetrates deep into the planet’s atmosphere.

That incoming light is absorbed by hydrogen atoms and converted to heat energy, NASA stated, and this steady conversion of light-to-heat makes the planet appear to be pitch-black to onlookers, the researchers found. To the human eye, the exoplanet appears to be as black as fresh asphalt.

“We did not expect to find such a dark exoplanet. Most hot Jupiters reflect about 40 percent of starlight,” said lead investigator Taylor Bell of McGill University and the Institute for Research on Exoplanets in Montreal. He added that the discovery “further demonstrates the vast diversity among the strange population of hot Jupiters,” some of which can be thousands of degrees cooler than WASP-12b.

Strange exoplanet is tidally locked, has extremely low albedo

Originally discovered in 2008, WASP-12b is an egg-shaped exoplanet that is about twice as big as Jupiter, and because of its close proximity to its sun, it is tidally locked– which, as Space.com explained, means that the same side of the planet (the day side) is always faces its host star.

Despite knowing of the planet’s existence for nearly a decade, though, scientists did not know just how dark WASP-12b was before observing it with the Hubble’s Space Telescope Imaging Spectrograph instrument last October. Those observations revealed that it had a reflectance (or albedo) of no more than 0.064 – much lower than that of Earth (0.37) or the moon (0.12).

nasa funny exoplanet graphic

That's one way to put it. (Credit: NASA)

The low reflectance means that the planet essentially consumes 94% of the light that it receives from its host star, converting it into heat and allowing it to maintain its scorching temperatures, NASA explained. For the sake of comparison, Bell noted that most hot Jupiters reflect about 40% of the sunlight they receive, making this distant gas giant (located around 1,400 light years from Earth) an oddity – even if it is not the only dark exoplanet of its kind to be discovered.

“There are other hot Jupiters that have been found to be remarkably black, but they are much cooler than WASP-12b,” Bell said, according to Space.com. “For those planets, it is suggested that things like clouds and alkali metals are the reason for the absorption of light, but those don't work for WASP-12b because it is so incredibly hot.”

“Past observations of hot Jupiters indicate that the temperature difference between the day and night sides of the planet increases with hotter day sides,” he added. “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.”


Image credit: NASA