Astronomers using the NASA/ESA Hubble Space Telescope have discovered that the well-studied hot Jupiter WASP-12b reflects almost no light, making it appear essentially pitch black. This light-eating prowess is due to the unique capability of WASP-12b to trap at least 94% of the visible starlight falling into its atmosphere.
This artist’s impression shows WASP-12b, an alien world as black as fresh asphalt, orbiting a star like our Sun. Image credit: NASA / ESA / G. Bacon, STScI.
WASP-12b was discovered by the UK’s Wide Area Search for Planets (WASP) in April 2008.
The exoplanet orbits WASP-12 (also known as Bergfors-6), a yellow dwarf star located 1,393 light-years away in the constellation Auriga.
With a mass 1.4 times that of Jupiter, a radius almost twice that of Jupiter and a year of just over one Earth day, WASP-12b is categorized as a hot Jupiter.
WASP-12b is so close to its parent star that its daylight side is superheated to nearly 4,600 degrees Fahrenheit (2,540 degrees Celsius) and stretched into a football shape by enormous tidal forces.
Since its discovery, several telescopes have studied the exoplanet, including Hubble, NASA’s Spitzer Space Telescope and the agency’s Chandra X-ray Observatory. Previous observations by Hubble’s Cosmic Origins Spectrograph (COS) revealed that WASP-12b may be downsizing. COS detected material from the planet’s super-heated atmosphere spilling onto the star.
Using Hubble’s Space Telescope Imaging Spectrograph (STIS), McGill University astronomer Taylor Bell and co-authors have now measured how much light WASP-12b reflects (its albedo) in order to learn more about the composition of its atmosphere.
“The results were surprising! The measured albedo of WASP-12b is 0.064 at most,” Bell said.
“This is an extremely low value, making the planet darker than fresh asphalt!”
This makes WASP-12b two times less reflective than our Moon which has an albedo of 0.12.
“We measured the optical geometric albedo of WASP-12b, which measures the light that is scattered back towards the source of light, and can have values above 1. This is in contrast to the Bond albedo, which describes the total amount of energy reflected across all wavelengths and always falls in the range of 0 to 1,” the researchers explained.
“The low albedo shows we still have a lot to learn about WASP-12b and other similar exoplanets,” Bell added.
The new Hubble data indicate that the exoplanet’s atmosphere is composed of atomic hydrogen and helium.
According to the team, the high surface temperature on WASP-12b is also the most likely explanation for the exoplanet’s low albedo.
“There are other hot Jupiters that have been found to be remarkably black, but they are much cooler than WASP-12b,” Bell said.
“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.”
The daylight side of WASP-12b is so hot that clouds cannot form and alkali metals are ionized. It is even hot enough to break up hydrogen molecules into atomic hydrogen which causes the atmosphere to act more like the atmosphere of a low-mass star than like a planetary atmosphere. This leads to the low albedo of the exoplanet.
But the planet’s nighttime side is a different story. It is 2,400 degrees Fahrenheit (1,340 degrees Celsius) cooler, which allows water vapor and clouds to form. Previous Hubble observations of the day/night boundary detected evidence of water vapor and possibly clouds and hazes in the atmosphere.
According to the astronomers, WASP-12b is only the second planet to have spectrally resolved albedo measurements, the first being HD 189733b, another hot Jupiter.
The data gathered by the scientists allowed them to determine whether the planet reflects more light towards the blue or the red end of the spectrum.
While the results for HD 189733b suggest that the exoplanet has a deep blue color, WASP-12b, on the other hand, is not reflecting light at any wavelength. WASP-12b does, however, emit light because of its high temperature, giving it a red hue similar to a hot glowing metal.
“The fact that the first two exoplanets with measured spectral albedo exhibit significant differences demonstrates the importance of these types of spectral observations and highlights the great diversity among hot Jupiters,” Bell said.
The results are published in the Astrophysical Journal Letters.
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Taylor J. Bell et al. 2017. The Very Low Albedo of WASP-12b from Spectral Eclipse Observations with Hubble. ApJL 847, L2; doi: 10.3847/2041-8213/aa876c
