The well-studied exoplanet WASP-12b reflects almost no light, making it appear essentially pitch black, astronomers, using the Hubble space telescope, have found. The discovery sheds new light on the atmospheric composition of the planet, about 1,400 light-years away, and also refutes previous hypotheses about WASP-12b’s atmosphere. Astronomers at McGill University in Canada, and the University of Exeter, UK, measured how much light the exoplanet WASP-12b reflects – its albedo – in order to learn more about the composition of its atmosphere. “The measured albedo of WASP-12b is 0.064 at most. This is an extremely low value, making the planet darker than fresh asphalt!” said Taylor Bell, a master’s student at McGill University.
“This makes WASP-12b two times less reflective than our Moon which has an albedo of 0.12. The low albedo shows we still have a lot to learn about WASP-12b and other similar exoplanets,” said Bell, lead author of study published in The Astrophysical Journal Letters. WASP-12b orbits the Sun-like star WASP-12A and since its discovery in 2008 it has become one of the best studied exoplanets. With a radius almost twice that of Jupiter and a year of just over one Earth day, WASP-12b is categorised as a hot Jupiter. Since it is so close to its parent star, the gravitational pull of the star has stretched WASP-12b into an egg shape and raised the surface temperature of its daylight side to 2600 degrees Celsius. The high temperature is also the most likely explanation for WASP-12b’s low albedo, researchers said.
The daylight side of WASP-12b is so hot that clouds cannot form and alkali metals are ionised, they said. 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. To measure the albedo of WASP-12b, the scientists observed the exoplanet last year during an eclipse, when the planet was near full phase and passed behind its host star for a time. This is the best method to determine the albedo of an exoplanet, as it involves directly measuring the amount of light being reflected. However, this technique requires a precision ten times greater than traditional transit observations.
Using Hubble’s Space Telescope Imaging Spectrograph, the scientists were able to measure the albedo of WASP-12b at several different wavelengths.
The new data indicates that the WASP-12b atmosphere is composed of atomic hydrogen and helium. WASP-12b is only the second planet to have spectrally resolved albedo measurements, the first being HD 189733b, another hot Jupiter.