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Exoplanets, planets orbiting other stars, are now being discovered by the thousands, and in a wide variety of sizes, colours, and compositions. Now, new studies of one of these worlds show that it reflects almost no light, making it appear nearly pitch black.
The new observations of exoplanet WASP-12b were made in October 2016 by an international team of astronomers using the Space Telescope Imaging Spectrograph (STIS) on the NASA/ESA Hubble Space Telescope. They were measuring how much light the planet reflects to learn more about 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 lead author Taylor Bell, a Master’s student in astronomy at McGill University. “The low albedo shows we still have a lot to learn about WASP-12b and other similar exoplanets.”
By comparison, our own Moon is about two times more reflective, with an albedo of 0.12.
WASP-12b is a "hot Jupiter," orbiting very close to its Sun-like star WASP-12A (its year is just over one Earth day in length), which is 1,400 light-years away. The surface temperature on the daylight side of the planet is about 2,600 degrees Celsius. The planet is also somewhat egg-shaped, due to the strong gravitational pull of the star. Its radius is almost twice that of Jupiter. It was discovered in 2008 and has already been well-studied. The new observations will now help astronomers learn even more about its origin and evolution.
So why is WASP-12b so dark? The extremely high temperature is at least part of the answer, according to astronomers. As Bell explained:
"There are other hot Jupiters that have been found to be remarkably black, but they are much cooler than WASP-12b. 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 atmosphere is so hot it behaves more like the atmosphere of a low-mass star than a planet, creating a low albedo.
“After we measured the albedo we compared it to spectral models of previously suggested atmospheric models of WASP-12b," said Nikolay Nikolov of the University of Exeter, UK and co-author of the new study. “We found that the data match neither of the two currently proposed models.”
According to current analysis, the atmosphere of WASP-12b is composed of atomic hydrogen and helium, and it is too hot for clouds to form, and hydrogen molecules are broken down into atomic hydrogen.
Measuring the albedo of exoplanets isn't easy, and this is only the second exoplanet where it has been done so far. The other is HD 189733b, another hot Jupiter. In that case, the results suggested that the planet had a deep blue colour. For WASP-12b, astronomers had to observe the planet during an eclipse, when it was behind its star for a short time. That way, the amount of light being reflected can be measured directly, as opposed to other methods.
As Bell noted, “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."
Hot Jupiters are fairly common among exoplanets, even though there are none in our own Solar System. The study of WASP-12b will help astronomers better understand other similar giant planets although their compositions may be quite different. Such searingly hot worlds are not too habitable for life, but they are still very interesting of course in their own right, and such planetary systems often have smaller, rocky planets similar in size to Earth. Comparing these systems to our own Solar System will provide more clues as to how it originated and developed to have some planets and moons potentially habitable (and indeed known to be inhabited, of course, in the case of Earth).