Hubble observes 'pitch black' exoplanet

The Hubble Space Telescope has been used to examine a large, hot exoplanet that appears pitch black.

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An artist’s impression of ‘pitch black’ exoplanet WASP-12b.
Credit: NASA, ESA, and G. Bacon (STScI)

 

A well-known ‘hot Jupiter’ exoplanet reflects almost no light, making it appear pitch black, “darker than fresh asphalt”, according to scientists who have observed it using the Hubble Space Telescope.

The exoplanet, known as WASP-12b, orbits a star like our own Sun 1,400 lightyears away, and was discovered in 2008.

Since then, it has been studied extensively and astronomers have learned much about it.

 


Read more about exoplanets from BBC Sky at Night Magazine:


 

A team of astronomers have used the Space Telescope Imaging Spectrograph on the Hubble Space Telescope to measure how much light it reflects: its albedo. Measurements like these can help astronomers learn much about the atmospheric makeup of exoplanets.

“The measured albedo of WASP-12b is 0.064 at most,” says study lead author Taylor Bell of McGill University, Canada. “This is an extremely low value, making the planet darker than fresh asphalt!”

WASP-12b is known as a ‘hot Jupiter’ because of its size and temperature. It is almost twice the size of Jupiter and its year lasts just over one Earth day.

This proximity to its host star has caused it to be stretched into an egg shape by the star’s gravitational pull, and raised the surface temperature of its daylight side to 2,600°C.

 

An artist’s impression of ten Jupiter-sized exoplanets, including WASP-12b, that were examined as part of a study published in 2015. The images are to scale.
Credit: ESA/Hubble & NASA

 

“There are other hot Jupiters that have been found to be remarkably black, but they are much cooler than WASP-12b,” says Bell.

“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 heat means that clouds cannot form and alkali metals are ionised, while hydrogen molecules are broken up into atomic hydrogen.

This, the study says, causes the atmosphere of the exoplanet to act more like the atmosphere of a low-mass star, causing its low albedo.

In order to carry out the study, the team made observations of the exoplanet in October 2016 during an eclipse, as the exoplanet passed behind its star.


 

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