Technology & Science

Astronomers discover pitch-black planet orbiting distant star

Dark planet absorbs light, reflecting about as much as fresh asphalt

The Pitch-Black Exoplanet WASP-12b

An artist’s impression shows the exoplanet WASP-12b — an alien world as black as fresh asphalt, closely orbiting a star like our sun. (NASA, ESA, and G. Bacon (STScI))


Astronomers using the Hubble Space Telescope have discovered that a planet orbiting a star 1,400 light-years away reflects almost no light.

The planet, WASP-12b, is orbiting a star similar to our own sun. In measuring the planet's albedo, or reflectivity, astronomers found the value to be just 0.064, or about as reflective as fresh asphalt, something that surprised the researchers.

"It's definitely not something we'd thought of initially," Taylor Bell, a master's student in astronomy at McGill, and co-author of the paper, told CBC News.

For comparison, the moon has an albedo of 0.12 and Earth 0.30.

First discovered in 2008, the planet is twice as large as Jupiter — a type of planet called a "hot Jupiter" — but orbits its parent star at a distance of just 3.2 million kilometres, a fraction of the distance between Earth and the sun. It's so close, astronomers believe the gravity of the star has stretched the planet into an egg shape, with some of its outer atmosphere spilling out onto the star. 

The planet is also tidally locked with its star, just as the moon is with Earth, meaning that one side is always facing the sun. And, as one can imagine, the day side of the planet is hot: a blistering 2,600 C. The night side is believed to be a much cooler 1,000 C.

Star devouring a planet (artist's impression)

WASP-12b is the hottest known planet in the Milky Way, and potentially the shortest lived. The planet is only 3.2 million kilometres from its sunlike parent star. Because its atmosphere is being eaten by the star, the planet may be completely devoured by the star in 10 million years. (Artwork Credit: NASA, ESA, and G)

Other planets have been found to have extremely low albedos, but they're usually cooler than WASP-12b. In those cases, astronomers believe, things like clouds and alkali metals may be responsible for absorbing the light. 

However, WASP-12b is far too hot and close to its star to form any alkali metals or clouds.

Instead, what astronomers believe is happening is that hydrogen molecules, which contain two atoms, break apart on the sweltering day side and rejoin on the night side. Hydrogen atoms absorb energy, in this case, the energy coming from the nearby star.

Bell said that the findings, published in The Astrophysical Journal Letters, could help astronomers understand the diversity of these hot Jupiters.

"This is perhaps the first confirmed group of a new class of hot Jupiters, maybe like ultra-hot Jupiters," he said.

And the new findings may also help astronomers better understand the mechanisms at work in these planets' atmospheres.

"By observing more planets like this we can better understand how this hydrogen breaks apart and goes back into a molecule at different points on the planet as it heats up and cools down. And in that way, we can learn something about the chemistry and physics that are happening."

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