Exoplanet Guide

Scienceline’s Guide to the Exoplanets: The Infernal Shadow

WASP-12b: The gas giant that's almost as dark as the void of space

March 9, 2020
A fake planet with "Scienceline's Guide to Exoplanets" labeled across its equator. Other planets zoom past behind it.
Your favorite planets, and you didn't even know they existed. [Credit: Curtis Segarra | CC BY-NC-ND 2.0]

WASP-12b: The Infernal Shadow
Discovered by: SuperWASP
Discovered: 2008
Distance from Earth: ~1400 light-years
Size: ~1.9 Jupiter radii (~21.3 Earth radii)
Mass: ~1.4 Jupiter masses (~445 Earth masses)
Surface climate: Black, glowing red
Habitability for humans: Do you like being vaporized and falling into an abyss at the same time?

No, it’s neither an insect nor a 1950s American suburbanite. It’s something else entirely: a “hot Jupiter.”

Hot Jupiters are worlds the size of Jupiter, or often much larger, which orbit very close to their parent stars — making them very hot indeed.  WASP-12b, in fact, orbits only about 3.4 million kilometers away from its yellow star: less than 3% the distance from the Earth to the Sun. That makes its surface temperature a boiling 4000 degrees Fahrenheit.

Hot Jupiters sound imposing and majestic — and, certainly, they are. But in the rank and file of exoplanets we know, they’re not that unique. In fact, especially before the launch of satellites like Kepler, hot Jupiters were the easiest exoplanets for astronomers to find.

Most hot Jupiters were discovered through the transit method: by detecting the drop in light that reaches Earth when the planet transiting the star: from our perspective, the planet passing in front of the star. Because hot Jupiters are both so big and so close to their parent stars, they can blot out more light than virtually any other sort of exoplanet, making them far easier to see.

That means astronomers have found hundreds upon hundreds of hot Jupiters. But what makes WASP-12b so special is something else: astronomers have found that it absorbs, rather than reflects, a staggering 94% of the light that touches it.

For comparison, Jupiter absorbs about 50%, blue Neptune absorbs 55-70%, Earth absorbs 60-70%, and dark-rock Mercury absorbs about 85-90%.

What this means is that WASP-12b is a hot Jupiter — a gaseous planet, with the thick atmosphere that entails — that’s pitch-black in color.

Scientists believe that this is because WASP-12b is a “carbon planet,” a world with a high carbon content. Astronomers believe that smaller, rocky carbon planets might be coated with rocks of graphite or even diamond. What a carbon-rich gas giant looks like, on the other hand, isn’t yet clear. It’s possible that there’s enough carbon that such rocks could exist deep within the core.

But time is running out for us to answer that question. WASP-12b is so close to its star that the star’s gravity is literally pulling the planet apart, tides literally pulling the atmosphere apart like Play-Doh. WASP-12b is in an agonizing, searing dance of death, and astronomers expect it has only 10 million years left to live.

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