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Scientists may need to rewrite the text books following the discovery of a carbon-rich exoplanet.

The discovery made using the Spitzer Space Telescope poses new questions about how planets are made and how their atmospheres work.

Reporting in the journal Nature, Dr Nikku Madhusudhan and colleagues from Princeton University, say the planet WASP-12B has an atmosphere with equal parts carbon and oxygen.

Madhusudhan says that's double the ratio of carbon to oxygen seen in planets in our solar system, and has implications for the planet's internal structure.

"The high carbon-to-oxygen ratio indicates a carbide or diamond interior rather than the silicate geology of the Earth," he says.

Madhusudhan says the planet's atmosphere is abundant in carbon monoxide, and has considerably more methane and less water than would be expected for a planet of this temperature in our solar system.

"If WASP-12B's host star has a carbon-to-oxygen ratio similar to the Sun, then it's difficult to explain the ratio in the planet's atmosphere," he says.

Because the planet passes in front of and behind its star as viewed from Earth, astronomers can use multi-wavelength spectroscopy to directly view the planet's atmospheric chemical composition and thermal structure.
Temperature questions

Madhusudhan says these observations have revealed the absence of a strong thermal inversion - the difference between night and day temperatures.

"The ability to also see the planet's day time side just before it passes behind its host star as seen from Earth, allows us to take accurate atmospheric temperature readings at different locations," he says.

According Madhusudhan, WASP-12B, which is 871 light years from Earth, orbits 40 times closer to its host star than our planet does to the Sun.

Being so close the star's tidal forces are distorting the planet into an egg shape which generates tidal heating.

The star is also slightly hotter than the Sun, giving the planet a surface temperature of over 2200²C.

Madhusudhan says,"this places it at the extreme end of hot Jupiters" (Giant gas planets orbiting close to their host stars).

"Theory tells us it should have thermal inversion sort of like the Earth's stratosphere, but it does'nt.

"It's still not known if this is related to the high atmospheric carbon ratio and is one of the issues that need to be solved."
Limited data

Dr Simon O'Toole from the Australian Astronomical Observatory says he's never seen carbon to oxygen ratios like this before.

"The findings are interesting, but are based on just four data points," says O'Toole. "They're using very sophisticated modelling to be sure, but they're still making assumptions based on very limited information, so I would proceed with caution."

He says there is still a lot we don't understand about planets in extreme conditions.

"Because of this planet's location so close to its host star, there's a lot of complex physics going on. We don't know enough about the chemistry of these hot Jupiters that are so close to their stars."

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