The rocky planet 55 Cancri e sits so close to its star that its equilibrium temperature runs near 2,000 kelvin, hot enough for magma at the surface. Earlier observations had already argued against a leftover hydrogen and helium envelope. Webb then measured the heat coming off the planet from 4 to 12 micrometers. What does the spectrum hide?
A real atmosphere, raised from the magma itself. The spectrum rules out a bare under a thin haze of vaporized rock. It points instead to a real envelope of volatiles, most likely rich in carbon dioxide or carbon monoxide. A can both outgas such an atmosphere and keep replacing it. This is an inference from a spectrum, not a picture of a sky.
Whether any rocky planet outside the solar system holds an atmosphere has stayed open for years, and most attempts have ended in upper limits rather than detections. The difficulty is double: the planets are small, and the glare of the star beside them is overwhelming.
A lava world is a strange but convenient candidate. It is extremely hot, so its own thermal emission is strong and can be isolated by comparing the moment it is in view with the moment it passes behind its star. 55 Cancri e, at about 1.95 times Earth’s radius and 8.8 times its mass, with an equilibrium temperature near 2,000 kelvin, is the best studied of them.
Earlier work had disfavored a primordial hydrogen and helium envelope, which is what makes this measurement interesting rather than routine. Any envelope found now has to be a , produced by the planet itself.
Webb recorded the thermal emission spectrum from 4 to 12 micrometers with NIRCam and MIRI. It does not match a bare surface under a tenuous veil of rock vapor. It does fit a volatile atmosphere likely rich in carbon dioxide or carbon monoxide, which a magma ocean could outgas and sustain. What remains unsettled is the exact mixture and how stable it is over time.
wat. The heat of 55 Cancri e suggests that even a world with a molten surface can carry an atmosphere above it.
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The search for air on rocky planets outside the solar system has mostly produced upper limits, so a positive case matters, and it suggests even an extreme world can keep renewing a sky.
How do we know?
JWST measured the planet’s thermal emission with NIRCam and MIRI across 4 to 12 micrometers, and the spectrum was compared against models of a bare lava surface and of an atmosphere.
What is still unknown
The exact composition and how long such an envelope lasts are still unknown. The case rests on which model fits a heat spectrum best, which is weaker evidence than detecting a molecule outright.
Learn something?
Where to next
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