:INFO
The first rocky planet outside our system with an atmosphere
55 Cancri e orbits its star in under 18 hours at 0.015 AU. Its surface is a magma ocean. The expectation for a rock that close was either bare stone or a thin veil of vaporised rock, neither of which counts as an atmosphere in any useful sense. In May 2024 JWST measured its thermal emission with two instruments and found something else: a genuine volatile atmosphere, likely rich in carbon dioxide or carbon monoxide. The likely source is the magma itself, outgassing volatiles dissolved in the melt.

:STATS
[icon:TARGET] Radius | 1.875 Earth radii, a super-Earth
[icon:CHART] Mass and density | 7.99 Earth masses at 6.66 g/cm3
[icon:CLOCK] One year | 17.7 hours
[icon:FIRE] Orbit radius | 0.015 AU, roughly a fortieth of Mercury's
[icon:STAR] Host star | 55 Cancri at 5172 K, close to a solar twin
[icon:BOLT] Distance | about 41 light-years

:CHECKLIST
What the thermal emission settled
[ ] A bare rock with no atmosphere. Ruled out.
[ ] A thin veil of vaporised rock, the default expectation for a lava world. Ruled out.
[ ] A genuine volatile atmosphere, likely rich in carbon dioxide or carbon monoxide. This is what the data supports.
[ ] Nitrogen, water vapour and sulfur dioxide in the mix. Possible, not established.
[ ] Clouds of condensed lava droplets. Proposed to explain the variability, not confirmed.

:TIMELINE
🌋🛰️🟠
2004-01-01 | Detected by radial velocity around a naked-eye star in Cancer, long before anyone could say what it was made of.
2011-01-01 | Seen in transit, which fixes its radius. Combined with its mass it comes out denser than Earth and therefore rocky.
2024-05-01 | JWST thermal emission rules out a bare rock-vapour planet and finds a secondary atmosphere rich in carbon dioxide or monoxide.

:NOTE
Secondary means the atmosphere is not the hydrogen the planet formed with. That original envelope is long gone, stripped by the star. What is there now came out of the planet itself. Earth's air is secondary in exactly the same sense, which is why a molten super-Earth turns out to be a reasonable place to study how our own atmosphere got started.

:NOTE.half
The archive lists no measured equilibrium temperature, so the 1783 K on the cover is derived from the star and the orbital distance and carries an asterisk. It is hot enough to melt every common silicate rock, which is the only part of the number that matters here.

:NOTE.half
The dayside brightness varies noticeably between observations, by amounts too large for a stable surface. Clouds of condensed rock, plumes from the magma ocean and shifting circulation have all been proposed. Nobody has settled which, and it makes every measurement here provisional.

:LINK
https://www.nature.com/articles/s41586-024-07432-x
A secondary atmosphere on the rocky exoplanet 55 Cancri e