NASA calls the evidence for gas around 55 Cancri e the best yet for an atmosphere on a rocky planet beyond our Solar System. The planet, also named Janssen, was first detected in 2004 with the Hobby-Eberly Telescope at McDonald Observatory. That team used the radial velocity method, which measures the tiny wobble a planet's gravity causes in its star. They put its orbit at 2.8 days. In 2011, after other astronomers predicted a much shorter orbit, the MOST space telescope caught the planet passing in front of its star every 0.74 days, about 18 hours. The star is bright enough to see with the naked eye, which makes the planet unusually easy to study.
55 Cancri e is a super-Earth, a rocky planet larger than Earth: about 8 times Earth's mass and nearly twice its width. It orbits about 25 times closer to its star than Mercury is to the Sun. It is tidally locked, so one side is in permanent daylight. Its surface is thought to be an ocean of magma.
In 2024 a team led by Renyu Hu of NASA's Jet Propulsion Laboratory used the James Webb Space Telescope's NIRCam and MIRI instruments to measure the infrared glow of the day side. Bare rock there should reach about 2,200 °C. Webb measured about 1,540 °C, which suggests an atmosphere is carrying heat away. The light also showed a dip consistent with carbon dioxide or carbon monoxide. The team proposes this gas bubbles out of the magma ocean and is constantly replenished.
The picture is not settled. A second Webb study, led by J. A. Patel and published the same year, watched five eclipses, when the planet passes behind its star. The planet's brightness changed sharply between visits, from undetectable to strong. Some visits favoured a thin carbon-rich atmosphere, others a thin vapour of rock, and some showed no clear signal. The authors suggest gas may be released in bursts, or that a patchy cloud of volcanic dust circling the star is involved. Which gas dominates, and whether the atmosphere is stable, remain open questions.
TerraForge's model, working only from its mass, orbit and star, classifies 55 Cancri e as a lava world, which fits the magma-ocean picture. It cannot capture an atmosphere that may come and go, and its modelled 2,000 K is a little hotter than the roughly 1,800 K that Hu's team measured on the day side.