A planet with a mass almost seven times greater than Jupiter exists at a distance of 12 light-years and contains unexpected cloud formations. The discovery, which researchers published in the Astrophysical Journal Letters, has compelled astronomers to change their methods for modelling extraterrestrial atmospheres.
The team from the Max Planck Institute for Astronomy (MPIA) under Elisabeth Matthews used the James Webb Space Telescope’s mid-infrared instrument MIRI to conduct direct imaging of Epsilon Indi Ab, which exists as a gas giant that orbits Epsilon Indi A in the southern constellation of Indus. Scientists have determined the planet’s mass to be 7.6 times that of Jupiter, while its diameter remains similar to that of Jupiter.
It was assumed that large amounts of ammonia gas would be found in the upper atmosphere of the planet. However, it was found that there were much lower amounts of ammonia than what was supposed according to calculations. This finding may be explained by the presence of thick but nonuniform layers of water ice clouds, similar to cirrus clouds on Earth.
“It’s a great problem to have,” said James Mang of the University of Texas at Austin, a co-author on the study. “What once seemed impossible to detect is now within reach, allowing us to probe the structure of these atmospheres, including the presence of clouds.”
The vast majority of exoplanetary atmospheres are discovered through planetary transits across the disc of their parent stars when observed from Earth. This technique inherently gives preference to hot, close-in planets. Epsilon Indi Ab is a planet orbiting at approximately four times the distance between Jupiter and the Sun.
This problem was solved by using the MIRI coronagraph to block out the star’s glare and spot the dim thermal radiation coming from the planet. By comparing the two images obtained with infrared light at wavelengths of 11.3 micrometres and 10.6 micrometres, one year apart, scientists calculated the amount of ammonia and determined how the molecule got suppressed.
The planet’s temperature is estimated to be in the range of 200 to 300 kelvin (from minus 70 to plus 20 degrees Celsius), hotter than that on Jupiter (140 K). The reason for such heating of the planet is leftover energy generated during its formation billions of years ago.
Computer models that usually help scientists to interpret their data about exoplanets don’t account for clouds. This is understandable, considering how complex clouds are to model.
“JWST is finally allowing us to study solar-system analogue planets in detail,” Matthews said. “If we were aliens several light-years away looking back at the Sun, JWST is the first telescope that would allow us to study Jupiter in detail.” Studying an Earth-like planet at that level of detail, she added, would require telescopes that don’t yet exist.
