A planet with a mass almost seven times greater than Jupiter is located 12 light-years away and contains unexpected cloud formations. The discovery, which researchers published in the Astrophysical Journal Letters, has prompted astronomers to change their methods for modeling alien atmospheres.
The Max Planck Institute for Astronomy (MPIA) team led by Elisabeth Matthews used the James Webb Space Telescope’s mid-infrared instrument MIRI to perform direct imaging of Epsilon Indi Ab, which exists as a gas giant orbiting Epsilon Indi A in the southern constellation of Indus. Scientists have determined that the planet’s mass is 7.6 times that of Jupiter, while its diameter remains similar to Jupiter’s.
It was believed that large amounts of ammonia gas would be found in the upper layers of the planet’s atmosphere. However, it was determined that much lower amounts of ammonia were present than expected according to the calculations. This finding can be explained by the presence of thick but non-uniform layers of water ice clouds, similar to cirrus clouds on Earth.
“It’s a big problem to have,” said James Mang of the University of Texas at Austin, co-author of the study. “What once seemed impossible to detect is now within reach, allowing us to investigate the structure of these atmospheres, including the presence of clouds.”
The vast majority of exoplanetary atmospheres are discovered through planetary transits across the disk of their parent stars, observed from Earth. This technique inherently favors hot, nearby planets. Epsilon Indi Ab is a planet orbiting about four times the distance between Jupiter and the Sun.
This problem was solved by using the MIRI coronograph to block the star’s glare and observe the planet’s weak thermal radiation. By comparing the two images obtained with infrared light at wavelengths of 11.3 micrometers and 10.6 micrometers, taken a year apart, scientists calculated the amount of ammonia and determined how the molecule was suppressed.
The planet’s temperature is estimated at 200 to 300 Kelvin (from minus 70 to plus 20 degrees Celsius), hotter than that of Jupiter (140 K). The reason for this warming of the planet is the leftover energy generated during its formation billions of years ago.
Computer models that usually help scientists interpret their exoplanet data do not take clouds into account. This is understandable, given the complexity of cloud modeling.
“JWST finally allows us to study analogous planets in the solar system in detail,” said Matthews. “If we were aliens looking at the Sun from a few light years away, JWST is the first telescope that will 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.
