James Webb Space Telescope has taken a closer look at an exoplanet, Gliese 1214b (GJ 1214b), a world very different from anything in our own planetary system. The planet is an example of a mini-Neptune, less massive than the ice giant Neptune in our Solar System, but four times larger than Earth.

Despite being the most common type of planet in the Milky Way, mini-Neptunes are curiously absent from our Solar System. As a result, we know very little about these worlds. However, Gliese 1214b has proven to be difficult to observe due to its highly reflective atmosphere, until the James Webb turned its powerful gaze towards it.

Clouds on Gliese 1214b

“The planet is completely covered by a sort of haze or layer of clouds” said Eliza Kempton, the lead author of the study (ref.) and an exoplanet astronomer at the University of Maryland. “The atmosphere remained completely hidden until this observation”. Kempton added that if GJ 1214b indeed possesses a water-rich atmosphere, it could have been a “waterworld” in the past.

However, the situation is very different now. The planet, located 48 light-years away from Earth, is instead very hot. As a result, astronomers do not expect to find oceans of liquid water on its surface. Instead, they believe its atmosphere may be composed of a large amount of water vapor, giving rise to the thick, reflective envelope that conceals it.

To achieve these results, the team used new approaches to peer through GJ 1214b’s atmosphere as it orbits around its star, a red dwarf called Gliese 1214. The exoplanet is tidally locked, meaning it has its daytime side always facing the star and a nighttime side always facing space. Observing the planet as it disappears behind its star and then reemerges (transit technique) allows astronomers to analyze both sides and better characterize its atmosphere.

A cold exoplanet

“The ability to obtain a complete orbit was crucial to understanding how the planet distributes heat from the daytime to the nighttime side. There is a lot of contrast between day and night” said Kempton. Using James Webb’s Mid-Infrared Instrument (MIRI), the team created a heat map of GJ 1214b as it orbits around the star.

Daytime temperatures on the planet reach 279 degrees Celsius, while temperatures on the nighttime side of GJ 1214b drop to 65 degrees Celsius. This significant temperature change is attributed to an atmosphere composed of heavier molecules, such as water or methane, rather than lighter hydrogen molecules. The observations from MIRI confirmed this, offering a clue about the formation of GJ 1214b.

“This is not its primordial atmosphere. It does not reflect the composition of the star around which it formed. It probably lost a lot of hydrogen over time or formed with heavier elements,” Kempton reiterated. Researchers were surprised to find that despite being much hotter than Earth, it is cooler than expected. They believe these low temperatures are due to the exoplanet’s atmosphere reflecting a large amount of light.

Future investigations

Astronomers have also found clues that GJ 1214b may have formed farther from its star than its current 1.6 Earth-day revolution orbit. During its spiral inward, temperatures on the planet could have increased dramatically, causing the evaporation of ice and liquid water and creating an atmosphere full of water vapor.

“The simplest explanation, if you find a planet that is very water-rich, is that it formed farther from the host star” Kempton said. The team will now attempt to gather more data on GJ 1214b to unravel further mysteries of this distant world. This will allow astronomers to better understand the formation and evolution of mini-Neptunes. Observing a broader population of these celestial bodies could even reveal why our Solar System has been deprived of this type of planet.

Stefano Gallotta
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