Vignette publicpreviewAn international scientific team, including Côte d'Azur University Professor Vivien Parmentier, has gained new insights into the atmosphere of a « mini-Neptune », a common type of planet in the galaxy but about which little is known. Vivien Parmentier's contribution was to develop the theoretical models that determined that a heavy atmosphere is necessary to reduce heat transport from the day side to the night side of the planet.

NASA’s James Webb Space Telescope has observed a distant planet outside our solar system – and unlike anything in it – to reveal what is likely a highly reflective world with a steamy atmosphere. It’s the closest look yet at the mysterious world, which was largely opaque to previous observations.

« After so many years of waiting, we can finally pierce the cloud veil of this mysterious object, » explain Vivien Parmentier, « We found that GJ1214b is unlike any of the planets in our solar-system, an intermediate between the gas giants such as Neptune and Earth. »

And while GJ 1214 b is too hot to harbor liquid oceans, water in vaporized form still could be a major part of its makeup. « The planet’s atmosphere is totally blanketed by some sort of haze or cloud layer, » said Eliza Kempton, a researcher at the University of Maryland and lead author of a new paper on the planet. « The atmosphere just remained totally hidden from us until this observation. » She noted that, if indeed water-rich, the planet could be a « water world, » with large amounts of watery and icy material at the time of its formation.

To penetrate such a thick barrier, the research team took a chance on a novel approach: In addition to making the standard observation – capturing light from the host star that’s filtered through the planet’s atmosphere – they tracked GJ 1214 b through its entire orbit around the star, demonstrating the power of Webb’s Mid-Infrared Instrument (MIRI).

Infrared radiation lies outside the part of the light spectrum that human eyes can see. But using MIRI, the research team was able to create a kind of « heat map » of the planet as it orbited the star.

The heat map revealed – just before the planet’s orbit carried it behind the star, and as it emerged on the other side – both its day and night sides, unveiling for the first time
details of the atmosphere’s composition.


© Thomas Müller (HdA/MPIA) )

« The ability to get a full orbit was really critical to understand how the planet distributes heat from the day side to the night side, » Kempton said. « There’s a lot of contrast between day and night. The night side is colder than the day side. » In fact, the temperatures shifted from 535 degrees to 326 degrees Fahrenheit (279 Celsius to 165 Celsius).

Such a big shift is only possible in an atmosphere made up of heavier molecules, such as water or methane, which appear similar when observed by the Webb telescope’s instruments. That means the atmosphere of GJ 1214 b is not composed mainly of lighter hydrogen molecules, Kempton said, a potentially important clue to the planet’s history and formation – and perhaps its watery start.

« This is not a primordial atmosphere, » she said. « It does not reflect the composition of the host star it formed around. Instead, it either lost a lot of hydrogen, if it started with a hydrogen-rich atmosphere, or it was formed from heavier elements to begin with – more icy, water-rich material. »

A companion paper to Kempton’s study, led by Peter Gao of the Carnegie Institution for Science, uses the same Webb data to focus on starlight filtered through the planet’s atmosphere – and further supports the idea that it is composed of heavier molecules. The paper has been accepted for publication.

And while the planet is hot by human standards, it is much cooler than expected, Kempton said. That’s because its unusually bright atmosphere, which came as a surprise to the researchers, reflects a large fraction of the light from its parent star rather than absorbing it and growing hotter.

The new observations could open the door to deeper knowledge of a planet type so far shrouded in uncertainty. Mini-Neptunes – or sub-Neptunes as they’re called in the paper – are the most common type of planet in the galaxy, but mysterious to us because they don’t occur in our solar system. Measurements so far show they are broadly similar to, say, a downsized version of our own Neptune. Beyond that, little is known.

« For the last almost decade, the only thing we really knew about this planet was that the atmosphere was cloudy or hazy, » said Rob Zellem, an exoplanet researcher who works with co-author and fellow exoplanet researcher Tiffany Kataria at NASA’s Jet Propulsion Laboratory in Southern California. « This paper has really cool implications for additional, detailed climate interpretations – to look at the detailed physics happening inside this planet’s atmosphere. »

The new work suggests the planet might have formed farther away from its star, a type known as a red dwarf, then spiraled gradually inward to its present, close orbit. The planet’s « year, » once around the star, takes only 1.6 days.

« The simplest explanation, if you find a very water-rich planet, is that it formed farther away from the host star, » Kempton said.

Further observations will be needed to pin down more details of GJ 1214 b as well as the formation histories of other planets in the mini-Neptune class. While a watery atmosphere seems likely for this planet, a significant methane component also is possible. And drawing broader conclusions about how mini-Neptunes form will require more of them to be observed in depth.

« By observing a whole population of objects like this, hopefully we can build up a consistent story, » Kempton said.

More About the Mission

The James Webb Space Telescope is the world’s premier space science observatory. Webb will solve mysteries in our solar system, look beyond to distant worlds around other stars, and probe the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency), and CSA (Canadian Space Agency).

MIRI was developed through a 50-50 partnership between NASA and ESA. NASA’s Jet Propulsion Laboratory led the U.S. efforts for MIRI, and a multinational consortium of European astronomical institutes contributes for ESA. George Rieke with the University of Arizona is the MIRI science team lead. Gillian Wright is the MIRI European principal investigator. Alistair Glasse with UK ATC is the MIRI instrument scientist, and Michael Ressler is the U.S. project scientist at JPL. Laszlo Tamas with UK ATC manages the European Consortium. The MIRI cryocooler development was led and managed by NASA’s Jet Propulsion Laboratory, in collaboration with Northrop Grumman in Redondo Beach, California, and NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Caltech manages JPL for NASA.

For more information about the Webb mission, visit :


Chercheur : Vivien Parmentier -

News Media Contacts : Calla Cofield - Jet Propulsion Laboratory, Pasadena, Calif. - 626-808-2469 -

Written by Pat Brennan