Fire and clouds: The ultra-hot Neptune that shouldn’t exist

A rarely encountered “ultra-hot Neptune” has revealed startling atmospheric details, from near-2,000°C surface temperatures to unexpected reflective clouds. 

The discovery is offering new insights into the extremes that can unfold on planets orbiting perilously close to their stars. 

Scientists have uncovered how these scorching conditions coexist with patches cool enough for cloud formation, suggesting a delicate balance between intense radiation and dynamic atmospheric processes.

A planet with relentless stellar heat 

The exotic atmosphere of LTT 9779 b came into sharper focus thanks to observations with the James Webb Space Telescope. The research was led by Louis-Philippe Coulombe, a graduate student at University of Montreal’s Trottier Institute for Research on Exoplanets (IREx). 

Orbiting its host star in less than one day, the planet endures relentless stellar heat while remaining tidally locked, such that one face constantly bakes in daylight and the other lies in endless darkness. 

Despite these formidable conditions, the dayside’s western hemisphere appears cool enough to host reflective clouds, while the planet’s eastern side remains significantly hotter. 

“This planet provides a unique laboratory to understand how clouds and the transport of heat interact in the atmospheres of highly irradiated worlds,” Coulombe said.

Atmosphere of an ultra-hot Neptune 

The analysis revealed that LTT 9779 b’s searing dayside is subject to powerful eastward winds that ferry energy around the planet. These winds seem to generate notable asymmetry on the daylight half, creating a bright, cloudier region on one side and a hotter, clearer domain on the other. 

By measuring both the planet’s emitted heat and the star’s light reflected by the clouds, the researchers developed a detailed atmospheric model. They found that the exoplanet’s capacity to redistribute energy plays a key role in allowing clouds to persist in the face of such harsh conditions. 

In addition, the experts detected water vapor in the atmosphere, a discovery that illuminates the composition and chemical reactions within this extreme environment. 

“By modeling LTT 9779 b’s atmosphere in detail, we’re starting to unlock the processes driving its alien weather patterns,” said Björn Benneke, an astronomy professor and co-author of the study.

Viewing exoplanets in unprecedented detail 

To investigate further, the researchers observed the planet across its entire orbit, capturing it at various phases to see how brightness and temperature changed over time. 

By combining visible-wavelength reflectivity with infrared heat signals, the experts could separate the portions of the planet lit by the star from those cloaked in darkness. This method included viewing two secondary eclipses – when LTT 9779 b passed behind its star – and a primary transit, when the planet crossed in front. 

Such comprehensive coverage enabled the team to pinpoint distinct temperature gradients and better understand how mineral-rich cloud layers might form and disperse on the marginally cooler western dayside.

Studies like this one rely on the James Webb Space Telescope’s ability to examine exoplanets in unprecedented detail, mapping both brightness and spectra at different wavelengths. 

“It’s remarkable that both types of analyses paint such a clear and consistent picture of the planet’s atmosphere,” said co-author Michael Radica, a postdoctoral researcher who had previously published a transit-focused analysis of LTT 9779 b. 

A planet that defies expectations

LTT 9779 b is notable for sitting in the so-called “hot Neptune desert,” a region unusually devoid of planets that are Neptune-sized but orbit their stars at extremely close distances. 

“Finding a planet of this size so close to its host star is like finding a snowball that hasn’t melted in a fire,” Coulombe said. Because it defies the norms of planetary migration and survival, LTT 9779 b offers a chance to examine how planets evolve when exposed to intense stellar forces. 

The presence of metallic clouds and the planet’s ability to maintain an atmosphere at such high temperatures raise questions about how these worlds form, what fraction of their mass is lost over time, and how they maintain any gaseous envelope in the face of unrelenting radiation.

“These findings give us a new lens for understanding atmospheric dynamics on smaller gas giants. This is just the beginning of what JWST will reveal about these fascinating worlds,” Coulombe noted. 

The data highlight how matter can condense into reflective clouds under formidable heat, while ferocious winds shuttle thermal energy and influence cloud distribution. 

Exploring other hot Neptunes

As astronomers continue exploring other “hot Neptunes” and related planetary classes, knowledge from LTT 9779 b will guide theories about how these systems emerge, adapt, and survive.

By showcasing the intricate dance of warmth, wind, and condensates, the study brings attention to just how robust – and delicate – an atmosphere can be in a realm nearing 2,000°C. 

Though extremely hostile by Earthly standards, LTT 9779 b stands as a testament to nature’s creativity in crafting astonishing cosmic wonders, reminding us that even conditions seemingly too severe might still allow clouds to glimmer and water vapor to persist against all odds.

The study is published in the journal Nature Astronomy. 

Image Credit: Benoit Gougeon, Université de Montréal

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