Peering with unprecedented detail at the Milky Way Galaxy, NASA's James Webb Space Telescope has allowed a team of astronomers co-led by a University of Arizona researcher to unlock answers about the composition of the most abundant type of planet in the galaxy. In the process, the researchers have unveiled a "new class of planet," according to Everett Schlawin, an assistant research professor at the U of A Steward Observatory. 

The solar system is home to a familiar array of planets: dense, metallic Mercury; colossal Jupiter, with ammonia storms swirling across a helium atmosphere; and our own watery Earth. However, more than 5,000 exoplanets have been confirmed around stars other than the sun, many of them entirely unlike any of the planets in the solar system, making it difficult to guess their true natures. One of the most common types of exoplanets falls in a size range between Earth and Neptune. Astronomers have debated whether these planets are Earthlike rocky planets with thick hydrogen-rich atmospheres, or Neptune-like icy planets surrounded by water-rich atmospheres.  

One of these "sub-Neptunes" is a planet named GJ 1214 b, located 48 light years from the sun. According to Schlawin, it stands out as an "archetype representing an entire population of planets that is unlike anything we have in the solar system." Yet, GJ 1214 b has remained indecipherable to Earth-based telescopes, shrouded by layers of haze.

An international team of researchers led by Schlawin and Kazumasa Ohno at the National Astronomical Observatory of Japan used the James Webb Space Telescope to unveil the nature of GJ 1214 b's atmosphere for the first time. The findings are published in two companion papers, one led by Schlawin, the other by Ohno, in The Astrophysical Journal Letters.

Discovered in 2009, the planet orbits a star in the constellation Ophiuchus, also known as the "serpent-bearer," in the northern sky.  At the time, it was thought to be an ocean planet, and in 2023 the International Astronomical Union formally dubbed the planet Enaiposha, which translates to "large body of water" in the Maasai language. But previous studies to probe its atmosphere have been inconclusive, said Matthew Murphy, a graduate student in the U of A Department of Astronomy and co-author on both studies. 

"It has been the prevailing puzzle in the field for a decade," he said. The Near-Infrared Spectrograph, or NIRSpec, instrumentation aboard JWST opened the possibility for Schlawin's team to use transmission spectroscopy to look for the unique fingerprints of different gases in the planet's atmosphere. NIRSpec spreads out the light from the host star and can detect miniscule changes as GJ 1214 b orbits in front of it. What the team found was not a steaming water world, as astronomers have long anticipated, but a carbon dioxide world. 

"The detected CO2 signal from the first study is tiny, and so it required careful statistical analysis to ensure that it is real," Ohno said. "At the same time, we needed the physical and chemical insights to extract the true nature of GJ1214 b's atmosphere from Schlawin's study." 

Ohno then used theoretical models to run a plethora of "what if" scenarios about the atmosphere of the planet. His models confirmed that, of all these possibilities, GJ1214 b's atmosphere is likely carbon-dominated, like a "super-Venus" – "an entirely new class of planet," as Schlawin put it.

The atmospheric signature detected in this work is very small, Schlawin said.

"It's equivalent to Leo Tolstoy's "War and Peace," he said. "If I gave you two copies and changed one sentence in one of the books, could you find that sentence?"

The team emphasized the need for future studies to confirm and expand their findings about this common yet mysterious type of exoplanet.

The results affirm that the universe makes many kinds of worlds – not only the kinds we have in the solar system, such as rocky planets, ice moons and gas giants, but also unprecedented highly "metallic" planets like GJ1214 b, unlike any of Earth's near neighbors. 

"Although we’re not setting foot on another planet and walking around, we're really pioneers – we're the first humans to see what's happening on this other planet," Murphy said. "It's amazing what you can do when you have a team from all over the globe." 

The findings are a testament to the unparalleled capabilities of the infrared instruments aboard JWST – some of which were designed at U of A – and also to the software and sophisticated modeling techniques used by Ohno and his team, known as "atmospheric retrieval." 

With this generation of instrumentation and theoretical modeling combined, astronomers are poised to unveil the compositions of the many diverse sub-Neptunes in the Milky Way and beyond. Understanding these highly common, highly mysterious planets may be a steppingstone toward analyzing smaller, rockier planets – "the types of habitable, Earthlike planets that could someday help address these big questions like, 'Are we alone in the Universe?'" said Schlawin.