IGRINS on Gemini South Detects Surprising Signatures in Dynamic Atmosphere of Exoplanet WASP-121b

As a member of the aptly named Roasting Marshmallows Program, Peter Smith, Graduate Associate at Arizona State University's School Of Earth and Space Exploration, is examining the atmospheric chemistry of hot and ultra-hot Jupiters to learn about the protoplanetary disks that they formed from.

This program utilizes the Immersion GRating INfrared Spectrograph (IGRINS), on the Gemini South telescope in Chile, one half of the International Gemini Observatory, funded in part by the U.S. National Science Foundation (NSF) and operated by NSF NOIRLab. IGRINS was built by the University of Texas and the Korea Astronomy and Space Science Institute.

Recently, the team observed the well-known gas giant WASP-121b, and their observations uncovered something unexpected about its formation history. Their research is presented in a paper appearing in The Astronomical Journal.

A planetary system forms from what is called a protoplanetary disk - a swirling disk containing a mix of rocky and icy material. Rocky materials, like iron, magnesium, and silicon, easily exist in their solidified state and require extreme levels of heat to vaporize into gas, whereas icy materials, like water, methane, ammonia, and carbon monoxide, are easily vaporized and require very low temperatures to condense.

With these observations, Smith and his team demonstrated for the first time measuring the rock-to-ice ratio for a transiting planet using a single instrument.

Because of their different temperature thresholds, the rocky and icy materials within the disk spread into a gradient, varying from vapor to solid depending on the distance from the star. As a result, astronomers can look for signatures of these elements in the composition of planets and their atmospheres, calculate the ratio of rocky to icy material, and determine how far from its star the planet formed.

Measuring this ratio typically requires multiple observations, using one instrument sensitive to visible light to detect the solid rocky elements and another sensitive to infrared light to detect the gaseous icy elements. But because WASP-121b is an ultra-hot Jupiter with extreme temperatures, both materials are vaporized into the atmosphere and are detectable with the high spectral resolution of IGRINS.

With these observations, Smith and his team demonstrated for the first time measuring the rock-to-ice ratio for a transiting planet using a single instrument. This unique capability allowed by IGRINS eliminates the potential errors introduced by instrumental differences and points an optimistic way forward for exoplanet chemical analysis. "Ground-based data from Gemini South using IGRINS actually made more precise measurements of the individual chemical abundances than even space-based telescopes could have achieved," says Smith.

Our measurement means that perhaps this typical view needs to be reconsidered and our planet formation models revisited.

The spectroscopic data show that WASP-121b has a high rock-to-ice ratio, indicating that it accreted an excess of rocky material while it was forming. This suggests the planet formed in a region of the protoplanetary disk where it was too hot for ices to condense, which is a surprising discovery since it's typically believed that gas giants need solid ices to form. "Our measurement means that perhaps this typical view needs to be reconsidered and our planet formation models revisited," says Smith.

The planet's dayside is so hot that elements typically thought of as 'metal' are vaporized into the atmosphere, making them detectable via spectroscopy.

Smith and his team also found remarkable characteristics of WASP-121b's atmosphere. "The climate of this planet is extreme, and nothing like that of Earth," he says. The planet's dayside is so hot that elements typically thought of as 'metal' are vaporized into the atmosphere, making them detectable via spectroscopy. Strong winds blow these metals to the planet's permanent nightside, where it is cool enough for them to condense and rain out - an effect that was observed on WASP-121b in the form of calcium rain.

"Our instrument sensitivity is advancing to the point where we can use these elements to probe different regions, altitudes, and longitudes to see subtleties like wind speeds, revealing just how dynamic this planet is," Smith says.

IGRINS was a visiting instrument at Gemini South when Smith observed WASP-121b during 2022 and 2023. It has since left the telescope to return to its home institution. The instrument was so successful that a new iteration of it - IGRINS-2 - was commissioned for the Gemini North telescope in Hawai'i and is now in its science calibration phase.

Smith cites IGRINS as a major factor in his team's detailed measurements of WASP-121b's atmosphere, and he looks forward to extending these investigations to other exoplanetary systems with IGRINS-2. Building a larger sample of hot and ultra-hot Jupiter atmospheres will allow scientists to refine their knowledge of how giant planets form.

Research Report:The Roasting Marshmallows Program with IGRINS on Gemini South. II. WASP-121 b has Superstellar C/O and Refractory-to-volatile Ratios