Citation – “The Roasting Marshmallows Program with IGRINS on Gemini South. II. WASP-121 b has Superstellar C/O and Refractory-to-volatile Ratios“ Peter C. B. Smith et al 2024 AJ 168 293
The study focuses on WASP-121b, a famous ultra hot Jupiter, using the IGRINS instrument on the Gemini South telescope. The goal is to understand the planet’s atmospheric composition and what it reveals about its formation history.
Exoplanets like WASP-121b provide a unique window into planetary formation. Scientists use atmospheric data to infer how and where a planet formed. A key metric is the carbon-to-oxygen ratio (C/O), which can indicate whether a planet formed closer to or farther from its star. However, C/O alone is not enough, measuring the refractory-to-volatile ratio (R/V) provides additional clues about a planet’s original location in the protoplanetary disk.
This study marks the first simultaneous measurement of both the C/O ratio and R/V ratio for an exoplanet using high-resolution infrared spectroscopy.
Observations and Methods
Using IGRINS (Immersion Grating Infrared Spectrometer), the team observed WASP-121b in thermal emission at two different times, before and after the secondary eclipse. This allowed them to analyse the light from the planet’s own atmosphere, rather than just the light from the star passing through it.
Spectroscopy is the key tool here, the researchers used a technique called cross-correlation spectroscopy, which compares observed light to simulated models of different atmospheric gases. This method allows them to detect molecules like H₂O (water vapor), CO (carbon monoxide), OH (hydroxyl), Fe I (iron), Mg I (magnesium), Ca I (calcium), and other elements.
By analysing how these gases shift in wavelength due to the planet’s motion, the study also investigated the dynamics motion within the planet’s atmosphere, possibly identifying high-speed winds and jet streams.
Key Findings from the study
- WASP-121b has a Superstellar C/O Ratio (0.70 ± 0.10)
- This means the planet has more carbon relative to oxygen than its host star, which strongly suggests the planet formed beyond the “soot line”, where carbon-rich materials will condense out of the protoplanetary disk.
- The Refractory-to-Volatile Ratio is Moderately Superstellar
- Volatile elements (like oxygen and carbon) are typically found in ices, while refractory elements (like iron and magnesium) condense at high temperatures. Thus, the observed high R/V ratio suggests that WASP-121b may have formed in an area rich in dust and migrated inward.
- Detection of H₂O, CO, and OH
- The presence of CO and OH supports the idea that water vapor is breaking apart (dissociating) due to the planet’s extreme heat.
- The planet’s day-side temperature is so high (~2500K) that molecules are being torn apart and recombining into new molecules.
- Velocity Shifts Suggest Atmospheric Circulation
- Different gases appear at slightly different velocities, strongly implying chemical variations across the planet’s surface.
- CO and OH follow the expected orbital motion, but H₂O appears to move at a different speed, possibly due to strong winds or uneven temperature distribution around the planet and perhaps in scaler height.
- Implications for Planet Formation
- The combination of a high C/O ratio and moderate R/V ratio suggests that WASP-121b likely formed between the soot line and the water snow line in the protoplanetary disk which challenges the traditional model that all hot Jupiter’s form beyond the water snow line and migrate inward.
This study is a step forward in understanding how planets form. By measuring both C/O and R/V ratios, researcher can better distinguish between different formation scenarios. WASP-121b serves as a test case for understanding how gas giants accumulate their atmospheres and migrate over time.
It also highlights the power of modern infrared spectroscopy in characterising exoplanets.
WASP-121b will continue to be an exoplanetary laboratory, offering new insights into the extreme chemistry and physics of planets orbiting close to their stars, shedding light on this phenomenon of giant planets being cooked by their star!