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Title: The Roasting Marshmallows Program with IGRINS on Gemini South. II. WASP-121 b has Superstellar C/O and Refractory-to-Volatile Ratios
Authors: Peter C. B. Smith et al.
First Author’s Institution: Arizona State University
Status: Published in AJ
Our solar system has a very nice, ordered structure to it. Close to the Sun are the rocky, terrestrial planets, and then once you cross the asteroid belt you find the big, surface-less gas giants. But over the past 30 years, astronomers have learned that this is not a universal setup. A majority of the exoplanets we have discovered to date fall in the gas giant category, but not all of these gas giant planets lie at the outer edges of their systems. In fact, we’ve found so many gas giants that are close to their host star that they’re given their own name: hot Jupiters. These are large, gaseous planets that orbit closer to their star than Mercury does to the Sun, and how they came to be there is a bit of a mystery.
Divvying Up the Protoplanetary Disk
Planets form out of a swirling ring of gas and dust called a protoplanetary disk. But this disk isn’t the same at all radii. As you move away from the star and the temperature drops, certain compounds change from gas to solid phase, which can have a big impact on the types of planets that form there. One way of dividing up the disk is via “snow lines.” Interior to a snow line, volatile materials (like water) are in gas form, but once you cross this line it is cold enough for them to freeze, forming small solid particles that provide seeds for planet growth. To highlight the role this could play in the types of planets that form, it’s thought that the snow line for water for our solar system was around 3 au, which places it right in between Mars and Jupiter. With the presence of solid particles, it’s easier for planetary cores to grow quickly and thus reach a point where they can hold on to the large gas envelopes that make a planet a gas giant. Another line that’s grown in interest recently is the soot line, which similarly marks the transition into gas specifically for carbon molecules. The properties of planets, including their potential for habitability, can depend greatly on where they formed relative to these lines.
So this is great and all, but when we go and observe exoplanets, we aren’t able to see a nice highlight reel of their formation history. We don’t know where they formed, and we can’t tell how they might have migrated through the disk. But if we can measure their compositions, we can start to get some clues as to where they might have formed and how they came to be where they are.
WASP-121b: An “Ultra-Hot” Planet
That’s where today’s article comes in. This work looks at an exoplanet with the beautifully poetic name of WASP-121b, which is a bit bigger than Jupiter but orbits so close to its host star that it’s actually classified as an “ultra-hot Jupiter.” (Side note: the title of the article is “The Roasting Marshmallows Program” because the planets studied in the program are all hot and puffy.) Ultra-hot Jupiters are, as you may have guessed, hotter than hot Jupiters, and thus they have fewer clouds in their atmospheres. This allows for a better understanding of their composition as you don’t have to deal with pesky and still relatively poorly constrained cloud models. Today’s work used observations from the Immersion Grating Infrared Spectrometer (IGRINS) instrument on the Gemini South telescope, observing the planet before and after its secondary eclipse — when the planet moves behind the host star. Figure 1 gives details about the system and shows the phases of the two observations.

Figure 1: Schematic of the system studied in this article. The blue and red highlighted regions show the phases of the planet’s orbit during the observations. Something fun about this diagram: it’s fully to scale, which really demonstrates just how close the planet is to its host star. [Adapted from Smith et al. 2024]
Ratios to the Rescue

Figure 2: Posterior distributions (which can be understood as the probability for each value) of the C/O and R/V ratios in the atmosphere of WASP-121b. By comparing the values to the stellar abundance we can constrain where in the protoplanetary disk the ultra-hot Jupiter formed. [Adapted from Smith et al. 2024]
Overall, WASP-121b has high values for both the C/O and R/V ratios. This suggests that the planet was formed exterior to the soot line but interior to the water snow line, which is kind of an unexpected result. As discussed above, it’s thought that massive gas giants form exterior to the snow line, as they need the ices to build a big enough core. Though surprising, it’s not impossible for this ultra-hot Jupiter to have formed interior to the snow line. One interpretation is that the planet actually started as a super-Earth (exoplanet categories are so silly sounding — this means a planet between the sizes of Earth and Neptune) that could form interior to the snow line but still be big enough to sustain the runaway gas accretion that got it up to Jupiter size. And then over time, it would have migrated in towards the host star, heating up until it became the roasted marshmallow we know and love. While this is one explanation, there’s still a lot to understand about this formation pathway and more study is definitely needed. Overall, these results provide an interesting challenge to our understanding of planet formation. They also demonstrate the usefulness of near-infrared spectroscopy when it comes to constraining the atmospheres of ultra-hot Jupiters.
Original astrobite edited by Lucas Brown.
About the author, Skylar Grayson:
Skylar Grayson is an Astrophysics PhD Candidate and NSF Graduate Research Fellow at Arizona State University. Her primary research focuses on active galactic nucleus feedback processes in cosmological simulations. She also works in astronomy education research, studying online learners in both undergraduate and free-choice environments. In her free time, Skylar keeps herself busy doing science communication on social media, playing drums and guitar, and crocheting!