Roasted Marshmallows Setting Fire to Our Understanding of Planet Formation?

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, Jorge A. Sanchez, Michael R. Line, Emily Rauscher, Megan Weiner Mansfield, Eliza M.-R. Kempton, Arjun Savel, Joost P. Wardenier, Lorenzo Pino, Jacob L. Bean, Hayley Beltz, Vatsal Panwar, Matteo Brogi, Isaac Malsky, Jonathan Fortney, Jean-Michel Désert, Stefan Pelletier, Vivien Parmentier, Sai Krishna Teja Kanumalla, Luis Welbanks, Michael Meyer, and John Monnier

First Author’s Institution: School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA

Status: Published in The Astronomical Journal (open access)

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 they’re not all at the outer edges of their systems. In fact, we’ve found so many 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 it 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 into where they might have formed and how they came to be where they are. 

WASP 121-b: An “Ultrahot” Planet

That’s where today’s paper comes in. This work looks at the exoplanet with the beautifully poetic name of WASP 121-b, which is a bit bigger than Jupiter but orbits so close to its host star that it’s actually classified as an “ultrahot Jupiter”. (Side note: this is why the title of the paper is “The Roasting Marshmallows Program”, because these planets are all hot and puffy.) Ultrahot Jupiters are, as you may have guessed, hotter than hot Jupiters, and thus they have fewer clouds in their atmosphere. 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 Gemini South, observing the planet before and after its secondary eclipse-when the planet moves behind the host star. Figure 1 gives details about the system as well as showing the phases of the two observations.

The observations of WASP-121 b consisted of high resolution spectra capturing the thermal emission of the planet. This data was then compared against models that included volatile (water, carbon monoxide, hydroxide) and refractory (Fe, Mg, Ca) species. Being able to constrain all these different compounds with the same observations is actually a pretty rare feat, as volatile and refractory species generally have to be studied in different portions of the electromagnetic spectrum. However, refractory species are possible to observe with high resolution spectroscopy in the near-IR, which is exactly what IGRINS provides. This is handy because it allows for direct comparisons without needing to worry about systematic effects from using different instruments.

Ratios to the Rescue

Thus by comparing IGRINS observations against planetary atmosphere models, the researchers were able to place constraints on the presence of both volatile and refractory species in WASP-121 b’s atmosphere. However, getting absolute abundance measurements was a trickier task given relative unknowns about other contributions to the continuum of the spectrum. So instead of trying to constrain absolute abundances, the research team calculated ratios. The two ratios that are important in this paper are carbon to oxygen (C/O) and refractory to volatile (R/V). Not only are ratios better constrained than straight up abundances, they can also help place where in the protoplanetary disk WASP-121 b formed.  The C/O ratio will increase past the soot line as oxygen incorporates into silicates, and further increase past the snow line as oxygen freezes into water ice. The R/V ratio tracks the behavior of solid accretion, and if the R/V ratio is high, this suggests that the planet formed interior to the snow line where most of the accreted solids did not contain volatile material. Thus a combination of the C/O and R/V ratio allows us to determine the location of planet formation relative to the water snow and soot lines.  Figure 2 shows the posterior distribution (which is a semi-complicated Bayesian statistical result that essentially just means probability) for the C/O and R/V ratios.

Overall, WASP-121 b 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 ultrahot 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 size of Earth and Neptune) that could form interior to the snow line but still be big enough to sustain 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 ultrahot Jupiters.

Astrobite edited by Lucas Brown

Featured image credit: NASA/JPL/S. Grayson