Could Rings Explain an Intriguing Exoplanet Spectrum?

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Title: A Circumplanetary Dust Ring May Explain the Extreme Spectral Slope of the 10 Myr Young Exoplanet K2-33b
Authors: Kazumasa Ohno et al.
First Author’s Institution: University of California, Santa Cruz
Status: Published in ApJL

Figure 1: Transit light curves of K2-33b using multiple instruments. The optical observations from K2 and MEarth show much deeper transits than those in the near-infrared observation, obtained with Spitzer IRAC’s Channels 1 and 2 and the Hubble Space Telescope. This could be because the planet’s atmosphere is blocking more light at bluer optical wavelengths than in the infrared. [Thao et al. 2022]

Interpreting the transmission spectra we observe from exoplanet atmospheres can be really tricky. While many of the features we see occur at characteristic wavelengths, the exact shapes and sizes of these features are controlled by a host of different factors, leaving a complex web of chemistry to untangle. For example, take what’s known as the “scattering slope”: a tilt in a planet’s transmission spectrum from more light transmitted at redder wavelengths to less light transmitted at bluer wavelengths. At bluer wavelengths, less light passes through the planet’s atmosphere, causing the depth of its transit at those wavelengths to appear deeper, as shown in Figure 1. This slope gets its name because it can be caused by the presence of clouds and hazes scattering light in the atmosphere, and how steep it is can provide information about such hazes. However, very steep slopes can also be caused by active regions on the host star’s surface, since transmission spectroscopy requires looking at the star’s light as it passes through the planet’s atmosphere.

K2-33b is one such planet with a very steep slope (check out this astrobite to find out more!), with ground- and space-based observations showing much deeper transits at bluer wavelengths, as seen in Figure 1. In this case, the host star probably isn’t active enough induce the slope we see, so a hazy atmosphere around a puffy, low-density planet is thought to be the culprit. But what if there were another possible explanation? The authors of today’s article consider whether a ring of dust around K2-33b could be responsible.

Ringing Out the Details

Using the presence of exoplanetary rings to interpret seemingly inexplicable observations isn’t a new idea. Rings could explain why some seemingly low-density exoplanets have flat transmission spectra; since the presence of rings would increase the radius obtained from the transit method while the mass of the system remains the same, rings could lead us to believe a planet’s density is lower than it is. But if rings produce a flat spectrum, how could they explain what’s happening with K2-33b? The authors of today’s article explain that the opacity of the ring is essential (take a look at Figure 2 for a handy guide!).

Cartoon illustrating the transmission spectrum of a ringed exoplanet

Figure 2: An illustration of how the presence of rings and their opacities can impact the transmission spectrum of an exoplanet’s atmosphere. [Ohno et al. 2022]

Too optically thick, as shown on the left-hand side of Figure 2, and the ring blocks light at optical wavelengths, producing a flat transmission spectrum. Too optically thin, as shown on the right-hand side of Figure 2, and the ring doesn’t interact with the star’s light at all, having no impact on the transmission spectrum. But as shown in the middle of Figure 2, if the ring’s opacity is just right, the star’s light passing through the ring will be absorbed more at bluer wavelengths. This creates a steep slope in the optical part of the transmission spectrum, similar to the slope that might be created by scattering from a hazy atmosphere. Crucially, a ring-induced slope can be much steeper than what might be produced by the atmosphere alone.

Does the Right Ring Make a Good Match?

To check whether this explanation could work for the transmission spectrum of K2-33b, the authors model both the atmosphere of the planet and rings of different mineral compositions.

Figure 3 demonstrates that with the right opacity, rings of all compositions are able to match the observations, reproducing the steep slope caused by the deeper transits at blue wavelengths. By comparing the coloured models to a model without the presence of a ring, shown by the grey line in each panel, it’s clear that the addition of rings is a big improvement! Many of the ring compositions also produce distinctive absorption features in the mid-infrared, which, if present, could be identified with JWST’s Mid-Infrared Instrument and would help confirm the existence of a ring.

K2-33b transmission spectrum compared to various models with and without rings of various mineral compositions

Figure 3: The transmission spectrum of K2-33b (black data points) along with models of the atmosphere without the presence of a ring (the flatter grey lines) and models including rings of different mineral compositions (coloured lines and shaded regions). Each panel highlights the impact of a ring made of a different mineral, as labelled in the bottom right of each panel. Note that the y axis shows the transit depth in parts per million (ppm), so a larger value here indicates less flux reaching the observer. [Ohno et al. 2022]

If the ring models provide a good match, does this mean K2-33b has a ring? Maybe! Since extremely low-density planets are expected to struggle to hold onto their atmospheres, and the ring scenario results in a higher-density planet than the hazy alternative, rings might start to seem like the more favourable option. But sustaining a dusty ring for long periods of time is also tricky. While mid-infrared observations will be helpful for understanding whether a ring is really present or not, until JWST points its hexagons at K2-33b, both scenarios remain perfectly reasonable.

Original astrobite edited by Jessie Thwaites.

About the author, Lili Alderson:

Lili Alderson is a second-year PhD student at the University of Bristol studying exoplanet atmospheres with space-based telescopes. She spent her undergrad at the University of Southampton with a year in research at the Center for Astrophysics | Harvard-Smithsonian. When not thinking about exoplanets, Lili enjoys ballet, film, and baking.