Could Rings Explain an Intriguing Exoplanet Spectrum?

Title: A Circumplanetary Dust Ring May Explain the Extreme Spectral Slope of the 10 Myr Young Exoplanet K2-33b

Authors: Kazumasa Ohno, Pa Chia Thao, Andrew W. Mann, Jonathan J. Fortney

First Author’s Institution: Department of Astronomy & Astrophysics, University of California, Santa Cruz, USA

Status: Published in ApJ Letters (open access)

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 often known as the “scattering slope” – a rise in the spectrum of an atmosphere towards bluer wavelengths. At these 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.

A plot of normalised flux from 1.002 to 0.996 on the y axis against orbital phase from -0.04 to 0.04, showing the U shaped transit light curves of K2-33b. The out of transit flux is level at 1. The light curves are centred in the plot so that the middle of the transit occurs at phase of 0. Near infrared observations from Spitzer IRAC Channel 1 and 2 are plotted in Purple and Orange, with HST plotted in green and all show a consistent dip in the normalised flux reaching roughly 0.999. The optical observations of K2 plotted in pink and MEarth plotted in blue are significantly deeper, reaching to almost 0.997.
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-infra 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. Figure 5 in Thao et al. 2022.

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 to be inducing the slope seen, so a hazy atmosphere around a puffy, low-density planet is thought to be the culprit. But what if there was another possible explanation? The authors of today’s paper consider whether a ring of dust around K2-33b could be responsible.

Ringing Out the Details

Using the presence of exoplanetary rings to understand seemingly unexplainable observations isn’t a new idea. Rings could explain why some very low-density exoplanets have very flat transmission spectra, since the presence of rings would increase the radius obtained from the transit method, but wouldn’t significantly increase the mass of the system. But if rings produce a flat spectrum, how could they explain what’s happening with K2-33b? The authors of today’s paper explain that the opacity of the ring is essential (take a look at Figure 2 for a handy guide!).

An illustration of how the presence of rings impacts a transmission spectrum. A ring free spectrum is shown in blue to be relatively flat, but with some bumps and wiggles across all wavelengths. On the left, the optically thick ring is described. "Ring's physical size limits transit depth, ring acts to make flat spectrum" is written out above a drawing of a black circle surrounded by a black ring to demonstrate the optically thick ring and planet. Below, the contribution of the ring to the spectrum is shown as a flat line. In the middle, the optically thin ring is described. "Ring of small particles cause spectral slope, spectrum shows absorption feature of ring" is written out above a drawing of a grey ring surrounding a black circle, illustrating the optically thin ring and planet. Below, the contribution of the ring to the spectrum show a steep slope which rises towards bluer wavelengths. On the right, the ring-free / extremely optically thin scenario is shown. "Ring does affect transmission spectrum" is written above a very faint grey ring surrounding a black circle, representing a planet with a ring that does not interact with the star light. Here, there is no contribution of the ring to the spectrum.
Figure 2: An illustration of how the presence of rings and their opacities can impact the transmission spectrum of an exoplanet’s atmosphere. Figure 1 in the paper.

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 one that might be created by scattering from a hazy atmosphere. Crucially, a ring-induced slope can be much steeper than 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. 

A 6 panel plot of the transmission spectrum of K2-33b showing different models with rings made of different materials. Each panel plots the transit depth from 500 to 3000 ppm on the y axis against wavelength from 0.2 to 30 microns on a log scale on the x axis. In each panel, the observed transmission spectrum of K2-33b is plotted in black data points, which shows several data points with large transit depths ~2500 ppm around 0.6 micron, quickly dropping to ~1000 ppm by 1 micron, with two additional data points around 5 micron also at ~1000 ppm. In each panel, a different ring model is plotted in a different colour, along with a ring free model in grey. The ring free model is flat with small bumps and wiggles at ~1000ppm across the full wavelength range, and does not fit the steep slope at all. Each ring model provides a good fit to the slope, and also shows large bumps between 10 and 30 microns. In the top row, of the 2x3 plot, Mg2SiO4 is plotted in one panel in red, Fe2SiO4 in the next panel in blue and Astronomical Silicate in the final panel in green. In the bottom row, Al2O3 is plotted in the first panel in yellow, FeO in the middle panel in purple and in the final MgO in silver.
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. Figure 4 in the paper.

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, would be easily identified with JWST’s MIRI instrument and would help confirm the existence of a ring.

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. 

Astrobite edited by Jessie Thwaites

Featured image credit: NASA/Cassini/James O’Donoghue 

About 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.

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