Title: Hazy with a Chance of Starspotting: Constraining the Young Planet’s Atmosphere, K2-33b
Authors: Pa Chia Thao, Andrew W. Mann, Peter Gao, et al.
Institution of First Author: Department of Physics and Astronomy, UNC Chapel Hill
Status: Accepted to the Astronomical Journal, available on arXiv.
As any astronomer on the ground can tell you, cloudy, hazy skies are the bane of good observations. Imagine hearing that there’s a new supernova that needs immediate tracking, and you can’t see it because there’s a thick layer of cloud above your observatory. You would think that space telescopes, which spend their entire productive lives above the Earth’s atmosphere, would be free from such frustrating phenomena, but no! Exoplanetary atmospheres are just as likely to be obscured by all manner of science-blocking aerosols, making it difficult to see identifiable characteristics of gases (like water vapor or carbon dioxide) in Earth’s atmosphere. the planet (a so-called “flat spectrum”). What if a planet’s flat spectrum isn’t really flat? What if there are no molecular features but there are puzzling differences at different wavelengths? Can we extract useful information from a featureless planet? Today’s writers argue that we absolutely can.
K2-33 b is the youngest known transiting planet (only 10 million years old!), a super-Neptune sized planet, orbiting a 3500K M3 dwarf, 140 parsecs from Earth in the constellation Scorpio. Since its discovery by the Kepler spacecraft in 2014, it had a significant number of follow-up observations, including 10 transits with Spitzera spectrum based on a partial transit of Hubble, and about ten transits from the two MEarth Project observatories. Taken together, all of these observations can be used to construct a low-resolution transmission spectrum of the planet ranging from 0.6 to 4.5 microns. Data from HubbleNear-infrared spectrographs show a relatively flat spectrum between 1.1 and 1.7 microns with no significant features indicating water vapor or any other identifiable molecule. However, the observed transits of this planet are almost twice as deep at short wavelengths as at long ones! Two major possibilities exist to explain the significant slope of the full spectrum: either it is the result of an inhomogeneous stellar surface, or the spectrum shows something physical in the atmosphere of the planet.
Figure 1: The different transits of K2-33 b. Optical transits (K2, MEarth) are almost twice as deep as near-infrared transits (Spitzer, Hubble), which raises interesting questions about the planet’s atmosphere. Figure 5 in the article.
Spots aren’t just for cheetahs
An important thing to consider when performing transmission spectroscopy is the stellar reference spectrum. Transit spectra rely on a good understanding of the stellar surface when the planet is not transiting, so stellar spots (where the surface is cooler and darker) and ranges (where the surface is hotter and brighter ) could have significant effects on these observations. For example, a transit crossing a star spot would seem shallower than normal, but a transit that coincides with visible star spots (but does not cross them) would seem deeper than normal. Starspots tend to come and go with the rotation of the star over long periods of time, so it is important to model them accurately to interpret data from transits separated by weeks and months.
Figure 2: Top – Starspots can change the observed spectrum of a star enough to significantly affect the transmission spectrum of an exoplanet, producing spurious absorption features. (Fig. 1 at https://arxiv.org/abs/1711.05691) Bottom – Clouds and haze at high altitudes prevent incident starlight from filtering through a planet’s atmosphere, flattening its transmission spectrum. (Fig. 1 in Kempton, EMR, 2014, Nature, 513, 493)
Alien Smog
The other major possible explanation for the steeply tilted spectrum is the effects of spectrally dependent hazes in the planet’s atmosphere. Hazes are expected to be present in planetary atmospheres as small molecules break up and polymerize under stellar radiation at high altitudes. While clouds are often modeled as uniform absorbers, small haze particles tend to absorb more strongly at shorter wavelengths, leading to steep slopes from optical to IR . Hot gas planets like K2-33b are expected to have complex carbon chemistry, with visible traces of methane (CH4) or carbon monoxide (CO) depending on the temperature and the amount of mixture in the atmosphere. Colder atmospheres should have more CH4 than CO, but warmer atmospheres or the mixing of warmer, deeper parts of the atmosphere up to higher altitudes can increase the amount of CO. These carbon species can be transformed into hazes like tholins and soot, similar to the smog produced by burning fossil fuels. The authors modeled the effects of the two types of haze produced by both CH4 and CO, leading to four possible atmospheres to fit the data.
Why not the two of them?
After detailed stellar and planetary atmospheric modeling, the authors found that neither star cover nor atmospheric haze was sufficient to explain the observed transmission spectrum of K2-33b. The inclusion of both spots (either a modest point coverage of <20% of the surface, or the spots should be similar to stellar surface temperature) and hazes (CO-derived tholins) gave the best fit to the data. K2-33 b isn't warm enough to expect much CO at high altitudes, so it's surprising that CO-derived tholins fit the data the best. The preferred explanation for the presence of CO on CH4 is the atmospheric mixture, but the planet may also have a higher metallicity than expected and a lower C/O ratio, which will require additional observations with JWST to confirm.
Figure 3: Atmospheric models best fitted to the observed spectrum of K2-33b. Models including stellar spots fit the data better than those not including spots, and the CO tholin model fits the observed data. Spitzer photometry better than soot. Figure 12 in the article.
put a ring on it
What if neither sunspots nor hazes were the real explanation? A final possibility is the presence of a large planetary ring, something often theorized but never observed outside of our own solar system. Young planets in close orbit can maintain rings, and annular particles around 1 micron in size could easily replicate deep optical transit while allowing for shallower parts of the near-infrared spectrum. There is a complementary article in the press discussing this possibility, so stay tuned!
Astrobite Edited by Aldo Panfichi
Featured Image Credit: ESA/A.Gerst
About Yoni Brande
I am a third-year PhD student at the University of Kansas, working on the discovery and characterization of exoplanets. I work primarily with TESS transit data and Exoplanet Transmission Spectroscopy data from the Hubble Space Telescope, and am also interested in enabling more collaborative science with open source astronomical software tools. When I’m not researching or writing astrobites, I can be found in a sci-fi streaming frenzy, running, lifting, cooking or on Twitter @YoniAstro.
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