Viewing Saturn as a Transiting Extrasolar Planet

This post was written by Paul Dalba, a 3rd year Ph.D. student in the Muirhead Group at BU:

What would a distant alien civilization learn from observations of Saturn transiting the Sun?  The most-studied planets in the Galaxy exist in our own solar system. With that thought in mind, we are publishing a paper in the Astrophysical Journal describing an investigation Saturn, as if it were a transiting exoplanet (currently available on the arXiv):

This study made use of a dataset taken by the Visual and Infrared Mapping Spectrometer (VIMS) onboard NASA’s Cassini Spacecraft, which has spent more than 10 years investigating Saturn and its many natural satellites. As seen in the raw image below, which was taken by the Imaging Science Subsystem onboard Cassini, the Sun occasionally appears to “set” over the limb of Saturn. When this occurs, light from the Sun travels through Saturn’s atmosphere before reaching the Cassini spacecraft. This light is encoded with information about the structure and chemical composition of Saturn’s atmosphere. Planetary scientists have used these types of “solar occultation” observations to learn about the atmospheres of solar system bodies for many years.


The geometry of a solar occultation is very similar to that of an exoplanet transit. We took advantage of this fact to simulate observations of a hypothetical exoplanet system containing Saturn and the Sun. We calculated the transit depth of Saturn (the decrement in brightness of the Sun caused by the hypothetical transit Saturn) at each wavelength sampled by the VIMS (1 to 5 microns, ~250 channels). The resulting “transit transmission spectrum” is shown below.

The first surprising result from this analysis is that the transmission spectrum is not flat! Clouds have been found to produce flat transmission spectra in exoplanets and Saturn is known to harbor clouds at nearly every latitude. Instead, absorption features from methane, acetylene and other hydrocarbons are clearly visible with sizes of up to 90 parts-per-million. 

The second interesting result of this transmission spectrum is that the baseline continuum is set by
atmospheric refraction. The radius of the star and planet-star distance physically determine a minimum altitude to which remote observations of exoplanet atmospheres can probe (green dashed line). We show that this minimum altitude is above the average altitude of Saturn’s clouds (grey dashed line). So an alien civilization observing this transmission spectrum of Saturn would not know about its clouds!

Our research suggests that transmission spectroscopy, a tried-and-true technique to study hot exoplanet atmospheres, can also be applied to cold ones. This result opens the door to many interesting questions: What varieties of cold giant exoplanets exist in our Galaxy? What types ongoing chemical cycles (i.e. the methane cycle on Saturn) can we observe with observatories like Hubble or JWST? What can cold giant exoplanet atmospheres teach us about planetary formation and evolution? Clearly, there is much work to be done in this burgeoning sub-discipline of exoplanetary science! 

-Paul Dalba