The Transit Transmission Spectrum of a Cold Gas Giant Planet

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arXiv:1510.03430v1 [astro-ph.EP] 12 Oct 2015 08.Acrigt h AAEolntAcieand Archive Exoplanet NASA the geometri- to Gaudi According are & (Beatty surveys solar 2008). planets long-period our transit against of biased in Unfortunately, cally atmospheres those resembling the system. planets characterizing long-period for cold, method known atmospheres. not planets’ do to but pertaining exoplanets information effec- giant provide observing microlensing) long-period, Other and discover 2013). radial-velocity tively al. et (i.e. (e.g. Birkby techniques emission 2010; in al. observed at- et have be Snellen to or observation enough warm single mospheres a K al. during changes et 800 large radial-velocity experience Carson in than that also 2010; planets hotter spectroscopy short-period al. favors ground-based and et High-resolution, Lagrange Myr 2008; 2013). 50 char- plan- al. than et to directly-imaged (Marois younger able most are been only exoplanets; ets also warm exoplan- Di- have that acterize self-luminous, require be which atmospheres. ets observations, are their many imaging characterize providing exoplanets rect frequently, heights, to hosts Close-in their scale opportunities al. transit et atmospheric they Kreidberg favorable and 2014). 2008; producing al. al. warm, et et Fraine Pont 2002; al. 2007; exoplan- 2014a; et al. of Charbonneau et (e.g. atmospheres Knutson only orbits the have short-period plan- observations characterize on Transit ets gaseous to system. used cold, solar been the our resembling in ets those targeted not alA Dalba, A. Paul rnmsinsetocp stems appropriate most the is spectroscopy Transmission have atmospheres exoplanet of investigations date, To rpittpstuigL using 2018 typeset 2, Preprint July version Draft eut,w reycnie h esblt fasre osac for search a to target-of-opportunity obs survey a using a future headings: Saturn of Subject and and feasibility current Jupiter the to with consider analogous possible briefly we be transmiss results, should that suggests exoplanets spectrum gaseous on spect transmission in transmission Saturn’s of located in interpretations is signal alter feature could large and This bandpasses hydrocarbon. aliphatic 3.4 a transmissio unknown near Saturn’s an refraction, feature mid-tran with absorption during atmospheric agreement large probed that a good be find show could p models also that 90 atmosphere We altitude to minimum up the clouds. of acet determines ammonia features ethane, of peak-to-peak methane, with ence from monoxide absorption carbon detect possibly We exoplanet. ing Cassini 1 eateto srnm,Bso nvriy 2 Commonwea 725 University, Boston Astronomy, of Department eueslroclain bevdb h iuladIfae Map Infrared and Visual the by observed occultations solar use We H RNI RNMSINSETU FACL A IN PLANET GIANT GAS COLD A OF SPECTRUM TRANSMISSION TRANSIT THE 1 pccatt xrc h o5 to 1 the extract to Spacecraft 2 hlpS Muirhead, S. Philip eateto srnm srpyis nvriyo Cali of University Astrophysics, & Astronomy of Department 1. A INTRODUCTION T E tl mltajv 5/2/11 v. emulateapj style X lnt n aelts topee lnt n aelts individu satellites: and planets — atmospheres satellites: and planets tr:paeaysystems planetary stars: 4 3 eateto srnm,CrelUiest,Ihc,N 14 NY Ithaca, University, Cornell Astronomy, of Department eateto hsc,Uiest fIao ocw D8384 ID Moscow, Idaho, of University Physics, of Department 1 oahnJ Fortney, J. Jonathan µ ieycue ygsosehn n - tecigmd of mode stretching C-H a and ethane gaseous by caused likely m rf eso uy2 2018 2, July version Draft µ rnmsinsetu fStr,a fi eeatransit- a were it if as Saturn, of spectrum transmission m ABSTRACT .Veyette J. evtoso / nhtJptr eg tvno tal. et Stevenson (e.g. ob- by Jupiters tested hot al. been in et has C/O (Owen which of accretion 1996), servations al. core et invoke includ- Pollack Jupiter, 1999; theories, for formation that planet formation ing Many planetary of evolution. tracers and are which ratios, O/H and Bie ta.20;Ws ta.20;Bgnle al. et Bagenal giants 2009; gas CH system al. et West solar 1983). al. the et 2005; Tokunaga which 2004; in of al. al. et observed et both (Baines Line in- chemistry, been temperature 2013; disequilibrium have as Yung such & and Line phenomena versions explore 2008; at- that Seager al. & 2014) full et Madhusudhan for Irwin (e.g. useful 2009; codes also retrieval are al. mospheric spectra et (Fortney Transmission profiles thermal 2010). which their of atmospheres, models exoplanet constrain in abundances molecular unexplored. relatively exo- system gone solar transiting far our of in so Hav- planets regime has giant the Jupiter. the to as member, analogous cold one planets as this nearly only it Bond an ing making al. a has 0.3), (assuming et of Kepler-421b K Jenkins albedo 200 only 2014; below 2012; these, temperature al. Of al. equilibrium et et Kipping Orosz respectively). 2014; 2015, 2012; al. al. et et Cabrera (Welsh Kepler-421b, Kepler-452b Kepler-90h, and Kepler-47c, Kepler-34b, AU: 1 ehd ny5(.% aeobtlsm-ao axes transit semi-major the orbital have via (0.4%) 5 discovered only 2014), exoplanets method, al. et confirmed (Han 1,230 Database Orbit Exoplanet the 2 1 topei bnacso oeue uha CO, as such molecules of abundances Atmospheric reveal to potential the have spectra Transmission ate .Hedman, M. Matthew 1 t v. otn A 21,UA [email protected] USA; 02215, MA, Boston, Ave., lth 4 ohdtbssacse 05Jl 23. July 2015 accessed databases Both CO , ona at rz A904 USA 95064, CA Cruz, Santa fornia, 2 ai o rprymdld h large The modeled. properly not if ra n H and , pproach. o pcrsoyo od long-period cold, of spectroscopy ion ln,aihtchdoabn,and hydrocarbons, aliphatic ylene, rsprmlindsietepres- the despite arts-per-million pcrmbtfi oreproduce to fail but spectrum n n hrceiecl exoplanets cold characterize and raois oiae ythese by Motivated ervatories. fthe of e igSetoee badthe aboard Spectrometer ping poe ocod rhaze, or clouds to opposed s 2 i.Sl-ossetexoplanet Self-consistent sit. 5,USA 853, lc osrit nCO C/H, C/O, on constraints place O ,USA 3, pte pc Telescope Space Spitzer 3 hlpD Nicholson, D. Philip l(aun — (Saturn) al 4 1 Mark fthe of a & 2 Dalba et al.

2014; Brogi et al. 2014; Line et al. 2014). Atmospheric (0.35-1.07 µm, 96 bands, ∼8 nm resolution) and near-IR abundance measurements of cold, giant exoplanets would (0.85-5.11 µm, 256 bands, ∼17 nm resolution), respec- provide a similar test of core accretion and could also be tively. Only the latter is used during solar occultations. used to improve the current understanding of how at- Solar occultation observations are obtained through a so- mospheric abundances respond to planetary migration lar port with an aperture of 30 mm by 5 mm that is orien- ◦ (Ida & Lin 2004; Madhusudhan et al. 2014; Oberg¨ et al. tated 20 away from the boresight direction of the main 2011). aperture. In the solar port, sunlight undergoes several Atmospheric abundances can be difficult or impossi- reflections that attenuate the solar flux by a factor of ap- − ble to determine for atmospheres that harbor clouds or proximately 2.5×10 7 before passing through the slit and haze, which produce flat transmission spectra across near in to the main optical path of VIMS-IR (Bellucci et al. infrared wavelengths (e.g. Bean et al. 2010; Berta et al. 2009). The nominal VIMS-IR observation produces a 2012; Kreidberg et al. 2014a; Knutson et al. 2014a,b; data cube comprised of two spacial dimensions (64 x 64 Ehrenreich et al. 2014). Clouds are present in the atmo- pixels) and one spectral dimension. The indium anti- spheres of each giant solar system planet (e.g. West et al. monide IR detector is a one-dimensional array (1 x 256 2009; Bagenal et al. 2004; Lindal et al. 1987; Smith et al. pixels), so it can only obtain the spectrum of a single 1989), but these planets are much colder and experi- spacial pixel at a time. Therefore, the IR telescope’s sec- ence different levels stellar insolation than previously- ondary mirror is scanned in two dimensions across the observed cloudy exoplanets. It is not clear how these target to construct a full data cube. For solar occultation differences would influence the effects of clouds on the observations, VIMS only acquires a 12 x 12 pixel field- ′ transmission spectrum of a cold giant exoplanet. of-view, which corresponds to an angular size of 20.6 x ′ To begin exploring the regime of cold, long-period 20.6 (each pixel having an angular resolution of 1.7 ar- exoplanets, we turn to an extensively studied gas cminutes). This reduction in field-of-view is acceptable giant in our own solar system: Saturn. High- since the solar disk as seen from Saturn only extends over quality solar system observations provide a unique op- approximately 2 x 2 pixels. portunity to study and “ground-truth” the methods 2.2. Occultation Data used to characterize exoplanets. Solar system bod- ies such as the Earth (e.g. Vidal-Madjar et al. 2010; We analyzed a Saturn solar occultation observation Garc´ıaMuñoz et al. 2012; Bétrémieux & Kaltenegger from UT 2007 November 17. The observation consisted 2013; Misra et al. 2014; Schwieterman et al. 2015), Ti- of 479 data cubes, each having an image dimension of tan (Robinson et al. 2014), Jupiter (Irwin et al. 2014; 12 x 12 pixels and an exposure time of 20 ms per pixel. Montañés-Rodr´ıguez et al. 2015), Uranus and Neptune VIMS began observing several minutes before ingress in (Kane 2011) have all been studied in the context of ex- order to establish a high-signal measurement of the solar trasolar planetary science in recent years. spectrum out of occultation. Similarly, the observation We use observations from the Visual and Infrared ended several minutes after the solar flux was completely Mapping Spectrometer (Brown et al. 2004) aboard the attenuated. The duration of an entire observation was Cassini Spacecraft to extract the 1 to 5 µm transmission approximately 0.5 hours. spectrum of Saturn, as if it were a transiting exoplanet. For each 12x12-pixel image, we determined a value of With this spectrum, we asses the feasibility of observing relative transmission (Tλ) by summing the signal over cold, gaseous exoplanets with current and future obser- the entire field-of-view and dividing by the total signal vatories. of the Sun prior to occultation. Outside of occultation, We present the Cassini-VIMS observations, data, and Tλ ∼ 1. Once the Sun’s flux was completely attenuated analysis procedures in Section 2. In Section 3 we develop by Saturn, Tλ ∼ 0. We followed this normalization pro- an occultation model that assumes the portion of Sat- cedure for each of the 256 wavelength bands in each of the urn’s atmosphere sampled by the observations is isother- 479 data cubes. This ratio removed systematic and in- mal and in hydrostatic equilibrium. We also fit for the strumental errors along with the the need to convert the effects of atmospheric refraction and absorption versus detector’s data numbers (or counts) into specific ener- ascribing them from previous observations. In Section gies. Data calibration was further simplified by the high 4, we calculate the transmission spectrum of Saturn and linearity of VIMS-IR detector (Brown et al. 2004) and by compare it to spectra of model atmospheres that are cur- the low background signal compared to that of the Sun rently applied to exoplanets. Lastly, in Section 5 we (less than 1%). The data we considered did not suffer discuss the implications of this work for exoplanet at- from contamination by stray light entering the boresight, mosphere models, and we briefly consider a strategy to spacecraft pointing instability, or other sources of spuri- locate and characterize cold, giant exoplanets in the near ous signal that warrant advanced calibration procedures future. (e.g. Maltagliati et al. 2015). A formal data reduction routine for VIMS exists (McCord et al. 2004); however, 2. OBSERVATIONS it is not appropriate for observations of solar occultations 2.1. that pass through a different optics chain than those ac- Cassini-VIMS quired through the main aperture. The Visual and Infrared Mapping Spectrometer When calculating Tλ, we defined the background as all (VIMS) aboard the Cassini Spacecraft has been observ- the pixels residing outside of a circular aperture centered ing Saturn and its satellites since arriving at the Sat- on the Sun with a radius of four pixels. The average back- urnian system in 2004 (Matson et al. 2002; Brown et al. ground was approximately 14 counts per pixel, only 0.6% 2004). VIMS has two imaging grating spectrometers, of the average integrated signal from the Sun (∼2,260 VIMS-VIS and VIMS-IR, that operate in the visible counts). Since the background level decreased unevenly Saturn as a Transiting Exoplanet 3 as Saturn’s limb entered the field-of-view, we could not accurately estimate the background signal across the detector simply by finding the mean number of counts in a circular annulus surrounding the central aperture. In- stead, we separated the detector into four 6x6-pixel quad- rants, and subtracted the mean background locally in each. When Saturn’s atmosphere was in view, the average background value was 1-2 counts per pixel. Consid- ering the minimal contribution of the background to the total count value of the entire field-of-view, this simple procedure was sufficient. After calculating Tλ, we median-filtered each occultation light curve to remove outliers due to other sources of spurious signal, most which were cosmic ray strikes on the detector. A data point was declared an outlier and Fig. 1.— Geometry of the Sun-Saturn-Cassini system (not to scale, rings of Saturn not pictured). Light from the Sun followed removed if it had a value of Tλ that was either 3σ above a curved path in Saturn’s atmosphere (solid red line). The tan- or below the median Tλ value of the six points on either gent radius (r) was measured from the center of Saturn to the side of it. point along the straight line-of-sight between Cassini and the Sun We assigned each T -value an uncertainty equal to the (dashed red line) that was tangent to the local horizon of Saturn. λ In the model, rays from the Sun entered Saturn’s atmosphere at an standard deviation of solar signal prior to occultation. At altitude of Rp + ztop before reaching Cassini with angle α to the redder wavelengths (λ > 4 µm), the solar intensity was Cassini-Saturn line. Rp was the “surface” of Saturn from which weak and the data became increasingly noisy. The red- the altitude z was measured. As the occultation progressed, the Sun appeared to move in the direction indicated by the black arrow dest 8 bands spanning 4.99–5.12 µm were used to record from the point of view of Cassini. Each value of r corresponded timing information for the observations and were not in- to a value of D (§3.3). There was some impact parameter between cluded in the following analysis. Some of the VIMS data the path of the Sun and the center of Saturn as seen from Cassini exhibited low-level, time-correlated noise, possibly due to (i.e. the Sun did not pass directly behind the center of Saturn). However, the occultation model only tracked the one-dimensional detector readout effects (McCord et al. 2004). Its mag- radial motion of the Sun. nitude was typically on the order of the uncertainty and did not greatly affect the signal or the analysis. We monitored the progress of the occultation with sure level. H is the atmospheric scale height defined by measurements of the “tangent radius” r. This was a k T measure of distance between the center of Saturn and H = B (2) the point on a straight line-of-sight between the Sun and µmpg Cassini that was tangent to the local horizon of Saturn where k is Boltzmann’s constant, T is temperature, µ is (see Fig. 1). We used r as a substitute for time since it B the mean molecular weight of Saturn’s atmosphere, mp included information about the relative positions of the is the proton mass, and g is the local acceleration due Sun, Saturn, and Cassini that was useful when modeling to gravity. H was a critical parameter to the occulta- the occultation. tion model as it controlled how steeply the transmission decreased during ingress. The scale height did not have 3. A SOLAR OCCULTATION MODEL a wavelength dependence per se, but we could not use 3.1. Parametrizing Saturn’s Atmosphere a single value of H in the model across the spectrum. Due to methane absorption, different wavelengths sam- A goal of this work was to measure Saturn’s transmis- pled portions of Saturn’s atmosphere that were separated sion spectrum as empirically as possible. Therefore, we by up to ∼450 km in altitude. This was readily observ- modeled the Saturn-solar occultations without directly able in the occultation data as a range in “half-light” using atmospheric chemical abundances, mixing ratios, r-values, where Tλ = 0.5. Over ∼450 km, variations in indices of refraction, and opacities available in the liter- temperature and therefore scale height necessitated that ature. Instead, we fit the VIMS occultation data to a we fit for H at each wavelength in the model. model atmosphere and estimated parameters describing We used two parameters to describe the wavelength- the structure and composition of Saturn’s atmosphere. dependent absorption and refraction of light in Saturn’s Each of Cassini-VIMS’ wavelength bands had its own atmosphere: the total absorption cross section σλ and 2 best-fit occultation light curve. For each wavelength the total refractivity νλ. Both parameters included con- band, we assumed the portion of Saturn’s atmosphere tributions from all atmospheric species. The other pa- sampled by the observation was ideal, isothermal, and in rameters in the model, Rp and n0, were not wavelength- hydrostatic equilibrium in order to acquire the familiar dependent so we adopted the following one-bar values 7 25 −3 number density profile for Saturn: Rp = 5.7×10 m and n0 = 5.5×10 m (Hubbard et al. 2009; Fouchet et al. 2009; West et al. −z/H 2009). This value of Rp accounted for Saturn’s oblate- n(z)= n0e (1) ness (∼0.0979) and the local Saturn-centric latitude of observation (∼49◦S), assuming Saturn to be an oblate where z is altitude, n(z) is particle number density as spheroid (e.g. Smith et al. 1983; Cox 2000). function of altitude, and n0 is the reference particle number density at the z = 0 m “surface” of Saturn (Rp), 2 The refractivity (ν) is related to the index of refraction (η) by which approximately corresponded to the one-bar pres- ν = η - 1. 4 Dalba et al.

Minimum Altitude [107 m] Minimum Altitude [107 m] In a vertically stratiﬁed atmosphere, refractivity ν(z) 5.73 5.75 5.77 5.79 5.81 5.83 5.73 5.75 5.77 5.79 5.81 5.83 can be deﬁned as 13.00 2 0 12.95 n(z) -2 ν(z)= fj (z)ν0,j (3) 12.90 L -4 0 j )

X 12.85 -6 ( where L0 is Loschmidt’s Number, fj(z) is the altitude- 10 [deg] 12.80 -8 α dependent mole fraction of the jth atmospheric species, log -10 and ν0,j is the refractivity of the jth species at stan- 12.75 dard temperature and pressure. Loschmidt’s Number is -12 12.70 merely a particle number density at some standard tem- -14 perature and pressure, both of which are functions of 12.65 2.7 2.8 2.9 3.0 3.1 3.2 3.3 2.7 2.8 2.9 3.0 3.1 3.2 3.3 altitude in Saturn’s atmosphere. We set L0 = n0, the D [1011 m] D [1011 m] reference particle number density, such that n(z)/L0 = −z/H e . We also assumed that Saturn’s atmosphere was Fig. 2.— Numerical relations between α, τλ, D, and minimum well-mixed such that f did not have a z-dependence. altitude at 1.25-µm in the occultation model. Each ray (black data j point) originated at the Sun with a D-value that corresponded to This allowed us to treat the summation term in Eq. 3 as a value of α and τλ, which governed brightness losses by refraction a parameter and rewrite the entire equation as and absorption, respectively. The minimum altitude in Saturn’s atmosphere (Rp + z) achieved by each ray is also shown. These −z/H smooth functions allowed for interpolation of any D-value. The ν(z)= νλe (4) red line in the left panel shows the relation tan α = D/(a + dSC) where D and (a+dSC) are the opposite and adjacent sides of a right where νλ is the wavelength-dependent total refractivity triangle, respectively, from Fig. 1. Rays that only traverse Saturn’s parameter described above. upper atmosphere (large α) lie along this red line because they do We note that νλ was evaluated at z = 0 m allowing for not experience high indices of refraction and therefore travel in ν(z) to be calculated elsewhere in the atmosphere with nearly straight lines. However, rays deviate from the red line at lower α as refractive bending becomes more significant. Eq. 4. For σλ, we assumed a well-mixed composition at the altitudes sampled by the observation at a given where s is the ray path length. The rays propagated wavelength so that σλ did not have a z-dependence. through Saturn’s atmosphere until one of two conditions was met: 1) z = ztop meaning that the ray reached the 3.2. Ray Tracing edge of the atmosphere on the Cassini-side, or 2) τλ = 50 in which case the ray’s energy had been fully attenuated. We traced rays between the Sun and Cassini accord- 3 ing to a ray tracing scheme developed by Kivalov (2007). 3.3. Generating a Transmission Model Each ray had finite energy and could be bent by refraction and attenuated by absorption. The density of rays We made the occultation model in a reference frame such that Saturn and Cassini were fixed relative to each in a given area and solid angle represented the specific 4 intensity from the Sun. other and the Sun appeared to move in a plane perpen- At the time of observation, we determined the orbital dicular to the Cassini-Saturn line (see the black arrow in distance of Saturn (a = 9.524 AU) and the distance be- Fig. 1). This plane will herein be referred to as the plane 8 of the Sun. Positions on this plane with respect to the tween Cassini and the center of Saturn (dSC = 2.59×10 m) using the JPL-HORIZONS solar system ephemeris Cassini-Saturn line were expressed with the coordinate computation service (Giorgini et al. 1996). We assumed D. Although D did not have a physical meaning, it al- these distances were constant during the 0.5-hour obser- lowed for direct comparison between the position of the vation period. Sun (from the data) and the rays’ points of origin (from We considered rays that reached the spacecraft at a the model). The D-values of the Sun were calculated by positive angle of α relative to the Cassini-Saturn line projecting r, the tangent radius, back to the plane of the Sun using the geometry of the system. that ranged from zero to arcsin[(Rp + ztop)/dSC] where z was the fiducial “top” of Saturn’s atmosphere equal Each ray considered by the model could be described top by three quantities: τ , the final optical depth the ray × 6 ∼ λ to 1.2 10 m or 20 scale heights (see Fig. 1). At achieved upon exiting Saturn’s atmosphere; α, the angle this altitude, the particle number density was reduced above the Cassini-Saturn line at which the ray reached by a factor of 2×10−9 from n and the atmosphere was 0 Cassini; and D, the height on the plane of the Sun above essentially transparent. the Cassini-Saturn line where the ray originated. Both The ray tracing scheme accounted for refraction by τ and α were important in determining the decrease modeling each step of a ray’s motion through Saturn’s λ in brightness during the occultation. Figure 2 shows atmosphere as a circle segment where the radius of cur- that these quantities had smooth, numerical relations vature was a function of the index of refraction (Kivalov amenable to interpolation. For any D-values occupied 2007; van der Werf 2008). At each step, we calculated by the Sun during the occultation, we could numerically the optical depth (τλ) experienced by the ray according determine the τ and α values of the Sun’s rays. We also to λ measured the minimum radial distance from the center dτ of Saturn achieved by each ray. This distance was im- λ = n(z)σ (5) portant in assessing the effects of refraction in the data ds λ 3 A concise summary of this ray tracing scheme was provided by 4 This choice increased the computational efficiency of the van der Werf (2008). model-fitting process. Saturn as a Transiting Exoplanet 5

a b Full Occultation Model 1.0 1.0 4 Refractive Losses Only

0.8 0.8 1.25 µm

3 0.6 0.6 1

0.4 2 0.4 on Detector [Solar Radii] [Solar Detector on

Relative Transmission Relative 0.2 1 2

3 Transmission Relative 4 0.2 0.0 1.38 µm 5.78 5.74 5.70 5.66 5.62 -2.0 1.0 0.0 1.0 2.0 Tangent Radius [107 m] Position on Detector [Solar Radii] 0.0 5.80 5.78 5.76 5.74 5.72 5.70 5.68 5.66 5.64 5.62 Fig. 3.— Two effects of atmospheric refraction captured by the Tangent Radius [107 m] occultation model. Panel a: Cassini-VIMS data (black data points) at 1.25 µm illustrating the decrease in transmission as the occultation progressed. Panel b: The apparent shape and posi- Fig. 4.— Cassini-VIMS data (black data points) and occultation of the Sun on the VIMS detector predicted by the occultation tion model fits at 1.25 µm — where CH4 was transparent — and model. The four numbered ellipses correspond to the four boxed 1.38 µm — where CH4 was opaque. Note that the tangent radius data points in Panel a. The dashed circle is the shape and position increases to the left. The dominant extinction process (refraction of the unocculted Sun for reference. Since Cassini always pointed or absorption) and the shape of the transmission curves in these towards the true position of the Sun, refraction caused the appar- two wavelength channels were different. At 1.25 µm, the flux loss ent position of the Sun to move against the gradient in ν (radially was almost entirely due to refraction, as shown by the blue curve away from the center of Saturn) as the occultation progressed. The which was found by ignoring absorption. At 1.38 µm, CH4 ab- refractive spreading of the Sun’s rays flattened the appearance of sorption attenuated the solar flux before refractive loses became the solar disk into an ellipse. Each of these effects was present in significant. the raw VIMS data cubes, although we did not use the image of shown in Fig. 4. the solar disk in the raw data cubes to estimate the parameters σλ and νλ. We display these phenomena simply to demonstrate 3.4. Bayesian Parameter Estimation that the occultation model correctly accounted for the effects of refraction. We fit the occultation model to the data and extracted (§3.5) and was physically more informative than D. the best-fit values of σλ, νλ, and H using emcee, an open source, pure-Python Markov Chain Monte Carlo ensem- The τλ-values allowed us to determine the energy attenuation due to absorption. The α-values allowed us ble sampler (Foreman-Mackey et al. 2013). In each of the 248 wavelength bands, we applied uniform priors to to determine flux losses due to refraction. Atmospheric −34 σλ and H that restricted the parameter space to 1×10 refraction caused an apparent shrinking of the solar disk 2 −29 2 4 4 in the vertical (or radial) direction (see Fig. 3). This m < σλ < 1×10 m and 2.0×10 m

bmin, at each wavelength. In regions of the spectrum with high methane opacity, bmin corresponded to rays with ﬁ- nal optical depths of ∼50. This meant that absorption limited the altitudes probed by the rays. Alternatively, in regions of the spectrum where methane was transparent, atmospheric refraction determined the value of bmin and the rays corresponding to bmin had optical depths 2.8 less than unity. The signiﬁcance of this result will be 2.6 discussed in §4.3. ]

4 2.4 − We calculated the wavelength-dependent eﬀective area 2.2 of Saturn’s disk (Aeﬀ,λ) using the expression [10 ν 2.0

1.8 Rp+ztop 2 Aeﬀ,λ = π (Rp + ztop) − 2 Tλ(b) b db , (6) 4.5 " bmin # 4.4 Z

m] 4.3 which neglects the effects of stellar limb darkening 4 4.2 (Bétrémieux & Kaltenegger 2014, 2015). The integral

H [10 H 4.1 term subtracts circular annuli of thickness db weighted

4.0 by their relative transmission Tλ(b) from the total com- 2 2.5 3.0 3.5 4.0 4.5 1.8 2.0 2.2 2.4 2.6 2.8 4.0 4.1 4.2 4.3 4.4 4.5 bined area of the atmosphere and planet π(Rp + ztop) . −32 2 −4 H [104 m] [10 m ] ν [10 ] We then determined the value of transit depth δλ triv- ially using Fig. 5.— Posterior probability distributions for the total absorption cross section (σλ), the total refractivity at one bar (νλ), Aeff,λ and the scale height (H) for a single wavelength band (1.25 µm). δλ = 2 (7) The one-dimensional histograms show the distributions for each pa- πR⊙ rameter marginalized over the others and the two-dimensional his- 8 tograms (with contours encompassing the 16th, 50th, and 85th per- where R⊙ is the solar radius (6.96×10 m). The resulting centiles) show the joint distributions for each parameter pair. The transmission spectrum of Saturn is displayed in Fig. 7. best-fit values and uncertainties found with these percentiles for σλ, +0.42 −32 2 +0.14 −4 We note that our method of removing the refrac- νλ, and H at 1.25 µm were 3.47−0.42×10 m , 2.38−0.18×10 , . tive flux losses intrinsic to occultation observations but +0 075 × 4 and 4.229−0.094 10 m, respectively. not transit observations differed from the methods of Robinson et al. (2014), who used Cassini observations The occultation model returned parameters σλ and νλ that described the opacity and refractivity of Saturn’s to measure the transit transmission spectrum of Titan. atmosphere between 1 and 5 µm. With these parame- Instead of modeling Titan’s atmosphere so that Tλ(b) ters, we shifted from an occultation geometry, where the could be calculated in the case of a Titan-Sun exoplane- observer (Cassini) was close to Saturn and relatively far tary system, Robinson et al. (2014) divided the Cassini from the Sun, to a transit geometry, where the observer data by the correction factor was located at an infinitely large distance away from a −1 Saturn-twin exoplanet orbiting a solar-twin star (see Fig. fref = (1+ dsc dθ/dzmin) , (8) 6). In the transit geometry, the observer only measured where θ is the bending angle swept out by a ray due to rays that left Saturn’s atmosphere parallel to the line- atmospheric refraction. This factor is simply the occulta- of-sight. These rays had a range of impact parameters tion light curve that would be produced for the case of a (b) relative to the center of the exoplanet. While in the completely transparent atmosphere such that brightness atmosphere, the rays still refracted according to Eq. 4 loss is only due to refraction. The expression for fref was and experienced attenuation according to Eq. 5, but the originally derived by Baum & Code (1953) under the as- refractive spreading of the rays did not cause the appar- sumption that θ was small or, equivalently, the index of ent shrinking of the stellar disk that was present in the the refraction was approximately unity (Baum & Code occultation observations. 1953; Wasserman & Veverka 1973). We considered the Saturn-Sun exoplanet system at the As a sanity check, we recalculated the transmission moment of mid-transit (Fig. 6). We traced rays with ∼4- spectrum of Saturn using the methods of Robinson et al. km vertical resolution in the upper 3% of Saturn’s atmo- 6 (2014). The resulting transmission spectrum closely sphere to determine relations between impact param- matched the one produced using the methods described eter, final optical depth (τλ), minimum altitude (zmin), in this work. and point of origin on the Sun. A ray was considered to be absorbed if it reached τλ ≥ 50. We calculated the 4. RESULTS −τλ relative transmission of each ray using Tλ = e . The 4.1. The Transmission Spectrum of Saturn 4-km vertical resolution yielded smooth numerical relations between each of the above parameters allowing us We generated the near-infrared transmission spectrum to determine the transmission as a function of impact of Saturn as if it were a transiting exoplanet (Fig. 7). parameter, Tλ(b), for Saturn at mid-transit. The spectrum displays several spikes in transit depth of We also calculated the minimum impact parameter, order 10 to 90 parts-per-million (ppm) corresponding to opacity from methane, ethane, acetylene, and possibly 6 Rays sampling lower altitudes were absorbed. carbon monoxide between 4.1 and 5.0 µm. The largest Saturn as a Transiting Exoplanet 7

Seidelmann et al. 2007) would be constrained, and the incident stellar flux around its G2V parent star at 9.524 AU (from JPL-HORIZONS, Giorgini et al. 1996) would be known. Furthermore, from the stellar age and planet mass the intrinsic flux from the planet’s interior could be assessed from evolution models (i.e. Fortney et al. 2007). With these parameters, the atmosphere code found a solution for the pressure-temperature-abundance profile Fig. 6.— Geometry of a Saturn-Sun exoplanet system at mid- that was in radiative-convective equilibrium given our transit (not to scale, rings of Saturn not pictured). The path of a maximally-deflected ray is shown in red. At mid-transit in regions knowledge of equilibrium chemistry and the wavelength- of the spectrum where methane was transparent, atmospheric re- dependent opacity of each molecule. The code ex- fraction determined the minimum altitude rays could probe (zmin). cluded photochemistry. Rather than solar abundances, Each zmin corresponded to a minimum impact parameter (bmin) we chose a metal-enhanced chemistry grid at 10× so- that set the continuum level of Saturn’s transmission spectrum. lar, as suggested from solar system and exoplanet trends feature, near 3.4 µm, is thought to be due to an asym- (Kreidberg et al. 2014b). The transmission spectrum of metric stretching mode of a C-H bond in an unknown the model was calculated using the one-dimensional code aliphatic hydrocarbon chain. Similar chains have been described in Fortney et al. (2010). identified in observations of Titan (Bellucci et al. 2009; The transmission spectrum of the self-consistent at- Robinson et al. 2014) and the diffuse interstellar medium mosphere model is shown in the bottom panel of Fig. 7. (Sandford et al. 1991). A recent analysis of Titan solar Since the self-consistent models did not not include the occultations by Maltagliati et al. (2015) suggested that limiting effects of refraction or the gray opacity source gaseous ethane may also contribute to the opacity be- near two bars, it was more appropriate to compare these tween 3.2 and 3.5 µm. Gaseous ethane is present in Sat- models to the version of Saturn’s transmission spec- urn’s atmosphere (Fouchet et al. 2009) and could there- trum that did not include refraction (see §4.3) than to fore be contributing to the absorption near 3.4 µm. the Saturn’s actual transmission spectrum (Fig. 7, top The uncertainties in Saturn’s transmission spectrum panel). To first-order, Saturn’s transmission spectrum are the standard deviations of 1,000 different transmis- and the spectrum from the self-consistent atmosphere sion spectra, each calculated using different values for model showed good agreement. Yet, at various loca- parameters σλ, νλ, and H. The 1,000 different parame- tions in the spectrum (i.e. 1.49 µm, 1.96 µm, and 2.93 ter sets formed a Gaussian distribution centered on the µm), the atmosphere model exhibited opacity where the best-fit parameters values and with standard deviations transmission spectrum of Saturn did not. These mis- equal to the uncertainties returned by emcee. The un- matches were due to the existence of gaseous ammonia certainty was higher in the 4- to 5-µm region where the at low pressures in the self-consistent model, which is not solar intensity was relatively weak. found in Saturn. The chemistry of the model naturally While most of the features in the transmission spec- allowed for NH3 condensation and depletion from the trum were due to absorption, the baseline was deter- gas phase when the temperature-pressure profile became mined by atmospheric refraction. This “critical transit sufficiently cold. However, if the temperature-pressure depth” corresponded to a critical minimum altitude in profile converged to warmer temperatures at low pres- Saturn’s atmosphere that rays could probe during mid- sures (a warm stratosphere), then the model included a transit. We found that the pressure level associated with reappearance of gaseous NH3 at low pressure. In real- the critical depth was 1.0 ± 0.5 bars. This value was con- ity, Saturn’s atmosphere acts like a cold trap, condens- sistent with a recent theoretical calculation of the critical ing most of the NH3 into a cloud layer near the two-bar pressure level for a Jupiter-sized planet with a 300-K at- pressure level. mosphere (Bétrémieux 2015). We note that we did not Therefore, we created a second model where all param- force this baseline; it is a simple geometric result of atmo- eters were kept the same but the gaseous NH3 abundance spheric refraction combined with the planet-star distance was forced to zero in the transmission spectrum calcula- and the stellar radius. The baseline of the spectrum was tion. This ammonia-free model yielded a substantially located above a gray opacity source near two bars, which better fit to Saturn’s transmission spectrum, although was presumably the top NH3 cloud deck. As a result, some inconsistencies remained: signatures of this feature were not detected in Saturn’s transmission spectrum. The value of the critical transit • At 1.27 µm, 1.58 µm, 2.08 µm, 2.96 µm, and be- depth varied slightly across the spectrum due to the un- yond 4.20 µm the ammonia-free model decreased certainty in νλ and the minor wavelength-dependence of to values of δλ below the critical depth set by re- refractivity in Saturn’s atmosphere. fraction (see §4.3) and even below the presumed location of the NH3 cloud deck. In reality, rays 4.2. Self-Consistent Atmosphere Models could not probe these depths during mid-transit. Having “reconstructed” the transmission spectrum of • The self-consistent models displayed continuum ab- Saturn using Cassini-VIMS, we next calculated the sorption due to scattering by aerosols and H2 at transmission spectrum of a self-consistent “off-the-shelf” wavelength shorter than 1.6 µm. Although haze is atmosphere model for Saturn, following Fortney et al. present in Saturn’s atmosphere, it was not detected (2005) and Fortney et al. (2010). The philosophy was in the transmission spectrum. not to search for a best fit, but rather to test how a model that was not tuned would fit the observations. As a tran- • The glaring disagreement near 3.4 µm resulted siting exoplanet, Saturn’s surface gravity (10.4 m s−2, from gaseous ethane and an asymmetric C-H 8 Dalba et al.

0.682 Transmission Spectrum Critical Depth CH4 0.680 2-bar Pressure Level

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%] 0.678 Self-Consistent Model, No NH3

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1 2 3 4 5 Wavelength [µm]

Fig. 7.— Top: The near-infrared, transmission spectrum of Saturn (black data points). The error bars are the 1σ uncertainties, which in some cases are smaller than the data point. The dashed green line and shaded green region correspond to the critical altitude and 1σ uncertainty range, below which rays cannot probe during mid-transit due to atmospheric refraction. The dashed black line corresponds to the two-bar pressure level in the models and presumably the top of Saturn’s global NH3 cloud layer. Bottom: Saturn’s transmission spectrum generated without the effects of refraction (see §4.3). In this scenario, the base of the spectrum is set by a gray opacity source near the two-bar level and not the critical depth. Two self-consistent atmosphere models (blue and red) are plotted with the transmission spectrum. The blue model allows for NH3 in gaseous form while the red model forces the gaseous NH3 content to zero. These models do not include the critical altitude set by refraction or the gray opacity source near two bars. stretching mode of an unknown aliphatic hydro- peratures, up to 1,500 K. We would not expect carbon chain (§4.1). these new line lists to be more appropriate for a model of Saturn’s atmosphere (at ∼140 K) than • Saturn’s transmission spectrum displayed opacity the Freedman et al. (2008) results, which specif- near 3.76 µm that the was not reproduced by the ically apply to cold atmospheres. Other expla- self-consistent atmosphere models. This feature nations for the discrepancies in the methane fea- may have been due to gaseous ethane (Sharpe et al. ture peaks include opacity from other unidentified 2004; Maltagliati et al. 2015). species or a disequilibrium process occurring in the region sampled by the observation. Photochemi- • The peaks of the methane features at 1.15, cal models and observations suggest that methane 1.38, and 2.30 µm were underestimated by the destruction occurs near the micro-bar level in Sat- self-consistent atmosphere models. This may urn’s atmosphere (Moses et al. 2005; Fouchet et al. have resulted from errors in either the methane 2009). Production of methane deeper in Saturn’s band Cassini data or the line-by-line opacities of atmosphere to replenish loss due to photolysis may methane used in the self-consistent atmosphere explain the observed excess. models (Freedman et al. 2008). Regarding the ExoMol latter, recent updates to the database 4.3. (Yurchenko & Tennyson 2014) could potentially Refraction and the Transmission Spectrum explain the observed discrepancies. However, Atmospheric refraction determined the minimum alti- the Yurchenko & Tennyson (2014) results primar- tude rays could probe during mid-transit and therefore ily explored the opacities of methane at high tem- the minimum value of transit depth in the transmission Saturn as a Transiting Exoplanet 9 spectrum. Consequently, the transmission spectrum did from methane-based photochemistry (Atreya & Wong not contain information about the structure or compo- 2005), was absent from the model. Having opacity be- sition of the atmosphere below the critical altitude. We tween 3.3 and 3.5 µm, this large feature is particularly recalculated the transmission spectrum forcing νλ = 0 alarming because it could influence the transit depth of at all wavelengths in order to determine what features, an exoplanet observed in Channel 1 of the Infrared Array if any, were blocked by refraction. In this scenario, rays Camera on the Spitzer Space Telescope, which is centered traveled in straight lines through Saturn’s atmosphere at 3.6 µm (Fazio et al. 2004). This suggests that exo- and the decrease in flux was entirely due to absorption planet atmospheres at all temperatures may harbor sur- (σλ). The resulting transmission spectrum is shown in prises that cannot be easily diagnosed with broad-band the bottom panel of Fig. 7. photometry. The methane features in this “νλ = 0” transmission Minor disagreements between the self-consistent mod- spectrum were nearly identical to those in the original els and the transmission spectrum such as the peak-to- transmission spectrum. This was not surprising since peak sizes of the methane features are also troubling. refraction effects were minimal in those portions of the These mismatches may be caused by local disequilibrium spectrum. Away from the methane features, however, processes (e.g. temperature variations, zonal winds) that rays probed deeper altitudes in Saturn’s atmosphere re- are difficult to predict and model. As observations of ex- vealing several features that were not present in the oplanet atmospheres progress to ever-greater precision, original spectrum. First, from comparison to the one- second-order effects such as these will become increas- dimensional atmosphere model we found empirical evi- ingly important. dence for not being able to probe deeper than approximately two bars, which appeared to be due to a gray 5.2. Clouds and Transit Transmission Spectra opacity source across all wavelengths, presumably the Although clouds are present in Saturn’s atmosphere at top of the NH3 cloud layer. Second, the minimum depth nearly every latitude (Baines et al. 2005), Saturn’s trans- near 2 µm did not appear to be set by the same fea- mission spectrum is not flat to 90 ppm. Furthermore, ture that limited the rest of the spectrum. Instead, the the lowest depth a ray can probe at mid-transit is deter- opacity at 2 µm was likely due to C2H2 absorption. mined by refraction and not clouds.7 Therefore, the role Since rays that experienced the greatest deflection in of clouds in the transmission spectra of cold, long-period Saturn’s atmosphere originated near the solar limb, the exoplanets may not be as restrictive as that of clouds in inclusion of limb-darkening could reduce the effects of warm Earth- and mini-Neptune-sized exoplanets. It is, of refraction on the transmission spectrum. As shown by course, possible that this solar occultation only probed Eq. 6, including limb-darkening would result in lower a relatively cloud-free portion of Saturn’s atmosphere. Tλ-values thereby increasing Aeff,λ. Consequently, the However, variability in Saturn’s cloud structure is ex- continuum level of the transmission spectrum may re- pected to develop gradually and over large ranges of lat- side slightly above the critical depth set purely by atmo- itude and longitude (Pérez-Hoyos et al. 2006), making it sphere refraction. This effect would be negligible for most unlikely that these observations were unique to a specific of Saturn’s near-infrared spectrum where the variation in time or location. intensity across the solar disk is minimal. However, limb- darkening could not be neglected at shorter wavelengths 5.3. Transmission Spectroscopy of Cold Gas Giants and could alter the optical transmission spectra of plan- Saturn’s transmission spectrum displays molecular ab- ets with highly refractive atmospheres. For composite sorption features on the order of 90 ppm, suggesting that transmission spectra that span multiple regimes of the transmission spectroscopy is a viable technique to study electromagnetic spectrum, special care must be taken to the atmospheres of cold giant exoplanets. Cold atmo- account for stellar limb-darkening. spheres can be hosted by planets with extremely long Calculating Saturn’s transmission spectrum with νλ = orbital periods (such as Saturn) or by those on shorter or- 0 revealed that refraction can suppress features in trans- bits around cooler stars. Despite their rarity, giant plan- mission spectra. This result has been discussed in several ets orbiting later-type stars represent an accessible start- previous studies involving refraction and transmission ing point for studies of cold giant-planet atmospheres spectroscopy (i.e. Sidis & Sari 2010; Garc´ıaMuñoz et al. outside of the solar system. Of all the known transiting 2012; Misra et al. 2014; Bétrémieux & Kaltenegger 2013, exoplanets, very few are expected to have cold atmo- 2014, 2015). Although the effects of refraction have been spheres with methane-dominated chemistry akin to Sat- largely unimportant in previous observations of hot gi- urn. The best candidate may be Kepler-421b, a Uranus- ant exoplanet atmospheres (e.g. Hubbard et al. 2001), sized exoplanet orbiting a G9 dwarf star with a period our results suggest that refraction may be critical to fu- of ∼704 days (Kipping et al. 2014). Assuming a Ura- ture investigations of giant, long-period exoplanet atmo- nian albedo, the equilibrium temperature of Kepler-421b spheres. would be ∼185 K. Although the mass of this planet is unknown, its supposed formation location within its 5. DISCUSSION protostellar disk suggests that it is likely to be an icy 5.1. Implications for Exoplanet Atmosphere Models gas giant versus a rocky planet with a gaseous envelope (Kipping et al. 2014). Typical models from the exoplanet atmosphere context A full investigation of the detectability of molecular reproduce most of the major features, due to methane ab- features in Kepler-421b’s atmosphere is beyond the scope sorption, across the entire wavelength range. However, the single largest absorption feature, likely due to gaseous 7 We note that the rays could likely reach the cloud deck at times ethane and an unknown aliphatic hydrocarbon derived before and after mid-transit (e.g. Misra et al. 2014). 10 Dalba et al. of this work. However, if the atmospheric chemistry of exoplanets assuming that the probability (dp) of a star Kepler-421b is similar to that of Jupiter or Saturn, we hosting a planet with mass spanning [M, M + dM] and might expect to see substantial methane features in the orbital period spanning [P , P + dp] is transmission spectrum. Such a detection would benefit − − theories of planet formation and migration and would M α P β dM dP dp = C (9) also be the first identification of an active methane cycle M P M P occurring in an exoplanet atmosphere. 0 0 Additional giant exoplanets with cold atmospheres where C, α, and β are constants and M0 and P0 are may be discovered in the near future. Based on ex- fiducial values chosen to be 1 MJ and 1 day, respectively pected yields from Sullivan et al. (2015), the Transiting (Tabachnik & Tremaine 2002). We adopt the values C = Exoplanet Survey Satellite (TESS, Ricker et al. 2015) is 1.04 × 10−3, α = 0.31 ± 0.2, and β = −0.26 ± 0.1 for expected to find around a half-dozen giant planets with FGK dwarf stars from Cumming et al. (2008), a radial- radii of 6 to 22 R⊕ and periods of several hundred days. velocity survey complete in the ranges 2 days

105) number of stars to make a detection on a exoplanets in the near future. practical time scale.

5.4.1. Detectability and Occurrence 5.4.2. Survey for Long-Period, Giant Exoplanets The a priori probability of observing a cold, long pe- We estimate the number of long-period (4.33×103 days 4 riod exoplanet in transit can be estimated by multiply-

101 ingress, the survey telescopes would issue an alert call- Jupiter analogs Saturn analogs ing for the activation of the target-of-opportunity pro- grams. Under ideal conditions, HST could begin observ- 0 10 ing the transit 24 hours after activation,11 capturing the

year) final ∼29 hours of the transit. Spitzer normally requires 2 12 10-1 48 hours to initiate a target-of-opportunity program, which leaves an insufficient amount of time to characterize the planet’s atmosphere. For the purposes of this 10-2 thought-experiment, however, we will consider Spitzer’s ability to characterize a Saturn-twin exoplanet atmosphere regardless of the 48-hour turnaround time. 10-3 HST Detections / (100 deg (100 / Detections In Fig. 9, we show the expected -WFC3 and Spitzer-IRAC transmission spectrum of a Saturn-analog exoplanet derived from the Cassini-VIMS transmission 10-4 spectrum. The spectral resolution of HST -WFC3 nearly 6 - 8 8 - 10 10 - 12 12 - 14 14 - 16 16 - 18 18 - 20 20 - 22 22 - 24 24 - 26 26 - 28 matches that of Cassini-VIMS in this wavelength range, H-band magnitude so a convolution to match resolution is unnecessary. We bin the high signal-to-noise ratio portion of the HST - Fig. 8.— Expected number of long-period, giant planet transit detections per 100 deg2 after a single year of observation. WFC3 spectrum by 2 resolution elements yielding 30 We estimate these detection rates using a synthetic catalog of data points between 1.13 to 1.65 µm. Then, by repeat- FGK dwarf stars generated with the TRILEGAL simulation code edly scattering the data points with random Gaussian (Girardi et al. 2005) and the a priori transit probabilities from noise of 1 to 50 ppm, we determine that a minimum preci- §5.4.1. A horizontal line is drawn at unity for reference. For a Saturn-analog exoplanet, the Wide Field Camera 3 (WFC3) sion of 12.8 ppm is required to distinguish features in the aboard the Hubble Space Telescope (HST) could make a 5σ de- HST -WFC3 spectrum from a flat line to 5σ confidence. tection of transmission spectrum features for stars with H-band For the simulated Spitzer observations, we integrate the magnitudes mH . 9.2. If the rates in this figure are extrapolated IRAC bandpasses over the transmission spectrum of Sat- to cover the whole sky, we would expect one detection per year suitable for characterization with HST -WFC3. urn to determine the 3.6 and 4.5 µm data points. We estimate that an uncertainty of 9.4 ppm is required in space-based telescope. Instead, arrays of ground-based, each Spitzer data point to rule out a flat spectrum to 5σ. robotic telescopes, akin to MEarth (Irwin et al. 2009) or It is critical to note that Spitzer cannot observe both MINERVA (Swift et al. 2015) could be used to detect IRAC channels simultaneously. Each data point must transit events. An all-sky, array telescope such as the be obtained individually. “Evryscope” would also be a highly appropriate instru- These uncertainties set upper limits on the magnitudes ment for this type of survey (Law et al. 2015); the con- of Saturn-hosting stars that are amenable to character- struction of the Evryscope is in part motivated by the ization with HST -WFC3 and Spitzer-IRAC. To deter- ability to observe giant planets transiting nearby bright mine this limit for HST , we consider a large variety of ob- stars. Further characterization of these cold giant exo- serving strategies (e.g. staring versus spacial scan modes, planets would, however, require more powerful observing various slew rates (McCullough & MacKenty 2012), sub- facilities. array sizes, and readout configurations) over a range of H-band magnitudes matching the output from the TRI- 5.4.3. Target-of-Opportunity Follow-Up Observations LEGAL simulation (6 < mH < 28). In each case, we The long periods of these exoplanets necessitate imme- assume HST observes the transiting system for 36 con- diate follow-up characterization. Fortunately, long peri- secutive orbits: 18 during the final half of transit, and ods also result in long transit durations. From the point- 18 out-of-transit orbits to establish a precise baseline for of-view of a distant observer, the transits of Jupiter and the stellar flux. We assume the host star is visible for 56 Saturn across the solar disk would last ∼23 and ∼57 minutes of the 96-minute orbit before Earth occultation, hours, respectively.10 These transit durations are long similar to the stars in the Kepler field. The nominal ex- enough such that target-of-opportunity campaigns with posure time is set by the chosen readout configuration, facilities such as HST or Spitzer Space Telescope could and the corresponding signal-to-noise ratio per resolution be activated in time to characterize the exoplanet’s at- element per exposure is estimated using the HST -WFC3 mosphere. The infrastructure for this type of observing exposure time calculator.13 For each observing configu- program is already in place in the field of gamma ray ration, we use the Phase II Astronomer’s Proposal Tool14 bursts (GRBs). Since 2004, the Swift Mission has been Orbit Planner to determine the number of exposures we observing GRBs and relaying the coordinates and data can obtain per HST orbit and make a final estimate of to the GRB community worldwide in just a matter of the precision of the transmission spectrum. seconds (Gehrels et al. 2004). The result of this calculation is that with only a half To demonstrate the ability of current facilities to char- transit, HST -WFC3 can make a 5σ detection of at- acterize the atmospheres of cold giant exoplanets, we mospheric features in the transmission spectrum of a specifically consider the case of a Saturn-Sun analog ob- Saturn-analog if the host star has mH . 9.2. As dis- served with the Wide Field Camera 3 (WFC3) aboard 11 http://www.stsci.edu/hst/HST_overview/documents/uir/ToO-UIR.pdf HST and the Infrared Array Camera (IRAC) aboard 12 Spitzer ∼ http://ssc.spitzer.caltech.edu/warmmission/ddttoo/whattoo/ . Upon observing the slow ( 3.3-hour), deep 13 http://etc.stsci.edu/etc/input/wfc3ir/spectroscopic/ 14 http://www.stsci.edu/hst/proposing/apt/ 10 Assuming circular orbits with inclinations of 90◦. 12 Dalba et al.

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Transit Depth [ Depth Transit Saturn Transmission Spectrum Flat Line 0.670 Spitzer-IRAC Channels HST-WFC3, binned 0.668 1.0 2.0 3.0 4.0 5.0 Wavelength [µm]

Fig. 9.— Simulated HST -WFC3 and Spitzer-IRAC observations of Saturn’s transmission spectrum. The spectrum is solid in regions sampled by either HST or Spitzer and dotted elsewhere. The red circles are the expected HST -WFC3 data points, binned by 2 resolution elements and scattered with random Gaussian noise of ∼13 ppm. The red diamonds are the expected Spitzer-IRAC data points with an uncertainty of ∼9 ppm. Note that Spitzer cannot observe both channels simultaneously. In each case, the quoted precision is the requirement to distinguish the features in the transmission spectrum from a flat line (black, dashed line) to 5σ confidence. To achieve this precision in 0.5 transits of a Saturn-twin exoplanet across a solar-type star (see text), HST would be limited to stars with H-band magnitudes mH . 9.2. Spitzer could achieve the displayed precision in a single channel (either 3.6 or 4.5 µm) for stars with mH . 3.4 but would require additional observing time to observe the the other channel. Therefore, Spitzer observations of this type of target are infeasible. played in Fig. 8, fewer than one Saturn-analog detection wide-field transit survey, false alarms may result from is expected per 100 deg2 per year. However, if we ex- variations in instrument sensitivity, weather, or other trapolate these detection rates to cover the entire sky, astrophysical sources such as stellar variability or un- we would expect approximately one detection per year known stellar companions. To the extent that it is pos- amenable to characterization with HST -WFC3. sible, explicit target selection and advance “snapshot” Considering that Spitzer cannot respond quickly observations could limit astrophysical false positives. To enough to characterize a transiting Saturn-analog exo- reduce the false positives due to eclipsing binary stars, planet and each channel must be observed individually, the target-of-opportunity program could also involve ob- we estimate its limiting host-star magnitude in less detail taining a spectrum of the target in search of two sets of than for HST. If we assume photon-limited observations, spectral lines. we can loosely estimate uncertainties by scaling those 6. obtained for a previous Spitzer-IRAC observation of a CONCLUDING REMARKS solar-type star. For 55 Cancri (mH =4.14), Demory et al. Studies of solar system analogs provide a useful method (2011) achieved 63-ppm-precision in IRAC’s 4.5 µm band of “ground-truthing” the techniques and models fre- over 4.97 hours of observation. If Spitzer-IRAC could quently applied to exoplanets. Exhausting the resources only observe 0.5 transits (29 hours in transit + 29 hours provided by decades of work in the planetary sciences out of transit) of a Saturn-twin exoplanet orbiting a will greatly aid the burgeoning field of exoplanetary sci- solar-type star in a single channel, then the 9.4 ppm ence. The usefulness of missions such as Cassini and precision requirement would limit the host star H-band Juno, which is currently en route to Jupiter, extends be- magnitudes to mH . 3.4. Spitzer would then have to yond the solar system to the cold, long-period regime of wait until the following transit event to obtain observa- exoplanets. tions in the other channel. This first-order approxima- The Kepler Mission has discovered a great variety of tion demonstrates that HST is by far the most appropri- Earth-sized exoplanets. Future efforts to discover and ate currently-operational instrument for characterizing characterize cold Jupiters and Saturns may find that the atmospheres of cold, long-period exoplanets. a similar diversity exists among giant gaseous planets. The success of this hypothetical survey is contingent These efforts will put the giant members of our solar upon the ability of the survey telescopes to quickly and system in a greater context, thereby allowing for a bet- accurately identify long-period, giant exoplanet transits. ter understanding of the formation and evolution of our This would require immediate, automatic data reduction entire solar system. and analysis. For stars brighter than mH ≈ 9.2 and transit durations longer than ∼57 hours, there is some flex- We wish to thank the anonymous referee for thought- ibility that would allow for human intervention. Still, ful suggestions that improved this work. We also wish distinguishing false-positives from actual events would to thank Brandon Harrison, Andrew Vanderburg, Bryce be a major challenge to this approach. As in any other Croll, and the Cassini-VIMS team. P.A.D. gratefully ac- Saturn as a Transiting Exoplanet 13 knowledges support from the Massachusetts Space Grant nology, under contract with NASA under the Exoplanet Consortium. This paper includes data collected by the Exploration Program. This research made use of the Cassini Mission, funding for which was provided by USGS Integrated Software for Imagers and Spectrome- NASA and ESA. This research made use of the Exo- ters. This research also made use of the Massachusetts planet Orbit Database and the Exoplanet Data Explorer Green High Performance Computing Center. at exoplanets.org and the NASA Exoplanet Archive, Facilities: Cassini (VIMS) which is operated by the California Institute of Tech-

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