Numerous studies have been published on the use of wavelength-dependent primary transits and secondary eclipses to characterise the atmospheres of exoplanets including GJ 1214b (e.g. Charbonneau et al. 2009; Bean et al. 2010; Berta et al. 2012; de Mooij et al. 2012) and 55 Cancri e (e.g. Crossfield 2012; Demory et al. 2012; Ehrenreich et al. 2012). Highlights of these studies include claimed detections of molecular features in the infrared (e.g. Knutson et al. 2011; Sing et al. 2016), the inferred presence of clouds/hazes in the visible in the atmospheres of hot Jupiters (e.g. Pont et al. 2013), and the detection of exoplanet’s exospheres that contain atoms of hydrogen, carbon, and oxygen (Vidal-Madjar et al. 2003; Lecavelier des Etangs et al. 2012; Ben-Jaffel & Ballester 2013). More recent discoveries of transiting exoplanets in suitable orbital and nearby configurations for atmospheric characterisation such as GJ 1132 b (Berta-Thompson et al. 2015) and the TRAPPIST-1 b, c, d planets (Gillon et al. 2016; de Wit et al. 2016) bear the promise of new transmission studies for these Earth-sized planets.
Visible data are particularly useful for determining the planet’s albedo (e.g. Evans et al. 2013), the identity of the major spectroscopically inert molecules, and the relative abundance of clouds/hazes of the atmosphere.
Clouds have long been an obstacle to our understanding of the atmospheres of Earth, other solar system planets, and brown dwarfs. Now they are rapidly emerging as a major theme in the study of hot Jupiters, super-Earths, and directly imaged exoplanets (e.g. Gibson et al. 2013; Barstow et al. 2014; Nikolov et al. 2015; Sing et al. 2015; Parmentier et al. 2016; Sing et al. 2016; Skemer et al. 2016; Apai et al. 2016). For small exoplanets, visible data help to determine if a thick, gaseous atmosphere is present, and thus can identify the exoplanet as a prime candidate for follow-up, atmospheric spectroscopy with JWST, E-ELT, and future L-class missions. Potentially, the pre- and post-transit halo caused by visible-wavelength starlight refracted in the planet atmosphere may reveal whether the atmosphere is cloudy/hazy or not (García Muñoz et al. 2012).
The albedo measures the fraction of starlight reflected by an atmosphere, and therefore participates in its energy budget. The geometric albedo is a measure of the reflectivity of the planet when viewed fully illuminated, whereas the spherical albedo is phase-averaged measure of the reflectivity and therefore more relevant for energy balance considerations and for the determination of the atmospheric thermal structure. Measuring the secondary eclipse (occultation depth) in the visible directly yields the geometric albedo (e.g. Demory et al. 2011b, 2014; Esteves et al. 2013, 2015; Angerhausen et al. 2015). By detecting reflected light over the full planet orbit, the spherical albedo can also be derived (García Muñoz & Isaak 2015). For the hottest objects (~2000 to 3000 K), thermal emission from the exoplanet may contaminate the broadband visible data, thus confusing the measurement of reflected light versus thermal emission. In these situations, the two broad-bands of the fast cameras of PLATO will be useful in decontaminating the occultation depth measurements for the brightest stars. Indeed, simultaneous measurements with the two filters of the fast cameras will provide insight into the reflected starlight-vs.-thermal emission contributions from the exoplanet atmosphere, and into the cloud properties of the object. In reflected starlight for planets around the brightest targets, the shorter-wavelength photons are expected to penetrate less than the longer-wavelength ones, which entails that different atmospheric depths are being probed.
The spectroscopically active molecules of an atmosphere typically contribute spectral features in the infrared, but these molecules are often minor constituents (by mass) of an atmosphere. Of central importance in interpreting an exoplanetary atmosphere is the knowledge of the pressure scale height, which is set by the mean molecular weight. This is determined by the dominant (by mass) inert molecule, and the gravity and temperature of the planet. On Earth, the dominant inert molecule is nitrogen; in gas giants like Jupiter, it is believed to be molecular hydrogen. Analyses of the spectra of hot Jupiters often assume the atmosphere to be hydrogen dominated (Madhusudhan & Seager 2009), but for rocky or terrestrial exoplanets with secondary atmospheres, the mean molecular weight cannot be assumed. First indications of the mean molecular weight can be obtained by measuring the primary transits at two visible wavelengths (Benneke & Seager 2012), which can be accomplished using the two broad-bands of the fast cameras of PLATO. The method is complicated by the presence of clouds, but still provides first hints (strong/weak Rayleigh slope) as to the nature of the atmosphere, which can be followed up with spectroscopic observations. If only one broad-band measurement is made, then one may be able to distinguish between hypothesized atmospheres (e.g. hydrogen-dominated versus water-dominated models; de Mooij et al. 2012). Alternative methods include the detailed analysis of the line shape of a certain molecular species, or the relative strength of its features at different wavelengths (Benneke & Seager 2012), but such an approach requires a robust line opacity list, which is not always the case. Visible data thus provide an important check on the analysis of infrared data of exoplanetary atmospheres. Identifying the dominant, inert molecule in an atmosphere has significant implications for inferring its thermal structure and spectrum, as the inert component often exerts an indirect influence on the spectroscopically active molecules via processes such as pressure broadening and collision-induced absorption.
Phase curves show the exoplanet’s flux as a function of orbital phase, which may be deconvolved to obtain the flux versus longitude, known as a “brightness map” (Cowan & Agol 2008; Demory et al. 2016). Infrared phase curves contain information about the efficiency of heat redistribution from the dayside to the night side of an exoplanet (Showman & Guillot 2002; Cooper & Showman 2005; Showman et al. 2009; Cowan & Agol 2011; Heng et al. 2011), as previously demonstrated for hot Jupiters (e.g. Knutson et al. 2007, 2009). To a lesser extent, infrared phase curves also constrain the atmospheric albedo and drag mechanisms (shocks, magnetic drag). In contrast, visible phase curves encode the reflectivity of the atmosphere versus longitude (e.g. García Muñoz & Isaak 2015; Shporer & Hu 2015), which in turn constrains the relative abundance of clouds or hazes if they are present (e.g. Webber et al. 2015). The cloud/haze abundance depends on the size and mass density of the particles, as well as on the local velocity, density, pressure and temperature of atmospheric flow, implying that a robust prediction of cloud properties requires one to understand atmospheric chemistry and dynamics in tandem. The physics relevant to the phenomena of reflected starlight and planet thermal emission, which dominate the phase curve measurements at visible and infrared wavelengths respectively, is complementary. In the optical, the scattering properties of condensates depend more strongly on the particles’ refractive index (and therefore composition) and sizes. This consideration is useful in the inverse step of trying to pin down potential condensates in exoplanet clouds.
Examples of exoplanets where clouds are likely present include Kepler-7b, which has a high albedo (~0.3, comparable to the solar system giant planets) and a phase curve containing a surprising amount of structure (Demory et al. 2011a; García Muñoz & Isaak 2015), as well as numerous hot Jupiters detected from the ground. The feasibility of obtaining visible phase curves has already been demonstrated for the CoRoT (Alonso et al. 2009a,b; Snellen et al. 2009, 2010) and Kepler(Borucki et al. 2009; Batalha et al. 2011) missions, and it has been demonstrated they that can be used for planet confirmation (Quintana et al. 2013) and bulk parameter determination (Barclay et al. 2012). A few comprehensive studies of phase curves and secondary eclipses with Keplerdata have recently been published. They investigated both giant planets (Angerhausen et al. 2015; Esteves et al. 2015) as well as super-Earths (Demory 2014) and suggest that, on a statistical basis, super-Earths are comparatively more reflective than hot Jupiters. This information is key towards validating theoretical models of exoplanet clouds and towards the preparation of both transmission and phase curve observations at infrared wavelengths.
Information from the phase curve of the exoplanet can also be used to constrain the temporal evolution of the temperature distribution of its upper atmosphere (e.g. Stevenson et al. 2014), and to set constraints on the dynamics and clouds coverage (see Parmentier et al. 2016) of its atmosphere (e.g. Knutson et al. 2009). An interesting goal would be to establish the frequency of planets showing super-rotation on their atmospheres, a phenomenon which involves displacement of the hottest atmospheric spot of a tidally locked planet by an equatorial super-rotating jet stream (see Faigler et al. 2013, and references therein). Optical data from Kepler have indeed enabled a first investigation of atmospheric super-rotation. It is now known that the hottest giant planets tend to exhibit their brightness peak east of the sub-stellar point, possibly as a result of a thermal hot spot shifted by atmospheric dynamics (Esteves et al. 2015; Hu et al. 2015; Shporer & Hu 2015). In contrast, the cooler objects tend to exhibit their brightness peak westwards of the sub-stellar point, a configuration that is likely explained by clouds formed on the planet night side and reaching onto the planet’s western dayside hemisphere. The optical phase curves of some of these exoplanets have also revealed higher-order harmonics whose physical origin remains uncertain (e.g. Esteves et al. 2015). PLATO’s long temporal coverage and its high photometric precision are the best mean to detect and accurately model phase curves for a large number of targets. Neither CHEOPS nor TESS will match these capabilities.
PLATO will provide targets to carry out the investigations described above. Furthermore, among PLATO’s detections will be nearby giant planets on wide orbits, for which both transit spectroscopy and direct imaging spectroscopy will be possible. The comparison of these two approaches will then allow us to study the vertical structure of exoplanet atmospheres. PLATO’s fast cameras, equipped with two colour filters, will also provide unique insight into the dusty envelope of disintegrating planets (e.g. KIC 12557548b, KIC 8639908, K2-22b; DeVore et al. 2016) and planetesimals orbiting white dwarfs (e.g. WD 1145+017; Vanderburg et al. 2015). Simultaneous measurements at two colours will impose key constraints on the optical properties and particles sizes of the dusty envelopes for these systems and for new similar systems to be discovered.
As the scientific community prepares for the launch of the JWST (which will be in its extended operations phase when PLATO is launched), as well as ground-based telescopes such as E-ELT (first light planned for 2024), it is vital that we correctly identify the best targets for follow-up, atmospheric spectroscopy of small exoplanets. Earth-like exoplanets with sizes ~2 REare believed to be either composed predominantly of rock, or to be scaled-down versions of Neptune with thick gaseous envelopes. If the bulk composition of an exoplanet cannot be made from a material lighter than water, then one can calculate the thickness of the atmosphere, relative to the measured radius, by utilising the mass-radius relation of pure water (Kipping et al. 2013). It was shown that such a simple approach can be used to imply a mostly rocky composition (e.g. Earth, Kepler-36b; Kipping et al. 2013). By quantifying this metric for the entire PLATO catalogue of small exoplanets, one could construct a valuable database of optimal follow-up targets. Knowledge of the fraction of small exoplanets with and without thick atmospheres, as a function of their other properties, will provides new constraints on planet formation theories.
In summary, the key science questions that PLATO can answer about the atmospheres of exoplanets are:
- What is the diversity of albedos present in exoplanetary atmospheres? How does the albedo correlate with other properties of the exoplanet (incident flux, metallicity, etc.)? Are these albedos associated with the presence of clouds or hazes?
- What are the dominant, inert molecules present in exoplanetary atmospheres? What are the mean molecular weights?
- When are clouds present in exoplanetary atmospheres? What is the diversity of the cloud properties (particle size, reflectivity, etc.)? This information will be key for the validation of theoretical models and in the interpretation of infrared observations.
For small exoplanets (~2REin size), what are the best targets for follow-up, atmospheric spectroscopy? Here, small planets at intermediate orbital separations are of particular interest.
PLATO – Revealing habitable worlds around solar-like stars
Definition Study Report, ESA-SCI(2017)1, April 2017