SPECIAL FEATURE: PERSPECTIVE PERSPECTIVE SPECIAL FEATURE: Spectra as windows into exoplanet atmospheres Adam S. Burrows1 Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544 Edited by Neta A. Bahcall, Princeton University, Princeton, NJ, and approved December 2, 2013 (received for review April 11, 2013) Understanding a planet’s atmosphere is a necessary condition for understanding not only the planet itself, but also its formation, structure, evolution, and habitability. This requirement puts a premium on obtaining spectra and developing credible interpretative tools with which to retrieve vital planetary information. However, for exoplanets, these twin goals are far from being realized. In this paper, I provide a personal perspective on exoplanet theory and remote sensing via photometry and low-resolution spectroscopy. Although not a review in any sense, this paper highlights the limitations in our knowledge of compositions, thermal profiles, and the effects of stellar irradiation, focusing on, but not restricted to, transiting giant planets. I suggest that the true function of the recent past of exoplanet atmospheric research has been not to constrain planet properties for all time, but to train a new generation of scientists who, by rapid trial and error, are fast establishing a solid future foundation for a robust science of exoplanets. planetary science | characterization The study of exoplanets has increased expo- by no means commensurate with the effort exoplanetology, and this expectation is in part nentially since 1995, a trend that in the short expended. true. The solar system has been a great, per- term shows no signs of abating. Astronomers An important aspect of exoplanets that haps necessary, teacher. However, most solar- have discovered and provisionally studied makes their characterization an extraordinary system spectra are angularly resolved with a more than a hundred times more planets challenge is that planets are not stars. They long time baseline and high signal-to-noise. outside the solar system than in it. Statistical have character and greater complexity. A Exoplanets will be point sources for the and orbital distributions of planets across star’s major properties are determined once foreseeable future, and signal-to-noise will their broad mass and radius continuum, in- its mass and metallicity are known. Most remain an issue. Perhaps more importantly, cluding terrestrial planets/Earths, “super- stars have atmospheres of atoms and their much solar-system research is conducted by Earths,”“Neptunes,” and giants, are emerging ions. However, planets have molecular atmos- probes in situ or in close orbit, with an array of instruments for direct determination of, at a rapid pace. pheres with elemental compositions that be- for example, composition, surface morphol- However, understanding its atmosphere is speak their formation, accretional, and (where ogies, B-fields, charged-particle environments, a necessary condition for understanding not apt) geophysical histories. Anisotropic stellar and gravitational moments. Masses and radii only the planet itself, but also its formation, irradiation, clouds, and rotation can break planetary symmetry severely, with the clouds can be exquisitely measured. Orbits are evolution, and (where relevant) habitability, known to standard-setting precision. More- and this goal is far from being realized. themselves introducing multiple degrees of complexity, still unresolved even for our over, when comparing measured with theo- Despite multiple ground- and space-based retical spectra, the latter are often informed campaigns to characterize their thermal, com- Earth. Molecules have much more com- plicated spectra than atoms, with a hun- by direct compositional knowledge. positional, and circulation patterns (mostly dred to a thousand times more lines, and The exoplanet scientific landscape will be for transiting giant planets), the data gleaned irradiated objects experience complicated more challenging. Exoplanet science is an to date have (with very few exceptions) been photochemistry in their upper reaches. It observational science that must rely on the of marginal utility. The reason is that most astronomical tools of remote spectroscopic took stellar atmospheres ∼100 y to evolve of the data are low-resolution photometry at sensing to infer the physical properties of as a discipline, and it still is challenged by individual planets. Therefore, there is a pre- a few broad bands that retain major system- uncertainties in oscillator strengths and issues mium on obtaining spectra and developing atic uncertainties and large error bars. More- with Boltzmann and thermal equilibrium. interpretative toolkits in the tradition of clas- over, the theory of their atmospheres has yet Furthermore, the spectroscopic databases sical astronomy, without the luxury of direct, to converge to a robust and credible inter- for molecules (1), particularly at the high in situ probes. Therefore, although solar- pretive tool. The upshot of imperfect theory temperatures (500–3,500 K) experienced by in support of imprecise data has been system variety will continue to inform exopla- close-in transiting planets, are much more net thinking and motivate many calculations, ambiguity and, at times, dubious retrievals. incomplete than those for atoms, and the To be fair, (i) telescope assets are being used relevant collisional excitation rates are all withgreateffortat(and,sometimes,beyond) but nonexistent. Therefore, it can reasonably Author contributions: A.S.B. wrote the paper. the limits of their designs; and (ii)most be suggested that the necessary theory for The author declares no conflict of interest. planet/star contrast ratios are dauntingly detailed studies of exoplanets is in its This article is a PNAS Direct Submission. small. As a consequence, the number of early infancy. 1E-mail: [email protected]. hard facts obtained over the last 10 y con- One might have thought that the study This article contains supporting information online at www.pnas.org/ cerning exoplanet atmospheres is small and of our solar system had prepared us for lookup/suppl/doi:10.1073/pnas.1304208111/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1304208111 PNAS | September 2, 2014 | vol. 111 | no. 35 | 12601–12609 Downloaded by guest on September 27, 2021 the methodology of solar-system research is As direct imaging techniques mature, more no doubt involved in their formation, their not the best model for conducting exoplanet and smaller directly imaged planets will be elemental abundances should reflect the research. Rather, we must determine the most discovered. However, as articulated earlier, it most abundant elements in the Universe. ‡ robust and informative methods with which is only with well-calibrated spectral meas- For giant exoplanets (like brown dwarfs ), to interpret remote spectra and perform urements at useful resolutions that we can this fact means H2,He,H2O, CO, CH4,NH3, credible spectral retrievals of physical prop- hope to characterize wide-separation exopla- PH3,H2S, Na, and K predominate, with most erties. Therefore, the science of exoplanet net atmospheres robustly. Polarization mea- of the metals sequestered in refractories at characterization is better viewed as a science surements will also have an important diagnos- depths not easily penetrated spectroscopi- of spectral diagnostics, and developing this tic role, particularly for cloudy atmospheres, cally. However, titanium and vanadium art should be our future focus. which at quadrature should be polarized in oxides (TiO and VO), identified in cool-star To date, planets that transit their stars due the optical to tens of percent. and hot-brown-dwarf atmospheres, have to the chance orientation of their orbit planes Currently, due to their larger size, the been suggested to reside in quantity in the have provided some of the best constraints photometric and spectroscopic techniques upper atmospheres of some hot Jupiters to on hot exoplanet atmospheres. The variation mentioned above have been applied mostly heat them by absorption in the optical and † of transit depth and, thus, apparent planet to giant exoplanets. Earths are ten times create inversions (37). However, TiO and λ radius (Rp), with wavelength ( )isanersatz smaller in radius and one hundred times VO too are likely condensed out (38). Be- spectrum and can be used to infer the smaller in mass. Therefore, while astrono- cause such inversions require an optical ab- presence of chemical species with the cor- mers and theorists hone their skills on the sorber at altitude, what this absorber is, responding cross-sections.Water,sodium, giant exoplanets, fascinating in their own molecule or absorbing haze/cloud, remains and potassium have been unambiguously right, these giants are also serving as stepping a major mystery (39). stones to the smaller planets, in anticipation detected by this means. Approximately 180° For terrestrial planets, the molecules N2, of future routine campaigns to characterize out of phase with the primary transit, when CO2,O2,O3,N2O, and HNO3 must be added them as well. Therefore, I concentrate in this the same planet is eclipsed by its star, the to the list above, with O2,O3 (ozone), and article on the giant population, but all of the difference between the summed light of N2O considered biosignatures, along with the planet and star and that of the star alone basic methodologies used in their study can “chlorophyll red edge” (or