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Exoplanetary Atmospheres: Key Insights, Challenges, and Prospects AA57CH15_Madhusudhan ARjats.cls August 7, 2019 14:11 Annual Review of Astronomy and Astrophysics Exoplanetary Atmospheres: Key Insights, Challenges, and Prospects Nikku Madhusudhan Institute of Astronomy, University of Cambridge, Cambridge CB3 0HA, United Kingdom; email: [email protected] Annu. Rev. Astron. Astrophys. 2019. 57:617–63 Keywords The Annual Review of Astronomy and Astrophysics is extrasolar planets, spectroscopy, planet formation, habitability, atmospheric online at astro.annualreviews.org composition https://doi.org/10.1146/annurev-astro-081817- 051846 Abstract Copyright © 2019 by Annual Reviews. Exoplanetary science is on the verge of an unprecedented revolution. The All rights reserved thousands of exoplanets discovered over the past decade have most recently been supplemented by discoveries of potentially habitable planets around nearby low-mass stars. Currently, the field is rapidly progressing toward de- tailed spectroscopic observations to characterize the atmospheres of these planets. Various surveys from space and the ground are expected to detect numerous more exoplanets orbiting nearby stars that make the planets con- ducive for atmospheric characterization. The current state of this frontier of exoplanetary atmospheres may be summarized as follows. We have entered the era of comparative exoplanetology thanks to high-fidelity atmospheric observations now available for tens of exoplanets. Access provided by Florida International University on 01/17/21. For personal use only. Annu. Rev. Astron. Astrophys. 2019.57:617-663. Downloaded from www.annualreviews.org Recent studies reveal a rich diversity of chemical compositions and atmospheric processes hitherto unseen in the Solar System. Elemental abundances of exoplanetary atmospheres place impor- tant constraints on exoplanetary formation and migration histories. Upcoming observational facilities promise to revolutionize exo- planetary spectroscopy down to rocky exoplanets. The detection of a biosignature in an exoplanetary atmosphere is conceivable over the next decade. In the present review, we discuss the modern and future landscape of this frontier area of exoplanetary atmospheres. We start with a brief review of the area, emphasising the key insights gained from different observational 617 AA57CH15_Madhusudhan ARjats.cls August 7, 2019 14:11 methods and theoretical studies. This is followed by an in-depth discussion of the state of the art, challenges, and future prospects in three forefront branches of the area. Contents 1. INTRODUCTION . 618 2. OBSERVATIONALMETHODS.............................................. 623 2.1. TransitSpectroscopy...................................................... 623 2.2. High-Resolution Doppler Spectroscopy . 628 2.3. DirectImaging............................................................ 629 2.4. SummarizingPoints....................................................... 630 3. THEORETICALADVANCEMENTS......................................... 631 3.1. Self-ConsistentModels.................................................... 631 3.2. Atmospheric Retrieval . 633 3.3. Disequilibrium Models . 633 3.4. SummarizingPoints....................................................... 635 4. ATMOSPHERICCHARACTERIZATIONOFEXOPLANETS................ 635 4.1. ChemicalCompositions................................................... 635 4.2. Clouds/Hazes . 638 4.3. TemperatureStructures.................................................... 640 4.4. AtmosphericDynamics.................................................... 641 4.5. AtmosphericEscape....................................................... 642 4.6. SummarizingPoints....................................................... 644 5. IMPLICATIONSFORPLANETARYFORMATION.......................... 645 5.1. TheBasicPicture......................................................... 645 5.2. CompositionsofAccretedMaterial......................................... 646 5.3. End-to-End Studies ....................................................... 646 5.4. SummarizingPoints....................................................... 650 6. HABITABLEPLANETSANDBIOSIGNATURES............................ 650 6.1. HabitablePlanets......................................................... 650 6.2. Biosignatures............................................................. 651 6.3. ObservationalProspects................................................... 652 6.4. SummarizingPoints....................................................... 652 7. FUTURELANDSCAPE...................................................... 652 Access provided by Florida International University on 01/17/21. For personal use only. Annu. Rev. Astron. Astrophys. 2019.57:617-663. Downloaded from www.annualreviews.org 7.1. Exoplanetary Atmospheres with Current Facilities . 653 7.2. Exoplanetary Atmospheres with the James Webb Space Telescope ............... 654 1. INTRODUCTION Planetary atmospheres serve as Rosetta Stones for planetary processes. Encoded in the spec- trum of a planetary atmosphere is information about its various physical and chemical proper- ties, which in turn provide key insights into myriad atmospheric processes as well as the for- mation and evolutionary history of the planet. Over a century of spectroscopic observations of planets and moons in the Solar System have revealed a vast treasury of information on their diversity in all these aspects. From the giant storms and NH3 clouds on Jupiter to the dense 618 Madhusudhan AA57CH15_Madhusudhan ARjats.cls August 7, 2019 14:11 sulfuric clouds on Venus, from H2-rich giant planets to CO2-rich Venus and Mars, and then to the unique Earth, the atmospheric diversity of the Solar System is a sight to behold for the intrepid explorer. Yet, all the breathtaking diversity of the Solar System arises from a surpris- ingly limited range of macroscopic planetary parameters from a cosmic context. The equilib- rium temperatures of the Solar System planets lie between 50 and 500 K, with only Venus and Mercury being above 300 K. The planetary sizes and masses fall in three broad ranges—the gas giants (8–11 R⊕, ∼100–320 M⊕), the ice giants (4 R⊕, ∼14–17 M⊕), and the terrestrial planets (≤1R⊕, ≤ 1M⊕). In contrast, the thousands of exoplanets known today span a range in bulk prop- erties that would have been considered impossible two decades ago, with temperatures ranging between 200 and 4,000 K and radii and masses spanning continuously over a large range (∼0.5– 20 R⊕, ∼1–104 M⊕). It is only natural then that the atmospheres of these exoplanets may also be expected to be similarly diverse. As such, exoplanetary atmospheres serve as excellent laboratories to study planetary processes and formation mechanisms over the entire possible range of macro- scopic properties—masses, radii, temperatures/irradiation, orbital architectures, and host stellar types. The information obtained about an exoplanetary atmosphere depends on the nature of obser- vations. Figure 1 shows a schematic of atmospheric processes that can be observed in exoplanets with different spectral ranges probing different regions, and hence different processes, in the at- mosphere. The different atmospheric processes can be understood as a function of the pressure (P) 10–7 UV 10–6 Lyα, CII, I III Atmospheric escape Mg , Si 10–5 Photochemistry 10–4 Optical Na, K, –3 Thermal inversions 10 Li, TiO (bar) Clouds/hazes P 10–2 Infrared –1 Vertical mixing 10 H2O, CO, CH4, HCN, Atmospheric circulation CO2, NH3 100 Access provided by Florida International University on 01/17/21. For personal use only. Chemical equilibrium Annu. Rev. Astron. Astrophys. 2019.57:617-663. Downloaded from www.annualreviews.org 101 102 800 1,200 1,600 2,000 2,400 Temperature (K) Figure 1 Processes active in exoplanetary atmospheres and how they are probed by different parts of the electromagnetic spectrum. These processes are prominent in different regions of the atmosphere and are shown accordingly. On the right, the typical penetration depths of UV, optical, and IR radiation are shown, indicating which processes can be probed by observations in each wavelength range. The chemical species whose signatures can be detected in each wavelength range are also indicated. On the left are shown three types of temperature profiles, which can arise as a result of atmospheric processes: a highly irradiated planet with a thermalred inversion( line), an irradiated planet without a thermal inversion (cyan line), and a poorly irradiated planet (gray dashed line). www.annualreviews.org • Exoplanetary Atmospheres 619 AA57CH15_Madhusudhan ARjats.cls August 7, 2019 14:11 in the atmosphere. Deep in the atmosphere (P 1 bar), the pressure and temperature, and hence density and opacity, are high enough that thermochemical equilibrium and radiative–convective equilibrium prevail; i.e., chemical reactions occur faster than kinetic processes. The resulting com- HST: Hubble Space Te l e s c o p e position then is that which minimizes the Gibbs free energy of the system for the given temper- ature, pressure, and elemental abundances. Higher up in the atmosphere, between ∼1mbarand Spitzer : Spitzer Space ∼1 bar, various processes become more prevalent, including atmospheric dynamics, clouds/hazes, Te l e s c o p e and temperature inversions, as a result of complex interplay between the incident radiation field, chemical composition, and other planetary properties. These processes strongly influence, and are influenced by, the atmospheric chemical composition and temperature structure, both
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