Addressing the Challenges in Understanding the Nature of Exoplanet Atmospheres from their Spectra

Luis Welbanks [email protected] @luis_wel luiswelbanks.com 1 Addressing the Challenges in Understanding the Nature of Exoplanet Atmospheres from their Spectra 51 Pegasi b Fellow → Assistant Professor What is our place in the Universe? Is Earth special? How unique is our own Solar System? Are we alone? Credit: NASA's Earth Observatory 1. The Importance of Exoplanet Atmospheres 2Towards new frontiers What is our place in the Universe? Is Earth special? How unique is our own Solar System? Are we alone? Credit: NASA's Earth Observatory 1. The Importance of Exoplanet Atmospheres Decadal for Planetary Science and Astrobiology Decadal for Astronomy and Astrophysics What do our planetary systems reveal about exoplanetary systems? What can exoplanetary systems teach us about the solar system? What other potentially habitable environments exist? What are the properties of individual planets, and which processes lead to planetary diversity? How can signs of habitable life be identified and interpreted in the context of their planetary environments? The coming decades will set humanity down a path to determine if we are alone. 3 Large Population with Large Diversity Over 5000 1. The Importance of Exoplanet Atmospheres 4Towards new frontiers Large Population with Large Diversity Over 5000 1. The Importance of Exoplanet Atmospheres 5 Large Population with Large Diversity Over 5000 Hot Jupiters Rocky Planets Lava Worlds Ocean Worlds & Ice Giants 1. The Importance of Exoplanet Atmospheres 6 Wide Orbit Giants Super Earths Large Population with Large Diversity What are exoplanet atmospheres made of? What do compositions tell us about planet formation? What is the diversity of planetary climates? What makes our Solar System unique? Credit: ESA/Hubble, N. Bartmann 1. The Importance of Exoplanet Atmospheres 7 Large Population with Large Diversity What are exoplanet atmospheres made of? What do compositions tell us about planet formation? What is the diversity of planetary climates? What makes our Solar System unique? Credit: ESA/Hubble, N. Bartmann 1. The Importance of Exoplanet Atmospheres 8 The atmosphere is the most observable portion of an extra-solar body, therefore the information we learn about the processes and history of that object will come from its atmosphere. Condie 2022 How should atmospheric metallicity change with planetary mass? 2. The Legacy of HST 9 Leveraging the Population to Understand Planet Formation Solar Composition The HST Spectra Family Portrait Madhusudhan 2019 2. The Legacy of HST 16 ≳50 transmission spectra (Zhang 2020) Water everywhere Welbanks, et al. 2019b 2. The Legacy of HST 17 Water everywhere ~ 5849 planets discovered (NASA Exoplanet Archive) ≳50 transmission spectra (Zhang 2020) ~ 12 different chemical species have been identified in ~ 30 different planets 2. The Legacy of HST 18 Welbanks, et al. 2019b Testing Hypothesis on the Primordial Formation Pathways of Exoplanets. •Hypothesis: Metallicity of a planetary atmosphere should increase with decreasing mass (coreaccretion model of planet formation) •Evidence: A trend that appears within the solar system Jovian planet population. 19 Kreidberg et al. 2014 See also, Madhusudhan 2014, Pinhas 2017, Chachan 2019 2. The Legacy of HST 19 WASP43b 2 Jupiter masses 1500 K Testing Hypothesis on the Primordial Formation Pathways of Exoplanets. •Hypothesis: Metallicity of a planetary atmosphere should increase with decreasing mass (coreaccretion model of planet formation) •Evidence: A trend that appears within the solar system Jovian planet population. 20 Kreidberg et al. 2014 See also, Madhusudhan 2014, Pinhas 2017, Chachan 2019 2. The Legacy of HST 20 WASP43b 2 Jupiter masses 1500 K Testing Hypothesis on the Primordial Formation Pathways of Exoplanets. •Hypothesis: Metallicity of a planetary atmosphere should increase with decreasing mass (coreaccretion model of planet formation) •Evidence: A trend that appears within the solar system Jovian planet population. 21 Kreidberg et al. 2014 See also, Madhusudhan 2014, Pinhas 2017, Chachan 2019 2. The Legacy of HST 21 WASP43b 2 Jupiter masses 1500 K Remote Sensing of Exoplanets 101 •To test our theories we need constraints on the chemical composition, vertical/horizontal temperature structure, aerosol properties •We use Bayesian parameter estimation involving a large ensemble of models (i.e., running radiative transfer thousands or millions of times) to explore the parameter space that can explain a set of observations. Welbanks 2021 22 2. The Legacy of HST Planet Mass (M⊕) Planet Mass (MJ) 23 Welbanks et al. 2019b Comparing Exoplanets compositions to Solar System Solar System 2. The Legacy of HST Solar Composition Atreya et al. 2016 Elemental enrichment Planet Mass (M⊕) Planet Mass (MJ) Comparing Exoplanets compositions to Solar System Welbanks et al. 2019b Solar System 24 2. The Legacy of HST Solar Composition Atreya et al. 2016, Li et al. 2020 Juno Elemental enrichment Planet Mass (M⊕) Planet Mass (MJ) Exoplanets 25 Comparing Exoplanets compositions to Solar System Welbanks et al. 2019b Solar System 2. The Legacy of HST Solar Composition Atreya et al. 2016, Li et al. 2020 Elemental enrichment Planet Mass (M⊕) Planet Mass (MJ) JWST does provide better data Madhusudhan 2023 Yet our models are imperfect, and our inferences model driven Moran et al 2023 3. The JWST Revolution 32 GJ486b JWST does provide better data Madhusudhan et al 2023 Yet our models are imperfect, and our inferences model driven 3. The JWST Revolution 33 K218b This 6-sigma detection of cheese could happen if you try to explain the composition of the moon with a cheese model and a sponge model 34 3. The JWST Revolution This 6-sigma detection of cheese could happen if you try to explain the composition of the moon with a cheese model and a sponge model 35 How do we assess what in the data drive our inferences? How do we ensure our models are ‘good’? How do we prevent falling into a trap of comparing two ‘bad’ models? 3. The JWST Revolution Building Physical Insights from a Data Driven Perspective H− opacity was detected in HAT-P-41b at 2.9𝜎, but this detection defies our physical intuition. Using Bayesian Leave-One-Out Cross-Validation we show that this inference was driven by a single data point. 36 Welbanks et al. 2023 3. The JWST Revolution Building Physical Insights from a Data Driven Perspective H− opacity was detected in HAT-P-41b at 2.9𝜎, but this detection defies our physical intuition. Using Bayesian Leave-One-Out Cross-Validation we show that this inference was driven by a single data point. 37 Welbanks et al. 2023 This new metric has given us context to our inferences 3. The JWST Revolution 3. The JWST Revolution 38 Addressing Key Questions in the era of JWST To answer these questions we need constraints on: •Chemical composition •Vertical/Horizontal temperature structure •Presence of clouds/hazes How should atmospheric metallicity change with planetary mass? What is the range of atmospheric elemental ratios? What physical processes govern planetary climate? 4. Early Science with JWST Early Release Science with JWST 39 Welbanks et al. in prep WASP39b Radius 1.3 Jupiter Radii Mass 0.3 Jupiter Masses (~Saturn mass) Temperature ~1200 K Early Release Science with JWST Welbanks et al. in prep 4. Early Science with JWST 40 WASP39b Radius 1.3 Jupiter Radii Mass 0.3 Jupiter Masses (~Saturn mass) Temperature ~1200 K Welbanks et al. in prep for Nature Dalba et al. 2015 Saturn from Cassini Early Release Science with JWST 4. Early Science with JWST 41 Detecting All Major Element Reservoirs in an Exoplanet Welbanks et al. 2024, Nature 48 WASP107b Radius 0.9 Jupiter Radii Mass 0.09 Jupiter Masses Temperature ~700 K 4. Early Science with JWST We see many carbon species ⇒ Can’t be that low of a C/O ⇒ SO2 implies not in equilibrium ⇒ Likely disequilibrium! 4. Early Science with JWST 49 Internal heating inflates the low-density planet WASP-107b Leveraging atmospheric constraints to find a unique solution for the planet’s composition and interior properties High internal temperature Low internal temperature Equilibrium expectation Welbanks et al. 2024, Nature 4. Early Science with JWST 50 Internal heating inflates the low-density planet WASP-107b Leveraging atmospheric constraints to find a unique solution for the planet’s composition and interior properties High internal temperature Low internal temperature Equilibrium expectation Welbanks et al. 2024, Nature WASP-107 b is a “popcorn planet” Eccentricity driven tidal heating is a critical process governing atmospheric chemistry and interior structure inferences for a majority of the cool super-Earth-toSaturn mass exoplanet population 51 This is Just the Beginning of 20 Years with JWST 4. Early Science with JWST JWST December 2021 ELTs ~2030 HWO >2040 We can now obtain a holistic view of exoplanet atmospheres answering not only what are exoplanet atmospheres made of, but which data drive our inferences and how reliable are these inferences. The Future is Bright New Knowledge Better Data Better Model 5. The Future is Bright 52 New Frontiers for Astrochemistry in Exoplanet Atmospheres 5. The Future is Bright 53 Signal validation Is the signal robust? Is the ‘bump’ really there? Signal attribution Are the spectral features correctly attributed to the appropriate gases? Interpretation How reliable are the derived planetary properties? New Frontiers for Astrochemistry in Exoplanet Atmospheres 5. The Future is Bright 54 Inspired and adapted from the works of Matteo Brogi. see e.g., Smith et al. 2024, Kanumalla et al. 2024, Gandhi et al. 2024, Brogi et al. 2023 How do we ush the boundaries of our knowledge of gas giants and rocky exoplanets with HRS and LRS? Chemical identification with HRS e.g., Prinoth 2025 6. Concluding Thoughts 55 The search for chemical species with JWST has resulted in detections of CH4, SO2, CO2, CO, SO2, H2O, NH3 In the era of JWST detecting chemical species means more than a sigma model preference Signal validation Signal attribution Interpretation We can connect atmospheric constraints to the interior structure of the planet and the planet dynamics. Questions: How is our discovery space limited by our model assumptions? What does it mean to ‘detect’ a gas? Can we really constrain planet formation? evolution? Are we ready to find ____ as we don’t know it?