Sara Seager - Lecture3 - MIT
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This document discusses biosignatures and the search for habitable exoplanets. It begins by reviewing Earth's biosignatures like oxygen, ozone, methane, and vegetation reflectance. It then explains that life exploits chemical energy gradients and metabolic reactions have an energy yield that can be quantified. The search for habitable worlds involves finding transiting exoplanets using ground and space telescopes to measure atmospheres. Direct imaging from space is also needed to find and characterize Earth analogs, with external occulters showing promise to block starlight. The overarching goals are to understand possible biosignatures on non-Earth worlds and find the most promising planets for atmospheric study.
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Sara Seager - Lecture3 - MIT
- 1. Biosignatures and the Search for “All Roads Lead to TPF/ Darwin” Habitable Worlds Sara Seager MIT
- 5. The exoplanet atmosphere is the only way to infer whether or not a planet is habitable or likely inhabited The planetary atmosphere is our window into temperatures, habitability indicators, and biosignature gases
- 6. Biosignatures and Habitable Planets Overview Review Introduction: Earth’s Biosignatures The Thermodynamics of Biosignatures The Search for Habitable Worlds
- 7. Known Planets 1995 Based on data compiled by J. Schneider
- 8. Known Planets 1996 Based on data compiled by J. Schneider
- 9. Known Planets 2000 Based on data compiled by J. Schneider
- 10. Known Planets 2005 Based on data compiled by J. Schneider
- 11. Known Planets 2010 Based on data compiled by J. Schneider
- 12. Planets 2000
- 13. Planets 2005
- 14. Planets June 14 2010
- 15. Planet Candidates June 15 2010 Kepler Planet Candidates announced June 15, 2010
- 16. Atmosphere InterpretaRon H2O and CH4 in transmission from HST Swain et al. (2008) See also Grillmair et al. 2008. IdenficaRon of atoms and molecules Day‐night temperature gradients Possible variability Advances in retrieval techniques See Seager and Deming ARAA 2010
- 17. Overview Points Monumental exoplanet achievements were made despite widespread skepRcism The huge promise for the future is based on the achievements of the past decade+ Keep this in mind for the rest of the talk
- 18. Biosignatures and Habitable Planets Introduction: Earth’s Biosignatures The Thermodynamics of Biosignatures The Search for Habitable Worlds
- 19. Earth, Venus, Mars 1D models from Vikki Meadows Virtual Planetary Laboratory
- 20. Earth, Venus, Mars 1D models from Vikki Meadows Virtual Planetary Laboratory
- 21. Earth’s Spectrum Turnbull et al. 2007 Pearl and Christensen 1997
- 22. VegetaRon as a Surface Biomarker S. Seager InsRtute for Advanced Study, Princeton, July 2002
- 23. VegetaRon as a Surface Biomarker S. Seager S. Seager
- 24. The VegetaRon Biosignature • Chlorophyll causes strong absorpRon at wavelengths shorter than 0.7 μm • Light scacering in air gaps between water‐filled plant cells causes strong red reflectance • Plants absorb energy at short wavelengths for photosynthesis; reflect and transmit radiaRon at long wavelengths for thermal balance Seager et al. 2005 Data from Clark 1993
- 25. Earth Biosignature Summary • O2 /O3 : the smoking gun • N2O: biological but weak signature • H2O: evidence for liquid water • CH4: biological/abiological • (VegetaRon red edge) But, we are stuck in a world of terracentric biosignatures.
- 26. Biosignatures and Habitable Planets Introduction: Earth’s Biosignatures The Thermodynamics of Biosignatures The Search for Habitable Worlds
- 27. Nothing would be more tragic in the American exploraRon of space than to encounter alien life and fail to recognize it NRC report 2007
- 28. Constraints for Life in the Universe All Life Liquid water Carbon‐based Chemical PotenRal Energy biologist chemist physicist
- 31. Fuels CH2O→CO2 Redox PotenRal (V) Oxidants H2→H+ CO2→CH2O H+→H2 NH4 +→N2 N2→NH4 + CH4→CO2 CO2→CH4 H2S→ SO4 2‐ SO4 2‐→H2S N2→ NO3 ‐ NO3 ‐→N2 H2O→O2 O2→H2O Not to scale Electron Tower Aker Lane, Nature May 2006
- 32. Fuels CH2O→CO2 Redox PotenRal (V) Oxidants H2→H+ CO2→CH2O H+→H2 NH4 +→N2 N2→NH4 + CH4→CO2 CO2→CH4 H2S→ SO4 2‐ SO4 2‐→H2S N2→ NO3 ‐ NO3 ‐→N2 H2O→O2 O2→H2O Not to scale
- 33. Fuels CH2O→CO2 Redox PotenRal (V) Oxidants H2→H+ CO2→CH2O H+→H2 NH4 +→N2 N2→NH4 + CH4→CO2 CO2→CH4 H2S→ SO4 2‐ SO4 2‐→H2S N2→ NO3 ‐ NO3 ‐→N2 H2O→O2 O2→H2O Not to scale Electron Tower Aker Lane, Nature May 2006
- 34. Biosignature Thermodynamics • Metabolic reacRons are redox reacRons • The Gibbs free energy is used to determine the energy yield of a reacRon • Recall: the Gibbs free energy is an energy potenRal • The energy potenRal can be converted into volts via the Nernst equaRon. See “Biological Thermodynamics” by D. T. Haynie.
- 35. Seager and Schrenk, “An Astrophysical View of Earth‐Based Metabolism submiced to Astrobiology,. Supported by FQXI O2, O3, N2O Unique Generated by geology or photochemistry Not rapidly assimilated H2, CO2, H2S, CH4, SO2 Not highly soluble in ocean Gaseous metabolic byproducts O2, H2, CO2, N2, N2O, NO, NO2, H2S, CH4, SO2, H2O, NH3 All Earth‐based metabolic byproducts Different atmospheres/ stellar UV could have different biosignatures.
- 36. Atmospheric escape Photochemistry RadiaRve transfer Atmospheric ComposiRon Chemical equilibrium/disequilibrium Atmospheric circulaRon Biosignatures Clouds ConnecRon with observaRons
- 37. Biosignature Framework • For a spectral feature to be significant, what is the required biosignature flux? • This is not only a radiaRve transfer problem, but also related to photochemistry sources and sinks, and putaRve chemistry of the planet crust and atmosphere • Reality check: convert to biomass by equaRon the Gibbs free energy (yield) to maintenance energy of the organism Seager et al. in prep. Figure from De Wit
- 38. Biosignature Summary • Life uses and exploits chemical energy gradients • ReacRons that are energeRcally favorable but kineRcally inhibited • There is a quanRtaRve path forward for redox biosignature fluxes in the context of atmospheric radiaRve transfer and chemistry models – gives a chance to find new biosignatures.
- 39. Biosignatures and Habitable Planets Introduction: Earth’s Biosignatures The Thermodynamics of Biosignatures The Search for Habitable Worlds
- 40. Two‐Pronged Strategy Prong 1: TranrsiRng planets with a focus on M dwarfs Fast‐track ground‐based, and exisRng space assets Prong 2: direct imaging of F, G, K dwarfs Requires technology investments And new space‐based faciliRes
- 41. Transit Survey Telescopes TESS concept 6 to 9 lenses on the same plaqorm CNES/Corot 30 cm mirror Polar orbit NASA/Kepler 1 m aperture Earth‐trailing orbit ExoplanetSat concept A triple CubeSat constellaRon of nanosats Plato science payload Under study by ESO
- 42. All roads lead to … Terrestrial Planet Finder/Darwin
- 43. Direct Imaging A5V 40 pc Planets at: 24, 38, and 60 AU Planet‐star contrast 104 or 105 Marois, MacIntosh, Doyon et al. 2008
- 44. Direct Imaging Telescope Diffracted light problem
- 45. Direct Imaging Telescope Diffracted light soluRons … require a near perfect telescope
- 46. Cash 2006
- 47. Cash 2006 Led by Remi Soummer and Web Cash Recent SPIE paper R.A. Brown, I. Jordan, A. Roberge, T. Glassman, A. Lo, S. Seager, L. Pueyo and others Technical issues 70m occulter at 70,000 km StaRonk eeping ‐not opRmized at visible ‐interesRng features near a micron undersampling of PSF at visible ‐limits to high S/N limited Rme ‐9% of JWST Rme, limits search to about 30 stars
- 48. Principle of a starshade on a separate spacecrak to block the light from the star, while allowing the light from an exoplanet to pass the edge of the occulter unimpeded. Northrup Grumman See the NASA‐sponsored studies THEIA (led by David Spergel), NWO led by Webster Cash. See Cash 2006
- 49. SimulaRons for a 12‐day observing Rme for a super Earth orbiRng a sun‐like star at 10 pc. R = 1000 binned down to R = 200 R. Soummer et al. SPIE in prep. Models by S. Seager
- 50. Search for Habitable Worlds Summary • Transits or direct imaging will provide a valuable handful of potenRally habitable planets • Vast resources in terms of telescope Rme for transits and technology development for space‐based direct imaging are needed • A near‐term Terrestrial Planet Finder possibility is the occulter + the JWST
- 51. A` Summary Biosignature goal: understand possible metabolic byproducts and their lifeRmes on non‐Earth‐like exoplanets NWO TransiRng exoplanets: find them via ground‐based or space based transit searches. Measure spectra of transiRng super Earths orbiRng M stars using the James Webb Space Telescope Direct imaging from space to find and characterize true Earth analogs. The external occulter is promising technology to block out starlight of a sun‐like star to a level of 10 billion at visible wavelengths
- 53. seagerexoplanets.mit.edu Exoplanet Atmospheres and Interiors B. Benneke (Aero/Astro grad student) M. Braunstein (EAPS grad student) R. Hu (EAPS grad student) M. Nikku (EAPS postdoc) A.‐M. Piso (Physics UROP) L. Rogers (Physics grad student) J. De Wit SupAero masters intern Kepler Data Brice Demory (EAPS postdoc) Josh Carter (Physics postdoc) K. Berry (Physics UROP) K. Singh (Physics UROP) ExoplanetSat J. Villasenor (Kavli) G. Farmer (EAPS grad student) C. Pong (Aero/Astro grad student) M. Smith (Aero/Astro grad student) M. Knapp (Aero/Astro UROP) B. Stavely (Aero/Astro UROP) Thank you to my students and postdocs Supported by MIT, NASA, and FQXI