Are We Alone?! Exoplanet Characterization and Direct Imaging!

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1 From Cosmic Birth to Living Earths A Vision for Space Astronomy in the 2020s and Beyond Are We Alone?! Exoplanet Characterization and Direct Imaging! A Study Commissioned by the Associated Universities for Research in Astronomy The Beyond JWST Committee Co-Chairs: Sara Seager (MIT) Julianne Dalcanton (Washington) Presenter: Marc Postman (STScI)

2 Can we find another planet like Earth orbiting a nearby star? To find such a planet would complete the revolution, started by Copernicus nearly 500 years ago, that displaced the Earth as the center of the universe The observational challenge is great but armed with new technologies astronomers are poised to rise to it. - New Worlds, New Horizons (Astro 2010) 21st century astronomers are uniquely positioned to study the evolution of the Universe in order to relate causally the physical conditions during the Big Bang to the development of RNA and DNA. - Riccardo Giacconi This is a quest sought by all of humanity and the search will require international cooperation.

3 The path has been laid for characterizing Earth 2.0 Kepler Hubble Spitzer CoRoT Ground-based Coronagraphs Gaia WFIRST 30-m class telescopes TESS JWST PLATO What is a logical next step?

4 The High Definition Space Telescope (HDST) A space-based telescope at the Earth-Sun L2 point. Goal is for a 12 m effective aperture diameter. Motivated by exoplanet yield, high-res images of galaxies, cosmic gas flows, and high-definition stellar populations in many environments. A segmented, deployable mirror. Diffraction-limited performance at visible wavelengths Full complement of coronographic, imaging, and spectroscopic instruments. UV to near-ir wavelengths (non-cryogenic optics). Serviceability is a goal but not a requirement.

5 Exoplanet Discovery Space: Direct Imaging Venus and Earth look the same to all planet-finding techniques except those that enable planet atmosphere spectra: predominantly transits and direct imaging. Only direct imaging can reach and distinguish rocky planets around hundreds of sun-like stars via spectroscopic characterization of their atmospheres. Planet / Star Contrast! This is the region we need! Terrestrial to explore! Planets! From WFIRST SDT Interim Report (2014)! Angular Separation (arcsec)! Delta Magnitude (mag)! Transiting planets around bright stars are rare because of the low probability to transit. Transmission spectra of Earth analogs and the cross-correlation technique (Snellen et al.) might only work around the very brightest sun-like stars and, even then, would be extremely challenging.

6 How Many Planets Must We Search? Even Earth-like planets in their HZ may have a great diversity of atmospheric properties owing to differences in mass, solar irradiation, and complex history. Sub Neptune Planet Albedo Spectra Fig courtesy of Aki Roberge, data in part from Renyu Hu. We want to maximize our chances of detecting these biosignature gases on Earth-like planets. If biomarkers can be found on 10% of Earth-like planets, and we want to reduce the chance of randomly missing these systems to <1%, spectra of ~50 planets must be obtained. With N = 10, biosignatures must occur at 37% probability to have <1% chance of missing it. courtesy Aki Roberge" Searching hundreds of stars also insures against η Earth on low side of present estimates." To find signs of life, even if it is uncommon, we must search dozens of Earth-like planets orbiting in their habitable zones.

7 How Many Planets Can We Search? Obscurational and photometric completeness make direct exoplanet imaging more challenging than standard faint-object imaging and spectroscopy. In other words, planets are not always visible and may be too faint depending on the planet illlumination phase. Need to be How able to parameterize Many Planets: yield as a function the of aperture Yield and uncertain astrophysical parameters (particularly η Earth and exozodi brightness). Computer simulations of planetary systems around known stars can tell us how exoplanet yield scales with astrophysical and observatory parameters. Yield calculacons by Chris Stark (GSFC) arxiv:1409:1528

8 ExoEarth Yield Results (Stark et al.2014) Optimistic" η Earth = 0.2 IWA = 3λ/D n exozodis = 3 5 Pessimistic" η Earth = 0.05 IWA = 3λ/D N η Earth ( Zodis) 0.23 ( D ) 1.88 ( Tel IWA) 0.64 ( ExpTime) 0.36 ( QE) 0.39 ( Contrast) 0.09 The uncertainty in astrophysical constraints is primarily primarily η Earth and exozodi. There a surprisingly weak dependence of exoearth candidate yield on exozodi level. Yield scales linearly with η Earth.

9 ExoEarth Yield Results (Stark et al.2014) Optimistic" η Earth = 0.2 IWA = 3λ/D n exozodis = 3 5 Pessimistic" η Earth = 0.05 IWA = 3λ/D N η Earth ( Zodis) 0.23 ( D ) 1.88 ( Tel IWA) 0.64 ( ExpTime) 0.36 ( QE) 0.39 ( Contrast) 0.09 A 12-meter telescope can reach Earth-like planets: this is enough to detect or significantly constrain the incidence of biomarker molecules.

10 Other Advantages: Detecting Diurnal Photometric Variability in Exoplanets Ford et al. 2003: Model of broadband photometric temporal variability of Earth 0.09 Earth at 10 pc Reflectivity m 8-m 4-meter Earth at 20 pc ~9 days 12-m 8-meter 4-meter Time (days) Require S/N ~ 20 (5% photometry) to detect ~20% temporal variations in reflectivity. Reconstruction of Earth s land-sea ratio from disk-averaged time-resolved imaging with the EPOXI mission.

11 R=500 Spectrum of 1 Earth-mass Terrestrial Exoplanet at 10 pc 760 nm H 2 O H 2 O H 2 O H 2 O O 2 (α) O 2 (B) H 2 O O 2 (A) H 2 O 12 m: ~900 ksec O 2 (A) 750 nm We don t expect all potentially habitable worlds to have spectra like this but interpreting their spectra will likely require this kind of instrumental capability.

12 Good Statistics Provide the Answer to: Are We Alone? While we can already estimate the probability of Earth-like worlds orbiting other stars, we do not know how often life occurs on those planets. This is what we are trying to determine! The incidence of life and its biomarker molecules may be small: 10% or even 1% on otherwise Earth-like planets in their HZ. If so, a small sample of planets (~10 or less) is very likely to fail to answer our most important question. Only by surveying dozens of worlds do we make the chance of detecting life s signature a good one, even if it is uncommon. An HDST-like telescope will be able to detect dozens of Earth-like planets orbiting in their habitable zones and systematically search for biosignature gases to address Are We Alone? with a robust statistical sample.