Key Challenges for Exoplanet Biosignature Gas Studies Sara Seager, MIT Earth as seen from 4 billion miles away, by the Voyager I spacecraft

Key Challenges for Exoplanet Biosignature Gas Studies Sara Seager, MIT Earth as seen from 4 billion miles away, by the Voyager I spacecraft Good news: The activity towards a space- based direct imaging mission to find Earth 2.0 is accelerating Bad news: From the image of the pale blue dot (aka “blob”) can we identify an Earth 2.0 let alone one with biosignature gases?

“Nothing would be more tragic in the American exploration of space than to encounter alien life and fail to recognize it” NRC report on “The Limits to Organic Life in Planetary Systems” 2007

Biosignature Gases

The Revised Habitable Zone See Seager, “Exoplanet Habitability”, Science May 2013

Terrestrial Planet Finder Telescope

Biomass Model as a Plausibility Check for Biosignature Gases

Biomass Model Estimate  The minimum maintenance energy rate [kJ/g/s]  Empirically measured in the lab  Tijuis et al Gibbs Free energy yield [kJ/mole] Gas production rate [mole/g/s] Measured for lab cultures

Biomass Model Estimate F source : biosignature surface flux [mole/m 2 /s] would be derived from future exoplanet observations, considering photochemistry R [mole/g/s] can be broken down into relevant quantities Σ B : biomass surface density [g/m 2 ]

Cold Haber World: NH 3 Cold Haber World 3H 2 + N 2  2NH 3 – NH 3 as a biosignature gas on an 90% H 2 -10% N 2 planet with life enzymatically catalyzing the N 2 bond – NH 3 has a short lifetime and requires a surface flux for production in thin atmospheres – Detectable NH 3 around a quiet M star with 3.3 ppm, F source = 2 x molecules/m 2 /s, ΔG and Σ B ~ 3 x g/m 2 Figure shows synthetic transmission spectra for a 10 Earth mass, 1.75 Earth radius planet orbiting a quiet M5 dwarf star Seager et al. submitted to ApJ

Biosignature Gases in H 2 Atmospheres Proof of concept that biosignature gases can accumulate in an H 2 -rich atmosphere H is the dominant reactive species (akin to OH) The low UV environments of quiet M stars are most favorable Examples studied shown in Fig. Seager, Bains, Hu submitted to ApJ

Requires technology investments and new space-based facilities This decade: small planets transiting small stars Future: Earth-like planets orbiting sun-like stars Two-Pronged Strategy Fast-track ground-based, and space assets under construction

Transiting Planet Science Primary Eclipse Measure size of planet See star’s radiation transmitted through the planet atmosphere Secondary Eclipse See planet thermal radiation disappear and reappear Learn about atmospheric circulation from thermal phase curves

TESS Transiting Exoplanet Survey Satellite MIT-led NASA Mission (launch 2017) Plan to find a pool of short-period rocky planets transitng small stars then followup atmosphere observations with the James Webb Space Telescope

TermM Stars N*N* 30,000 FQFQ (0.2) F HZ 0.15 FOFO FLFL 1 FSFS 0.5 N2

Requires technology investments and new space-based facilities This Decade: Super Earths transiting small stars Future: Earth-like planets orbiting sun-like stars Two-Pronged Strategy Fast-track ground-based, and space assets under construction

Probe Class Starshade Mission STDT Progress Report Chair: S. Seager (MIT) W. Cash (U. Colorado) N.J. Kasdin (Princeton U.) W. Sparks (STSci) M. Turnbull (GCI) M. Kuchner, A. Roberge, and S. Goldman (NASA-GSFC) S. Shaklan and M. Thomson (NASA-JPL/Caltech) JPL Design Team: D. Lisman, S. Martin, E. Cady D. Webb, J. Henrikson D. Scharf, and R. Trabert March 4, 2014

Diffracted Light

Pont F et al. MNRAS 2013;432: © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society HD b

Transmission spectrum data, with data sets and visits indicated separately. Pont F et al. MNRAS 2013;432: © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society

The Search for Life Beyond Earth Small planets are extremely common The near-term approach to finding and characterizing transiting planets is ongoing For space-based direct imaging, technological milestones, gap list, and plan forward are in place The opportunity to find Earth 2.0 is here