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. 1993

• Gibbs Free energy yield [kJ/mole]

• Gas production rate [mole/g/s]

• Measured for lab cultures

Biomass Model Estimate

Fsource: biosignature surface flux [mole/m2/s] would be derived from future exoplanet observations, considering photochemistry

R [mole/g/s] can be broken down into relevant quantities

ΣB: biomass surface density [g/m2]

Cold Haber World: NH3

• Cold Haber World 3H2 + N2 2NH3

– NH3 as a biosignature gas on an 90% H2-10% N2 planet with life enzymatically catalyzing the N2 bond

– NH3 has a short lifetime and requires a surface flux for production in thin atmospheres

– Detectable NH3 around a quiet M star with 3.3 ppm, Fsource = 2 x 1013 molecules/m2/s, ΔG and ΣB ~ 3 x 10-5 g/m2

Figure shows synthetic transmission spectra for a 10 Earth mass, 1.75 Earth radius planet orbiting a quiet M5 dwarf starSeager et al. submitted to ApJ

Biosignature Gases in H2

AtmospheresProof of concept that biosignature gases can accumulate in an H2-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

http://phl.upr.edu/projects/habitable-exoplanets-catalog

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 EclipseMeasure size of planetSee star’s radiation transmitted through the planet atmosphere

Secondary EclipseSee planet thermal radiation disappear and reappear

Learn about atmospheric circulation from thermal phase curves

10-2

10-4

10-3

TESSTransiting 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

Term M Stars

N* 30,000

FQ (0.2)

FHZ 0.15

FO 0.001

FL 1

FS 0.5

N 2

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. CadyD. Webb, J. HenriksonD. Scharf, and R. Trabert March 4, 2014

Diffracted Light

Pont F et al. MNRAS 2013;432:2917-2944

© 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society

HD 189733 b

Transmission spectrum data, with data sets and visits indicated separately.

Pont F et al. MNRAS 2013;432:2917-2944

© 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