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
Good news: The activity towards a spacebased 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?
Earth as seen from 4 billion miles away, by the Voyager I spacecraft
“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
Pme » DG R
 The minimum
• Gibbs Free
maintenance energy
energy yield
rate [kJ/g/s]
[kJ/mole]
 Empirically measured
in the lab
 Tijuis et al. 1993
é -EA ù
Pme = Aexp ê
ú
ë RT û
• Gas production
rate [mole/g/s]
• Measured for lab
cultures
Biomass Model Estimate
Pme » DG R
Fsource » R SB
DG Fsource
SB »
Pme
R [mole/g/s] can be broken
down into relevant quantities
Fsource: biosignature surface flux
[mole/m2/s] would be derived
from future exoplanet
observations, considering
photochemistry
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 star
Seager et al.
submitted to ApJ
Biosignature Gases
in H2 Atmospheres
Proof 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
Two-Pronged Strategy
This decade: small planets transiting small stars
Fast-track
ground-based, and space assets
under construction
Future: Earth-like planets orbiting sun-like stars
Requires technology investments
and new space-based facilities
Transiting Planet Science
Secondary Eclipse
See planet thermal radiation
disappear and reappear
10-3
Primary Eclipse
Measure size of planet
See star’s radiation
transmitted through the
planet atmosphere
10-4
10-2
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
Term
M Stars
N*
FQ
FHZ
FO
FL
FS
N
30,000
(0.2)
0.15
0.001
1
0.5
2
Mission Goal
Time
3U
3U Prototype
3U Fleet
6U+
6U+ Prototype
6U+ Fleet
Science Evolu on
Technology Upgrades
The ExoplanetSat 3U prototype lays the technological and scien fic founda on for the graduated
growth of a modular, extensible
e
flet o f satellites observing bright stars for other Earths.
Two-Pronged Strategy
This Decade: Super Earths transiting small stars
Fast-track
ground-based, and space assets
under construction
Future: Earth-like planets orbiting sun-like stars
Requires technology investments
and new space-based facilities
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
HD 189733 b
Pont F et al. MNRAS 2013;432:2917-2944
© 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical
Society
Transmission spectrum data, with data sets and visits indicated separately.
© 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical
Society
Pont F et al. MNRAS 2013;432:2917-2944
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