an equation to estimate the probability of identifying … · an equation to estimate the...

Image: JWST @SXSW March 2013

An Equation to Estimate the Probability of Identifying an Inhabited World Within the Next Decade !Sara Seager, MIT!

Acknowledgements Exoplanet Atmospheres and Interiors B. Benneke (Atmospheric Retrieval) R. Hu (Photochemistry) S. Messenger(Biosignature Gases) J. De Wit (Planet Atmospheres) W. Bains (Biosignature Gases) B. Demory (Observations) V. Stamenkovic (Planet Interiors) A. Zsom (Clouds, atmospheres)

Space Engineering C. Pong (ADCS) M. Smith (Optics) M. Knapp (Systems/Camera) J. Nash (Avionics) A.  Babuscia (Comm.) I. Sergeev (Avionics) B. Corbin (Structures/Systems) N. Inamdar (Structuresl)

Thank you to my past and present students and postdocs Supported by MIT, NASA (HQ, JPL), Draper Lab, Lincoln Lab

N number of planets with detectable biosignature gasesN* number of stars within the sampleFQ fraction of quiet starsFHZ fraction with rocky planets in the HZFO fraction of observable systemsFL fraction with lifeFS fraction with detectable spectroscopic signatures A “revised” Drake Equation For any star types, any well defined survey

N = N*FQFHZFOFLFS

N number of planets with detectable biosignature gasesN* number of M stars with I < 13FQ fraction of quiet M starsFHZ fraction with rocky planets in the HZFO fraction of observable=transiting systems observable with JWSTFL fraction with lifeFS fraction with detectable spectroscopic signatures

For M stars: TESS/JWST

N = N*FQFHZFOFLFS

Number of Stars

N* should be those accessible to TESS and for planet-hosting stars accessible to atmosphere followup with JWST"•  I mag < 13 "•  30,000 to 50,000 stars""•  Bochanski 2007; Reid and Hawley 2006"

Term M Stars

N* 30,000

N = N*FQFHZFOFLFS

N*

Frac3on of Stars in the HZ

FHZ for M stars comes from Kepler data from Dressing and Charbonneau 2013. The quiet star fraction is folded in here""

Term M Stars

N* 30,000 FQ (0.2) FHZ 0.15

N = N*FQFHZFOFLFS

FHZ

Frac3on of Observable Systems

FO here means fraction of planets that transit and that are observable by JWST. Transiting planets are required for JWST atmosphere followup for small planets in the HZ"

N = N*FQFHZFOFLFS

FO

Term M Stars

N* 30,000 FQ (0.2) FHZ 0.15 FO 0.01 x 0.1

Frac3on with Life

FL is purely speculative"

Term M Stars

N* 30,000 FQ (0.2) FHZ 0.15 FO 0.001 FL 1

N = N*FQFHZFOFLFS

FL

Frac3on with Detectable Spectrscopic Signature

Does life generate a spectroscopic signature? "

Term M Stars

N* 30,000 FQ (0.2) FHZ 0.15 FO 0.001 FL 1 FS 0.5 N 2

N = N*FQFHZFOFLFS

FS

“Nothing would be more tragic in the … exploration of space than to encounter alien life and fail to recognize it” NRC report 2007

We appear stuck with a terracentric view of biosignature gases

Seager, Science 2013 Inner edge: Zsom, Seager, de Wit, arXiv: 1304.3714

The Habitable Zone

Terrestrial Planet Finder

Telescope

Biomass Model as a Plausibility Check for Biosignature Gases

Thermodynamicmodel predicts

necessary biomass

Is biomass neededto generate a

detectablespectrum a

plausible biomass?

!G = !Go – RTln(Qt)

Atmospherephotochemistry model

Source !ux necessaryto maintain thedetectable gasconcentration

Compute minimalspectral feature

needed for detection

Gas concentrationneeded for detection

Planetary scenario:P, T, base chemistry

Hypothesis:biosignature

gas to beevaluated Step 1:

Determineatmospheric gas

concentration

Step 2:Determinerelated gassurface !ux

Step 3:Determine

relatedbiomass

CO2 + H2 CH4 + H2O

Turnbull et al. 2007

"B = !GFSPme

10–12

101

N2

H2

H

NH3

NH2

N2H2

N2H4

H2O

OH

CO2CO

CH4HCN

102

103

104

105

10–8 10–4 100

Mixing ratio

Pres

sure

(Pa)

Biomass Model Es3mate

Pme≈ ΔG R

§  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

Pme= Aexp −EA

RT"

#$%

&'

Biomass Model Es3mate

Pme≈ ΔG R

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]

Fsource ≈ R ΣB

ΣB ≈ΔG Fsource

Pme

Cold Haber World: NH3 •  Cold Haber World 3H2 + N2 è 2NH3

–  NH3 as a biosignature gas on an 90% H2-‐10% N2 planet with life enzyma3cally catalyzing the N2 bond

–  NH3 has a short life3me and requires a surface flux for produc3on 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

1.75

1.755

1.76

1.765

1.77

Plan

et R

adius

[Ear

th R

adius

] CH3Cl

CH3Cl

1.75

1.755

1.76

1.765

1.77

Plan

et R

adius

[Ear

th R

adius

] DMS DMS

1.75

1.755

1.76

1.765

1.77

Plan

et R

adius

[Ear

th R

adius

] N2O

N2O

CO2

H2O

H2 - H2

H2 - H2

H2 - H2

CO2

CO2

H2O

H2O

CO2

CO2

H2O

H2O

H2O

CH4

CH4

CH4

0 ppm0.5 ppm0.5 ppm5 ppm5 ppm50 ppm50 ppm500 ppm500 ppm



Plan

et R

adius

/ St

ellar

Rad

ius

0.016

0.0161

0.0161

0.0162

0.0802

0.0804

0.0806

0.0808

0.081

Plan

et R

adius

/ St

ellar

Rad

ius

0.016

0.0161

0.0161

0.0162

0.0802

0.0804

0.0806

0.0808

0.081

Plan

et R

adius

/ St

ellar

Rad

ius

0.016

0.0161

0.0161

0.0162

0.0802

0.0804

0.0806

0.0808

0.081

0.2 0.5 1 2 5 10 20 50 1001.75

1.755

1.76

1.765

1.77

Wavelength [microns]

Plan

et R

adius

[Ear

th R

adius

]

Plan

et R

adius

/ St

ellar

Rad

ius

0.016

0.0161

0.0161

0.0162

0.0802

0.0804

0.0806

0.0808

0.081

H2 - H2

CO2

CO2

H2OCH4

NH3

NH3


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

1.75

1.755

1.76

1.765

1.77

Plan

et R

adius

[Ear

th R

adius

] CH3Cl

CH3Cl

1.75

1.755

1.76

1.765

1.77

Plan

et R

adius

[Ear

th R

adius

] DMS DMS

1.75

1.755

1.76

1.765

1.77

Plan

et R

adius

[Ear

th R

adius

] N2O

N2O

CO2

H2O

H2 - H2

H2 - H2

H2 - H2

CO2

CO2

H2O

H2O

CO2

CO2

H2O

H2O

H2O

CH4

CH4

CH4




Plan

et R

adius

/ St

ellar

Rad

ius

0.016

0.0161

0.0161

0.0162

0.0802

0.0804

0.0806

0.0808

0.081

Plan

et R

adius

/ St

ellar

Rad

ius

0.016

0.0161

0.0161

0.0162

0.0802

0.0804

0.0806

0.0808

0.081

Plan

et R

adius

/ St

ellar

Rad

ius

0.016

0.0161

0.0161

0.0162

0.0802

0.0804

0.0806

0.0808

0.081

0.2 0.5 1 2 5 10 20 50 1001.75

1.755

1.76

1.765

1.77

Wavelength [microns]

Plan

et R

adius

[Ear

th R

adius

]

Plan

et R

adius

/ St

ellar

Rad

ius

0.016

0.0161

0.0161

0.0162

0.0802

0.0804

0.0806

0.0808

0.081

H2 - H2

CO2

CO2

H2OCH4

NH3

NH3


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

“It’s not going to be easy but we can dream” Dave Latham

N = N*FQFHZFOFLFS Term M Stars

N* 30,000 FQ (0.2) FHZ 0.15 FO 0.001 FL 1 FS 0.5 N 2

an equation to estimate the probability of identifying … · an equation to estimate the...

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