Biomarkers on EarthBiomarkers on Earth--like planets like planets around M starsaround M stars

Antígona Segura*, James Kasting*Pennsylvania State University

Victoria Meadows*, David Crisp*, California Institute of Technology/NASA Jet Propulsion Laboratory

Martin Cohen*University of California

John ScaloUniversity of Texas

Rebecca A. H. Butler*California Institute of Technology/NASA Jet Propulsion Laboratory

Giovana Tinetti*California Institute of Technology/National Research Council

*Members of the Virtual Planetary Laboratory a NASA Astrobioloy Institute project

Characteristics of M dwarfsCharacteristics of M dwarfs

Mass range: 0.07 MMass range: 0.07 M☼☼ to 0.7 Mto 0.7 M☼☼

Effective temperature: 2100 K to 3800 KEffective temperature: 2100 K to 3800 KThey account for They account for ~ 73% of all main~ 73% of all main--sequence stars in the solar neighborhood.sequence stars in the solar neighborhood.Spectra dominated by strong molecular Spectra dominated by strong molecular and atomic lines.and atomic lines.Highly variable due to chromospheric Highly variable due to chromospheric activity caused by strong magnetic fields.activity caused by strong magnetic fields.

Problems in detecting EarthProblems in detecting Earth--like like planets around M dwarfsplanets around M dwarfs

Ozone has been shown to be a good biomarker in Ozone has been shown to be a good biomarker in EarthEarth--like planets around F, G and K stars like planets around F, G and K stars (Segura et al., (Segura et al., AstrobiologyAstrobiology 2004).2004).

Ozone production depends on the amount of UV Ozone production depends on the amount of UV light (<240 nm).light (<240 nm).

Due to their low surface temperatures M dwarfs Due to their low surface temperatures M dwarfs are faint in the UV are faint in the UV butbut… during active periods M … during active periods M dwarfs emit substantial amounts of UV and soft dwarfs emit substantial amounts of UV and soft X rays.X rays.

MM--star planets may be tidally locked (we have star planets may be tidally locked (we have notnotdealt with this here!) dealt with this here!)

Star sampleStar sample

0.070.074.94.934003400M4.5VM4.5VAD AD LeoLeob

0.220.2236503650M1VM1VModelModelc0.160.1634003400M3VM3VModelModelc0.070.0731003100M5VM5VModelModelc

6.5 6.5

DistanceDistance(pc)(pc)

0.160.16

Planet major Planet major axis (AU)axis (AU)

3200 3200 M3.5VM3.5VGJ 643GJ 643a

EffectiveEffectivetemperature (K)temperature (K)

SpectralSpectraltypetypeStarStar

a Composite spectra from IUE, Henry et al. (1994), Leggett et al. (2000), NextGen models.

b Spectra from IUE, Pettersen and Hawley (1989), Leggett et al. (1996) and NextGen models.

c NextGen models from BaSeL website (www.astro.mat.uc.pt/BaSeL/).

Spectra were normalized in order to get a surface temperature of 288 K on each planet.

Stellar spectra at the top of the planetary atmosphereStellar spectra at the top of the planetary atmosphere

Spectra were normalized in order to get a surface temperature of288 K on each planet.

Stellar spectra at the top of the planetary atmosphereStellar spectra at the top of the planetary atmosphere

Atmospheric modelsAtmospheric modelsClimate modelClimate modelRadiativeRadiative--convective 1convective 1--D model (Pavlov et al., 2000, D model (Pavlov et al., 2000, JGRJGR 105, 11981).105, 11981).

Photochemical modelPhotochemical model11--D model for 55 chemical species linked by 219 reactions (Pavlov D model for 55 chemical species linked by 219 reactions (Pavlov and and Kasting, 2002, Kasting, 2002, AstrobiologAstrobiology 2, 27).y 2, 27).

SMART radiative transfer modelSMART radiative transfer modelGenerates highGenerates high--resolution, angle dependent synthetic planetary spectra resolution, angle dependent synthetic planetary spectra (Meadows and Crisp, 1996, (Meadows and Crisp, 1996, JGRJGR 101(E2), 4595).101(E2), 4595).

All planetary atmospheres have present Earth concentrations for All planetary atmospheres have present Earth concentrations for major species major species (N(N22, O, O22) at a surface pressure of 1 atm.) at a surface pressure of 1 atm.

Active MActive M--star planets have fixed surface fluxes for biogenic compounds (Hstar planets have fixed surface fluxes for biogenic compounds (H22, , CHCH44, N, N22O, CO, CHO, CO, CH33Cl).Cl).

For the quiescent MFor the quiescent M--star planets the boundary conditions were: fixed CHstar planets the boundary conditions were: fixed CH44mixing ratio (500 mixing ratio (500 ppmvppmv), fixed deposition velocities for H), fixed deposition velocities for H22 (2.4(2.4××1010--44 cm/s) cm/s) and CO (1.2and CO (1.2××1010--44 cm/s) and fixed surface flux for methyl chloride and cm/s) and fixed surface flux for methyl chloride and nitrous oxide.nitrous oxide.

ModelsModelsClimate model

Radiative transfer model

(SMART)

Temperature, pressure,

H2O, O3, CO2, CH4, N2O, CH3Cl

Photochemical model

Ozone,stratospheric

H2O

T, tropospheric

H2O

Spectra of habitable exoplanets

Temperature, HTemperature, H22O and OO and O33 profilesprofilesActive M-star planets

Quiescent M-star planets

Biogenic compoundsBiogenic compounds

Active M-star planets. Fixed surface fluxes

Quiescent M-starplanets. Fixed CH4

mixing ratio (5×104).

Chemistry on a habitable planet Chemistry on a habitable planet around an active M starsaround an active M stars

Methane destruction in Earth’s troposphereMethane destruction in Earth’s troposphere

OO33 + h+ hνν ((λλ < 310 nm) < 310 nm) →→ OO22 + O+ O11DDOO11D + HD + H22O O →→ 2 OH2 OH

CHCH44 + OH + OH →→ CHCH33 + H+ H22OOCHCH33 + O+ O22 + M + M →→ CHCH33OO22 + M+ M→→ … … →→ CO (or COCO (or CO22) + H) + H22OO

O2

O3

Photolysis of OPhotolysis of O22 and Oand O33

N2O

IR IR SpectrumSpectrum forfor AD Leo AD Leo planetplanet

Visible Visible spectrumspectrum forfor AD Leo AD Leo planetplanet

MethylMethyl chloridechloride

ConclusionsConclusions• Predicted concentrations of methane, nitrous oxide and

methyl chloride are higher for the M-star planets than for Earth, making these compounds more detectable on those planets.

• The planets around the active M stars developed ozone layers similar to that on Earth.

• The signature of O3 from habitable planets around active M dwarfs may be detectable by instruments like TPF or Darwin, along with the signatures of various reduced gases.

• The simultaneous detection of O2 or O3 and N2O, CH4, or CH3Cl in the atmosphere of an M-star (or other extrasolar) planet would provide convincing evidence for the existence of extraterrestrial life.