Mirage Earths - Rodrigo Luger · Mirage Earths Extreme Water Loss and Abiotic O 2 Buildup on...

Mirage EarthsExtreme Water Loss and Abiotic O2Buildup on Planets Throughout the

Habitable Zones of M Dwarfs

Rodrigo Luger and Rory Barnes

225th AAS — January 8, 2015

The Big Picture

M dwarfs: the best targets Terrestrial planets are easiest todetect around low-mass stars

Rapid planet formation Both in situ formation and disk-driven migration into the HZoccur in . 10 Myr

Extended pre-MS phase M dwarfs can take up to ∼ 1Gyr to reach the main sequence

Long activity timescales High X-ray/EUV fluxes in theHZ for up to a few Gyr

Scalo et al. (2007), Ricker et al. (2010)

Raymond et al. (2007), Lissauer (2007)

Baraffe et al. (1998), Lissauer (2007)

Scalo et al. (2007), West et al. (2008)

Pre-Main Sequence Evolution

106 107 108 109 1010

time (years)

0.01

0.10

1.00se

mi-

majo

r axis

(A

U)

Habitable

Too Hot

Earth today

Pre-Main Sequence Evolution

106 107 108 109 1010

time (years)

0.01

0.10

1.00se

mi-

majo

r axis

(A

U)

Habitable

Too Hot

Earth todayEarth formed

Pre-Main Sequence Evolution

106 107 108 109 1010

time (years)

0.01

0.10

1.00se

mi-

majo

r axis

(A

U)

1.001.001.001.00

Pre-Main Sequence Evolution

106 107 108 109 1010

time (years)

0.01

0.10

1.00se

mi-

majo

r axis

(A

U)

1.001.001.001.00

0.90

0.80

0.70

0.60

0.50

0.40

0.30

0.20

0.15

0.10

Pre-Main Sequence Evolution

106 107 108 109 1010

time (years)

0.01

0.10

1.00se

mi-

majo

r axis

(A

U)

1.00

0.10

Pre-Main Sequence Evolution

106 107 108 109 1010

time (years)

0.01

0.10

1.00se

mi-

majo

r axis

(A

U)

Earth today

Exo-Earth today

Pre-Main Sequence Evolution

106 107 108 109 1010

time (years)

0.01

0.10

1.00se

mi-

majo

r axis

(A

U)

Earth todayEarth formed

Exo-Earth todayExo-Earth formed

Duration of the Runaway Greenhouse

0.01 0.1 1.0

a (AU)

0.1

0.2

0.3

0.4

0.5

0.6

0.70.80.9

M(M

¯)

1 Myr

10 Myr

100 Myr

1 Gyr

5 Gyr

Atmospheric Escape Model

XUV power law decline (Ribas et al. 2005)

LXUV

Lbol=

{f0 t ≤ t0

f0(

tt0

)−β

t > t0(1)

XUV-driven energy-limited escape (Erkaev et al. 2007)

FH =εXUVFXUVRp

4GMpKtidemH(2)

Hydrodynamic mass fractionation (Hunten et al. 1987)

FO =XO

XHFH

(mc −mO

mc −mH

)(3)

Water Loss & O2 Buildup

RV 25% 50% 75% EM

Position in Habitable Zone

0.1

0.2

0.3

0.4

0.5

0.6

0.70.80.9

M(M

¯)

0.1

1

10

∆M

H2O

(TO

)RV 25% 50% 75% EM

Position in Habitable Zone

0.1

0.2

0.3

0.4

0.5

0.6

0.70.80.9

M(M

¯)

1

10

100

1000

5000

PO

2(b

ar)

A 5 M⊕ super-Earth can lose up to a few tens of Earth oceans ofwater and build up several thousands of bars of O2, particularlynear the inner HZ of low-mass M dwarfs.

O2 Buildup on Known Exoplanets

Summary & Conclusions

Planets in the HZs of all M dwarfs can lose several Earthoceans of water and build up hundreds to thousands of barsof O2. Both processes threaten the habitability of many ter-restrial planets

Water loss scales with planet mass. Super-Earths lose morewater than Earths because of inhibited oxygen escape

O2 buildup rates also scale with planet mass: ∼ 5 bars/Myron Earths and ∼ 25 bars/Myr on super-Earths. These rates arecontrolled by diffusion

Fast O2 production could overwhelm surface sinks, leading todetectable levels of atmospheric O2. Oxygen may not be areliable biosignature on M dwarf planets

Thank youarXiv:1411.7412