Habitable Exomoons

Rory Barnes

with lots of help fromRené Heller

Habitable Exomoonsare Awesome!

Rory Barnes

with lots of help fromRené Heller

What is an exomoon?

Exomoons!?

You’re gonna talk abouthabitable exomoons!?

We don’t even understandhabitable exoplanets!

The Habitable Zone is about Surface Energy Flux

~300 W/m2

~30 W/m2

Kepler couldfind an exomoon.

Kepler couldfind an exomoon.

See theexomoon?

Exomoon Transits and Timing Variations

Kipping et al. (2012)

Exomoon Transits and Timing Variations

Kipping et al. (2012)

DirectDetection

Exomoon Transits and Timing Variations

Kipping et al. (2012)

TTV

Exomoon HabitabilityI. Formation A. Inside Circumplanetary Disk B. Capture C. Planet MigrationII. Radiation A. Starlight B. Reflected Light C. Planetary Thermal Emission D. EclipsesIII. Tidal Heating

The Scale of the Galilean Satellites

Io6 RJup

Europa10 RJup

Ganymede16 RJup

Callisto27 RJup

Canup & Ward (`06)transform disks into

moons

Total mass ofmoons ~10-4 of planet

Earth = 0.003 Jupiter

Williams, AsBio, submitted

Capture Possibilities

Williams, AsBio, submitted

Capture Possibilities

Williams, AsBio, submitted

Capture Possibilities

Williams, AsBio, submitted

Planet has to moveto 1 AU!

Plan

etar

y Se

mi-M

ajor

Axi

s (AU

)

Sate

llite

Sem

i-Maj

or A

xis (

AU)

Time (Years)

Planet

Galilean Moons

Jupiter’s Radius

Namouni (2010)

Plan

etar

y Se

mi-M

ajor

Axi

s (AU

)

Sate

llite

Sem

i-Maj

or A

xis (

AU)

Time (Years)

Jupiter’s Radius

Instabilities due to planet’sshrinkinggravitational influence

Namouni (2010)

Plan

etar

y Se

mi-M

ajor

Axi

s (AU

)

Sate

llite

Sem

i-Maj

or A

xis (

AU)

Time (Years)

Jupiter’s Radius

Namouni (2010)

Moons still safeat 1 AU

Exomoon Formation/Composition

May form with planet (<10 Myr)- Icy worlds (volatile rich)- But small

May be captured- Requires precise encounters- Captured body must have water- Terrestrial planets need ~100 Myr to form

Moon must survive migration to HZ

The Radiation Environment of Exomoons

Heller & Barnes (2013)

Starlight Only – The Habitable Zone

Reflected Light – Almost Negligible

Multiply your HZ boundary by this factor

For F star, outer HZ pushed out by ~0.01 AUat aps < 5 RJup

Heller & Barnes (2013)

Reflected Light – Almost Negligible

Multiply your HZ boundary by this factor

For F star, outer HZ pushed out by ~0.01 AUat aps < 5 RJup

There is a “Reflection Correction”for habitable exomoons

Heller & Barnes (2013)

Thermal EmissionHeat from star (almost negligible)Heat from Contraction (important early)

Longitude Heller & Barnes (2013)

Planets Cool with Time*

* adopted from Baraffe+ (1997, 2003)

A Moon at Europa’s Orbit

Run. Grnhs Limit

Time in a Runaway Greenhouse

Time in a Runaway Greenhouse

The moon could lose itswater early.

There is a “Cooling Edge”for habitable exomoons

Eclipses

LongitudeHeller & Barnes (2013)

EclipsesEclipses No Eclipses

Stellar radiation dominatesWith eclipse -> sub-planetary point is coldNo eclipse -> sub-planetary point is hot

Heller & Barnes (2013)

Radiation

The HZ appliesReflection CorrectionCooling EdgeEclipses could affect local climate

Tidal Heating

Caused by gravitational flexing of the crustSource of tectonics on Io, Europa and EnceladusCould be very large for large moonsCould also produce exo-EuropasCould sustain plate tectonics indefinitely

Tidal GreenhouseTidal/RadiationGreenhouseSuper-IoTidal EarthNo Tidal Heating

Earth orbiting Jupiter orbiting the Sun

Earth orbiting Jupiter orbiting the Sun

Earth orbiting Jupiter orbiting the Sun

Earth orbiting Jupiter orbiting the Sun

There is a “Tidal Heating Edge”to exomoon habitability

Conclusions

Large exomoons probably rareKepler can detect, but hardPlanets add energy to the classical HZA reflection correction pushes HZ out (slightly)Thermal radiation causes a cooling edgeEclipses could alter weatherA tidal heating edge could sterilize close moonsTidal heating could sustain star-free habitats

For more info:

Heller & Barnes, 2013.“Exomoon Habitabilityconstrained by illumination andtidal heating.” AsBio, 13, 18-46.

Tidally Heated to Habitable?

Reynolds, McKay & Kasting (1987)

Radiative + Tidal HZs

Reynolds, McKay & Kasting (1987)

Orbits After Capture

Porter & Grundy (2011)

Reflected and Thermal Light (“inplanation”)

Heller & Barnes (2013)

Heller & Barnes (2013)

Heller & Barnes (2013)

Heller & Barnes (2013)

Heller & Barnes (2013)

Heller & Barnes (2013)

Heller & Barnes (2013)