Habitable Exomoons Rory Barnes with lots of help from René Heller Rory Barnes with lots of help from René Heller

Habitable Exomoons are Awesome! Habitable Exomoons are Awesome! Rory Barnes with lots of help from René Heller Rory Barnes with lots of help from René Heller

What is an exomoon?

Exomoons!? You’re gonna talk about habitable exomoons!? You’re gonna talk about habitable exomoons!? We don’t even understand habitable exoplanets! We don’t even understand habitable exoplanets!

The Habitable Zone is about Surface Energy Flux ~300 W/m 2 ~30 W/m 2

Kepler could find an exomoon.

Kepler could find an exomoon. See the exomoon?

Exomoon Transits and Timing Variations Kipping et al. (2012)

Exomoon Transits and Timing Variations Kipping et al. (2012) Direct Detection

Exomoon Transits and Timing Variations Kipping et al. (2012) TTV

Exomoon Habitability I. Formation A. Inside Circumplanetary Disk B. Capture C. Planet Migration II. Radiation A. Starlight B. Reflected Light C. Planetary Thermal Emission D. Eclipses III. Tidal Heating I. Formation A. Inside Circumplanetary Disk B. Capture C. Planet Migration II. Radiation A. Starlight B. Reflected Light C. Planetary Thermal Emission D. Eclipses III. Tidal Heating

The Scale of the Galilean Satellites Io 6 R Jup Europa 10 R Jup Ganymede 16 R Jup Callisto 27 R Jup

Canup & Ward (`06) transform disks into moons Total mass of moons ~10 -4 of planet Earth = Jupiter

Williams, AsBio, submitted

Capture Possibilities Williams, AsBio, submitted

Capture Possibilities Williams, AsBio, submitted

Capture Possibilities Williams, AsBio, submitted Planet has to move to 1 AU!

Planetary Semi-Major Axis (AU) Satellite Semi-Major Axis (AU) Time (Years) Planet Galilean Moons Jupiter’s Radius Namouni (2010)

Planetary Semi-Major Axis (AU) Satellite Semi-Major Axis (AU) Time (Years) Jupiter’s Radius Instabilities due to planet’s shrinking gravitational influence Namouni (2010)

Planetary Semi-Major Axis (AU) Satellite Semi-Major Axis (AU) Time (Years) Jupiter’s Radius Namouni (2010) Moons still safe at 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 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 AU at a ps < 5 R Jup Heller & Barnes (2013)

Reflected Light – Almost Negligible Multiply your HZ boundary by this factor For F star, outer HZ pushed out by ~0.01 AU at a ps < 5 R Jup There is a “Reflection Correction” for habitable exomoons Heller & Barnes (2013)

Thermal Emission Heat 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

The moon could lose its water early. There is a “Cooling Edge” for habitable exomoons

Eclipses Longitude Heller & Barnes (2013)

Eclipses No Eclipses Stellar radiation dominates With eclipse -> sub-planetary point is cold No eclipse -> sub-planetary point is hot Heller & Barnes (2013)

Radiation The HZ applies Reflection Correction Cooling Edge Eclipses could affect local climate

Tidal Heating Caused by gravitational flexing of the crust Source of tectonics on Io, Europa and Enceladus Could be very large for large moons Could also produce exo-Europas Could sustain plate tectonics indefinitely Caused by gravitational flexing of the crust Source of tectonics on Io, Europa and Enceladus Could be very large for large moons Could also produce exo-Europas Could sustain plate tectonics indefinitely

Tidal Greenhouse Tidal/Radiation GreenhouseSuper-Io Tidal Earth No Tidal Heating Earth orbiting Jupiter orbiting the Sun

There is a “Tidal Heating Edge” to exomoon habitability

Conclusions Large exomoons probably rare Kepler can detect, but hard Planets add energy to the classical HZ A reflection correction pushes HZ out (slightly) Thermal radiation causes a cooling edge Eclipses could alter weather A tidal heating edge could sterilize close moons Tidal heating could sustain star-free habitats Large exomoons probably rare Kepler can detect, but hard Planets add energy to the classical HZ A reflection correction pushes HZ out (slightly) Thermal radiation causes a cooling edge Eclipses could alter weather A tidal heating edge could sterilize close moons Tidal heating could sustain star-free habitats

For more info: Heller & Barnes, “Exomoon Habitability constrained by illumination and tidal heating.” AsBio, 13,

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)