Habitable Exomoons
Rory Barnes
with lots of help from
René Heller
Habitable Exomoons
are Awesome!
Rory Barnes
with lots of help from
René Heller
What is an exomoon?
Exomoons!?
You’re gonna
talk about
habitable exomoons!?
We don’t even understand
habitable exoplanets!
The Habitable Zone is about Surface Energy Flux
~300 W/m2
~30 W/m2
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
Direct
Detection
Kipping et al. (2012)
Exomoon Transits and Timing Variations
TTV
Kipping et al. (2012)
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
The Scale of the Galilean Satellites
Europa
10 RJup
Io
6 RJup
Callisto
27 RJup
Ganymede
16 RJup
Canup & Ward (`06)
transform disks into
moons
Total mass of
moons ~10-4 of planet
Earth = 0.003 Jupiter
Williams, AsBio, submitted
Capture Possibilities
Williams, AsBio, submitted
Capture Possibilities
Williams, AsBio, submitted
Capture Possibilities
Planet has to move
to 1 AU!
Williams, AsBio, submitted
Planetary Semi-Major Axis (AU)
Jupiter’s Radius
Time (Years)
Satellite Semi-Major Axis (AU)
Galilean Moons
Namouni (2010)
Jupiter’s Radius
Time (Years)
Satellite Semi-Major Axis (AU)
Planetary Semi-Major Axis (AU)
Instabilities
due to planet’s
shrinking
gravitational
influence
Namouni (2010)
Jupiter’s Radius
Time (Years)
Satellite Semi-Major Axis (AU)
Planetary Semi-Major Axis (AU)
Moons
still safe
at 1 AU
Namouni (2010)
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 AU
at aps < 5 RJup
Heller & Barnes (2013)
Reflected Light – Almost Negligible
Multiply your HZ
boundary by this factor
There is a “Reflection Correction”
For F star, outer HZ
for habitable exomoons
pushed out by ~0.01 AU
at aps < 5 RJup
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
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
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
Earth orbiting Jupiter orbiting the Sun
Tidal Greenhouse
Tidal/Radiation
Greenhouse
Super-Io
Tidal Earth
No Tidal Heating
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 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, 2013.
“Exomoon Habitability
constrained by
illumination and
tidal 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)