The nature and impact of obliquity evolution on the habitability of

The nature and impact of obliquity evolution on the
habitability of Earth-like exoplanets
Astrophysics of Planetary Habitability
Vienna, Austria
February 12, 2016
Russell Deitrick*
Rory Barnes*
Cecilia Bitz (UW Atmospheric Sciences)
Tom Quinn*
John Armstrong (Weber State University)
Victoria Meadows*
Benjamin Charnay*
*UW Astronomy
photo%by%Stephen%Hudson%(from%Wikipedia)
How do sibling planets affect
obliquity and habitability?
Alter orbit, obliquity
At the outer edge of the
habitable zone:
Ice ages & snowball states
hellocharlie.com
Earth’s(ice(ages(likely(
linked(to(orbital(and(
The%nature%and%impact%of%obliquity%evolution%on%the%habitability%of%Earth>like%
exoplanets%
obliquity(variations
Astrophysics%of%Planetary%Habitability%
Vienna,%Austria%
February%12,%2016%
3
Spin axis
Obliquity
Santa(Claus?
Equatorial bulge
Equ
ato
r
Things(that(affect(obliquity:(
Tidal(dissipation(
Torques(on(the(equatorial(bulge(
(star,(moons)(
Planetary(perturbers(
(changes(in(ecc,(inc)(
Rotation(rate(
Shape(of(object
Orbital plane
4
obliquity
inclination
eccentricity
long. perihelion
Eccentricity, inclination variations (due to other
planets) can strongly affect obliquity
Time([Myr]
Time([Myr]
5
Armstrong et al 2014.
long. perihelion
Eccentricity, inclination
planets) can strongly affect obliquity
Higher(eccentricity,(obliquity(
tend(to(extend(outer(habitable(
zone(distance((Spiegel(et(al(2009,(
Dressing(et(al(2010)(
Variations(in(ecc,(obl(extend(HZ(
even(further((a(few(to(20%)
obliquity
inclination
eccentricity
light blue = mean values of ecc, obl
variations
(due
to other
green = including
variability
Time([Myr]
Time([Myr]
6
Armstrong et al 2014.
long. perihelion
Eccentricity, inclination
planets) can strongly affect obliquity
Higher(eccentricity,(obliquity(
tend(to(extend(outer(habitable(
zone(distance((Spiegel(et(al(2009,(
Dressing(et(al(2010)(
Variations(in(ecc,(obl(extend(HZ(
even(further((a(few(to(20%)
However:((
Limited(to(a(small(number(of(simulations(due(to(
computation(time((
Climate(model(had(low(sensitivity(to(temperature
obliquity
inclination
eccentricity
light blue = mean values of ecc, obl
variations
(due
to other
green = including
variability
Time([Myr]
Time([Myr]
6
Armstrong et al 2014.
We need a fast, fully-coupled
model to explore parameter space
This(work
Orbital(
dynamics
Obliquity(
evolution
Climate
Other(
models:(
Tidal,(
Atmospheric(
escape,(
Geothermal
…(
VPLANET
Barnes%et%al%(in%prep)
7
4th order secular model
Gas giant
Planets as
“rings” of mass
Earth mass planet
Pericenters
Nodes
No need to resolve
orbital timescales
see%Murray%&%Dermott%1999
8
Obliquity model
Kinoshita%1975,%1977
Planets are oblate spheroids
"OblateSpheroid"%from%Wikipedia
"Spheroid"%by%Peter%Mercator
9
Spin axis
Obliquity model
Obliquity
Stellar torque on
equatorial bulge
(increases with
eccentricity)
Equ
Motion of
orbital plane
ato
r
Orbital plane
*We are currently focusing
on moonless planets…
10
▪ Star = 0.97 M⊙
au
Habitable Zone
au
11
Obliquity amplitude increases with
inclination
High(
▪ 0.97 M⊙
amplitude
▪ 1.5 M⊕
@0.19 au
▪ 1.8 M⊕
(in HZ)
Low(
amplitude
▪ Star = 0.98 M⊙
au
Habitable Zone
au
13
A more complex system
▪ 0.98 M⊙
High(
amplitude
▪ 19 M⊕
@0.13 au
▪ 1 M⊕
The
“arc
”
(in HZ)
▪ 1.5 MJ
@3.8 au
Low(
amplitude
A more complex system
▪ 0.98 M⊙
High(
amplitude
▪ 19 M⊕
@0.13 au
The
▪ 1 M⊕
“arc
”
(in HZ)
▪ 1.5 MJ
@3.8 au
Caused by secular resonance
(see e.g. Laskar et al. 1992, Pilat-Lohinger et al. 2008, Brasser et al. 2014)
Low(
amplitude
A more complex system
▪ 0.98 M⊙
High(
amplitude
▪ 19 M⊕
@0.13 au
The
▪ 1 M⊕
“arc
”
(in HZ)
▪ 1.5 MJ
@3.8 au
Caused by secular resonance
(see e.g. Laskar et al. 1992, Pilat-Lohinger et al. 2008, Brasser et al. 2014)
Axial precession rate matches slow orbital frequencies Low(
amplitude
(Earth-mass and 1.5 Jupiter mass planets)
A more complex system
▪ 0.98 M⊙
High(
amplitude
▪ 19 M⊕
@0.13 au
The
“arc
”
▪ 1 MSystem
⊕
architecture affects obliquity in
(in HZ)
complex
ways -> affects climate/habitability
▪ 1.5 MJ
@3.8 au
Low(
amplitude
In the arc (secular resonance in
effect)
15
Insolation
VPLanet’s
EBM
North%&%Coakley%1979
*%Snow%*/Melt
Ice flow
Land
T1
T2
…
Heat flow
Ocean
T1
Pole
Ice?
T2
…
Equator
Pole
16
ice(ages(during(low(obliquity(times
warm(during(high(
obliquity(times
Obliquity
17
Another look at parameter
space
18
Another look at parameter
space
18
Revising the
habitable zone
Without perturbations
average(albedo
Ice
Dynamics,%orbital%parameters%
may%influence%predictions/
interpretation%of%spectra
With perturbations
average(albedo
Snowball state
Snowball
Snowball
Outer edge of HZ can change by 2-3%
Similar to (or slightly smaller than) that found by Armstrong et al 201419
Secular resonances abound
20
Secular resonances abound
Vary%
rotation,%
obliquity
A(complex(set(of(factors(can(contribute(to(secular(resonances:(
orbital(parameters,(rotation(rate,(planet(shape
20
Summary
• Climate/surface(conditions(may(be(influenced(by(
system(architecture(in(complex(ways((e.g.(secular(
resonances)(
• Dynamics can modify the habitable zone
boundary (roughly consistent with
Armstrong et al. 2014)
• Orbital/obliquity evolution is a key aspect
of planetary habitability
Danke!
• My co-authors: Rory Barnes, Cecilia
Bitz, Tom Quinn, John Armstrong,
Victoria Meadows, & Benjamin Charnay
• Virtual Planetary Laboratory and NASA
Astrobiology Institute
• SOC and LOC
• …Everyone here today!
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