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Habitability Index for Transiting Exoplanets
Rory Barnes, Nicole Evans, Victoria S. Meadows
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Find nearby transiting exoplanets with TESS
Maybe interesting atmospheric molecules with JWST
Biosignatures with some mythical 12-20m class spacecraft
NASA’s Plan to Find Life on Exoplanets
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We could have 100s of planet candidates of bright, nearby stars
to observe
But only a few potentially habitable planets can be observed
with JWST
How do we prioritize??
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Transit Observables: Orbital Period Transit Duration Transit
Depth Times of Transit Impact Parameter*
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Transit Observables: Orbital Period Transit Duration Transit
Depth Times of Transit Impact Parameter*
}
*Not usually measured for Kepler
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Transit Observables: Orbital Period Transit Duration Transit
Depth Times of Transit Impact Parameter*
By some other means, we must determine: Stellar Mass Stellar
Radius Stellar Temperature
From these parameters we can calculate planetary properties
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Which Planet to Observe to Find Life?
Liquid
Water
Possib
le
Surfac
e
Kasting et al. (1993)
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Yes!No
No
Which Planet to Observe to Find Life?
Kasting et al. (1993)
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Which Planet to Observe to Find Life?
The Runaway Greenhouse ~300 W/m2
The HZ Limits are ∝ the Outgoing Radiation Flux
The Maximum Greenhouse
65 W/m2
Kasting et al. (1993)
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No
Which Planet to Observe to Find Life?
The HZ Limits are ∝ the Outgoing Radiation Flux
Kasting et al. (1993)
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No
Stellar Luminosity Albedo
EccentricitySemi-major axis
The HZ Limits are ∝ the Outgoing Radiation Flux
Which Planet to Observe to Find Life?
Kasting et al. (1993)
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No
Stellar Luminosity Albedo
EccentricitySemi-major axisNote Eccentricity-Albedo
Degeneracy
Which Planet to Observe to Find Life?
The HZ Limits are ∝ the Outgoing Radiation Flux
Kasting et al. (1993)
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Let us define a “habitable” exoplanet as one for which:
The emitted flux lies between Fmax = 300 W/m2 and Fmin = 65
W/m2
AND
Is terrestrial-like.
What is the likelihood that these conditions are met?
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Which Planet to Observe to Find Life?
Transit Depth Orbital Period
Transit Duration Impact Parameter*
Stellar Radius Stellar Mass Stellar Temp( )( )Luminosity
Semi-major Axis Eccentricity Albedo Density H
Where H = the likelihood of habitability “The Habitability Index
for Transiting Exoplanets”
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Luminosity from Radius, Temp., Stefan-Boltzmann Law Semi-major
axis from Period and Kepler’s 3rd Law Planetary Density poorly
constrained ->
“Rockiness” only assessed probabilistically
Eccentricity can be constrained by - Minimum: Duration, Period,
Stellar Radius, Mass
(difficult for Kepler; Impact Parameter helps a lot) - Maximum:
Orbital stability (if multi-planet)
Albedo is very difficult to constrained
Calculate H: Scan through permitted e-a parameter space [0,0.8]
H = (Fraction with right Flux) * (Probability of Rockiness)
Comparative Habitability of Transiting Exoplanets
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Comparative Habitability of Kepler planets
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5Scirc
0.0
0.2
0.4
0.6
0.8
1.0H
abita
bilit
y In
dex
Constrained by Maximum FluxConstrained by Minimum Flux
Both Limits Constrain
1 RE
1.75 RE
2.5 RE
E
V
M
Incident Radiation (circular orbit)
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Time to Get Your Hands Dirty!
Download HITE at:
http://vplapps.astro.washington.edu/vpltools.html Also available
at: https://github.com/RoryBarnes/HITE
Compile with “gcc -o hite hite.c -lm”
Try the two cases provide (earth.in, kepler452b.in)
Study Questions: Why doesn’t H=1.0 for Earth? Is Kepler-452 b
more habitable than Earth?
Now try it online: http://vplapps.astro.washington.edu/hite
