Habitability Index for Transiting Exoplanetsvplapps.astro.washington.edu/vpltools/hite/HITETutorial.pdfWhich Planet to Observe to Find Life? Transit Depth Orbital Period Transit Duration

Habitability Index for Transiting Exoplanets

Rory Barnes, Nicole Evans, Victoria S. Meadows

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

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??

Transit Observables: Orbital Period Transit Duration Transit Depth Times of Transit Impact Parameter*

Transit Observables: Orbital Period Transit Duration Transit Depth Times of Transit Impact Parameter*

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*Not usually measured for Kepler

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

Which Planet to Observe to Find Life?

Liquid

Water

Possib

le

Surfac

e

Kasting et al. (1993)

Yes!No

No

Which Planet to Observe to Find Life?

Kasting et al. (1993)

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)

No

Which Planet to Observe to Find Life?

The HZ Limits are ∝ the Outgoing Radiation Flux

Kasting et al. (1993)

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)

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)

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?

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”

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

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)

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