Potential for Characterization of Transiting Planets with JWST

Nick Cowan (Northwestern)

Study Analysis Group 10 (SAG-X)1. What is the full diversity of planet properties needed to

characterize and understand the climate of short-period exoplanets?

2. Which measurement suites and how much observing time are needed to characterize the climate of transiting planets?

3. Will JWST be able to characterize the atmospheres of transiting terrestrial planets?

4. Which critical measurements will be too expensive or inaccessible to JWST, and can these be obtained with planned observatories?

SAG-X Participants (so far)Daniel Angerhausen (RPI), Natasha Batalha (Penn State), Adrian Belu (Bordeaux), Knicole Colon (Lehigh), Bill Danchi (Goddard), Pieter Deroo (JPL), Jonathan Fortney (UCSC), Scott Gaudi (Ohio State), Tom Greene (Ames), Matt Greenhouse (Goddard), Joe Harrington (UCF), Lisa Kaltenegger (MPIA), Nikole Lewis (MIT), Chuck Lillie (Lillie Consulting), Mercedes Lopez-Morales (CfA), Avi Mandell (Goddard), Emily Rauscher (Princeton), Aki Roberge (Goddard), Franck Selsis (Bordeaux), Kevin Stevenson (Chicago), Mark Swain (JPL)

1D Climate CartoonHe

ight

Temperature Tsurf

Tropopause

Tstrat

ΓTroposphere

Stratosphere

Thermal Emission Probes Near Tropopause

Reflected Light Probes Near Surface

1D Climate Cartoon

Temperature

Heig

ht

Increase Insolationand/or Decrease Albedo

1D Climate Cartoon

Temperature

Heig

ht

Increase Longwave Opacity

1D Climate Cartoon

Temperature

Heig

ht

Decrease SpecificHeat Capacity

Advection

2D Climate Cartoon(eg: height + latitude)

Temperature

Heig

ht

EquatorPole

To learn from exoplanets, we need to measure:• Planetary Albedo• Albedo Gradients• Emitting Temperature• Temperature Gradients (vertical and horizontal)

Unfortunately we don’t have robust predictive models for:• Clouds• Atmospheric Loss• Photochemistry• Geochemical Cycling • Wind Velocities

Predicting vs Measuring Planetary Climate

Model Inputs• Insolation• Eccentricity• Obliquity• Surface Albedo• Greenhouse Gases• Specific Heat Capacity• Surface Gravity• Surface Pressure• Thermal Inertia

Model Outputs• Emitting Temperature• Temperature Gradients• Diurnal Response• Seasonal Response• Cloudiness• Surface Temperature• Precipitation

TransitPhase Variations

Archetypal Short-Period Planets

Hot Earth (CoRoT-7b, Kepler 10b, α Cen Bb)

Temperate Super-Earth (HD 85512 b, GJ 163 c)

Warm Neptune (GJ 1214b, GJ 436b)

Hot Jupiter (HD 209458b, HD 189733b, WASP-12b)

Transit

Thermal Light CurveBrightness

Time

100%

99%Transit

Transit Depth = (Rp/R*)2

Some Transit Depths

Hot Jupiter: 8.8E-3

Hot Earth: 7.3E-5

Temperate Super-Earth: 7.3E-3

Warm Neptune: 2.7E-2

Thermal Light CurveBrightness

Time

100%

99%Transit

Transit depth is different at different wavelengths

Annulus/Stellar Area = 2RpH/R*2

(where scale height H = kBT/μg)

Some Transit Spectral Features

Hot Jupiter: 1.2E-4

Hot Earth: 2.0E-6

Temperate Super-Earth: 6.5E-6

Warm Neptune: 2.4E-4

Eclipse

Thermal Light CurveBrightness

Time

100.3%

99%Transit

Eclipse100%

Eclipse Depth ≈ (Rp/R*)2 (Tday/T*)

Star + PlanetStar

Some Eclipse Depths

Hot Jupiter: 4.0E-3

Hot Earth: 3.3E-5

Temperate Super-Earth: 7.0E-4

Warm Neptune: 8.1E-3

Thermal Light CurveBrightness

Time

100.3%

99%

Transit100%

Star + PlanetStar

Eclipse depth is different at different wavelengths

Eclipse Spectrum ≈ (Rp/R*)2 (Tsurf - Ttrop)/T*

Thermal Phases

Thermal Light Curve

Brightness

Time

100%

TransitEclipse

100.3%

Star + PlanetStar

100.1%

Phase Amplitude ≈ (Rp/R*)2 (Tday - Tnight)/T*

Eclipse Spectroscopy vs. Phase Variations

• Differences in temperature – Vertical (33-200 K)– Zonal (60-2000 K)

• Spectroscopy has to achieve S/N with relatively narrow bandwidth (R>10)

• Phase variations require long-term stability

Predicting vs Measuring Planetary Climate

Model Inputs• Insolation• Eccentricity• Obliquity• Surface Albedo• Greenhouse Gases• Specific Heat Capacity• Surface Gravity• Surface Pressure• Thermal Inertia

Model Outputs• Emitting Temperature• Temperature Gradients• Diurnal Response• Seasonal Response• Cloudiness• Surface Temperature• Precipitation