Towards a Physical Characterization of Extrasolar Planets Sara Seager Carnegie Institution of Washington Image credit: NASA/JPL-Caltech/R. Hurt (SSC)

Towards a Physical Characterization of Extrasolar Planets

Sara Seager Carnegie Institution of Washington

Image credit: NASA/JPL-Caltech/R. Hurt (SSC)

Towards a Physical Characterization of Extrasolar Planets

Transiting PlanetsModelsDataHD209458bNear FutureEarths

Planet sizes are to scale. Separations are not.

Characterizing extrasolar planets: very different from solar system planets, yet solar system planets are their local analogues

The Solar System

Known Extrasolar Planets

Based on data compiled by J. Schneider

(As of 24 MAY 2005)

Direct Detection Challenge

Nearby M dwarf star with brown dwarf companion

Jupiter would be 10 x closer in 1 million times fainter

Gliese 229 and 229B - Hubble Space Telescope (Kulkarni, Golimowski, NASA)

Star

J

M

VE

Seager 2003

Hot Jupiters

Fp/F* = p Rp2/a2

Fp/F* = Tp/T* Rp2/R*

2

= (R*/2a)1/2[f(1-A)]1/4

Solar System at 10 pc

a

Zone where transit can be seen from

Geometric Transit Probability

P ~ (R*/a)

P(0.05 AU) = 10%P(1 AU) = 0.5%P(5 AU) = 0.1 %

1 radial velocity planet is known to transit its star

Transiting planets allow us to move beyond minimum mass and orbital parameters without direct detection.HD209458b. November

1999. Lynnette Cook.

Venus. Trace Satellite. June 8 2004.Schneider and Pasachoff.

Mercury. Trace Satellite. November 1999.

Transiting Planets

Planet Transit Surveys

Survey thousands of stars simultaneously

Measure drop in starlight due to transiting planet

Huge number of false positives Over 20 groups running planet

transit surveys Require radial velocity followup

to determine mass

Six short-period planets successfully discovered

Two OGLE transiting planets.

Planet Transit Surveys

Survey thousands of stars simultaneously

Measure drop in starlight due to transiting planet

Huge number of false positives Over 20 groups running planet

transit surveys Require radial velocity followup

to determine mass

Six short-period planets successfully discovered

Two OGLE transiting planets.Brown et al. ApJ 2001

Why Transiting Planets?

Planetary bulk composition H-He gas giant? Super Earth? Water world? Rocky planet?

Evolutionary history HD 209458b -- too big! HD 149026 -- too small!

Courtesy Jeremy Richardson

Seager, in preparation

Transiting Planets Transit [Rp/R*]2 ~ 10-2

Transit radius

Emission spectra Tp/T*(Rp/R*)2 ~10-3

Emitting atmosphere ~2/3 Temperature and T

Transmission spectra [atm/R*]2 ~10-4

Upper atmosphere Exosphere (0.05-0.15)

Reflection spectra p[Rp/a]2~10-5

Albedo, phase curve Scattering atmosphere Polarization

Before direct detection

Compelling Questions for Hot Jupiter Atmospheres

Do their atmospheres have ~ solar composition? Or are they metal-rich like the solar system planets? Has atmospheric escape of light gases affected the abundances?

Are the atmospheres in chemical equilibrium? Photoionization and photochemistry?

How is the absorbed stellar energy redistributed in the atmosphere? Hot Jupiters are tidally locked with a permanent day side And are in a radiation forcing regime unlike any planets in the solar

system

Towards a Physical Characterization of Exoplanets

Transiting PlanetsModels DataHD209458bNear FutureEarths

Giant PlanetSpectra

dI(s,,)/ds = -(s,)I(s,,) + j(s,,);(s,) ~ T,P;T,P ~ I(s,,);

1D models

Governed by opacities

“What you put in is what you get out”

Seager, in preparationFKSI Danchi et al.

20 pc

0.05AU0.1 AU0.5 AU

Hot Jupiter Spectra Scattered light at visible

wavelengths Thermal emission at IR

wavelengths Teff = 900 - 1700 K H2O, CO, CH4, Na, K, H2

Rayleigh scattering High T condensate clouds?

MgSiO3, Fe?

See also Barman et al. 2001, Sudarsky et al. 2003, Burrows et al. 2005, Fortney et al 2005, Seager et al. 2005

Seager et al. 2000

Clouds Spectra of every solar system body

with an atmosphere is affected by clouds

For extrasolar planets1D cloud models are being used

Cloud particle formation and subsequent growth based on microphysical timescale arguments

Cloud models have their own uncertainties

Homogenous, globally averaged clouds

Marley et al. 1999

Ackerman & Marley, Cooper et al. 2003; Lunine et al. 2001

Liang, Seager et al. ApJL 2004Liang et al. ApJL 2003

Photochemistry

Jupiter and Saturn have hydrocarbon hazes--mute the albedo and reflection spectrum Hot Jupiters have 104 times more UV flux = more hydrocarbons? Much higher hydrocarbon destruction rate

normal bottleneck reaction is fast less source from CH4

additional consequence: huge H reservoir from H2O

Karkoschka Icarus 1994

Large Range of Parameters

Forward problem is straightforward despite uncertainties

Clouds Particle size distribution, composition, and

shape Fraction of gas condensed Vertical extent of cloud

Seager et al. 2000

Opacities Non-equilibrium chemistry Atmospheric circulation of heat

redistribution Internal luminosities (mass and age

dependent)

Towards a Physical Characterization of Exoplanets

Transiting PlanetsModels DataHD209458bNear FutureEarths

Observations of HD 209458 b

Na (Charbonneau et al. 2001) Lyman-alpha (Vidal-Madjar et al. 2003) C and O* (Vidal-Madjar et al. 2004) CO upper limit (Deming et al. 2005a)

Thermal emission 24 m (Deming et al. 2005)

TrES-1 at 4.5 and 8 m (Charbonneau et al 2005)

CH4 upper limit 3.6 m (Richardson et al. 2003a)

H2O upper limit 2.2 m (Richardson et al. 2003b)

MOST albedo upper limit (Rowe et al. 2005)

Primary Eclipse Secondary Eclipse

Thermal Emission

Detected from two transiting planets during secondary eclipse

Brightness T HD 209458 b 24 m 1130 +/- 150 K TrES-1 4.5 and 8 m 1010 +/- 60 K/1230 +/- 110 K

Opens the door for many more measurements

Deming, Seager, Richardson, Harrington 2005Charbonneau et al. 2005

Richardson, et. al., in prep

Thermal Emission: NASA IRTF 2.2 m Constraint

Secondary eclipse Spectral peak at 2.2 m due

to H2O and CO Data from NASA IRTF

R = 1500 Richardson, Deming, Seager

2003;

Differential measurement only

Upper limit of the band depth on either side of the 2.2 micron peak is 1 x 10-4 or 200 Jy

Transmission Spectra: HST STIS and Keck Probes planetary limb Na (Charbonneau et al. 2002)

CO upper limit (Deming et al. 2005) Consistent with high

clouds Or low Na and CO

abundance

H Lyman alpha (Vidal-Madjar et al. 2003)

Transmission Spectra: HD209458b Exosphere

15% deep Lyman alpha transit 4.3RJ

Requires exospheric T ~ 10,000K!

High exospheric T on solar system giant planets are not well understood (order of magnitude)

EUV heating Upper atmospheric T,

atmospheric expansion, and mass loss are coupled

Escape rates are high but atmosphere is stable over billions of years

No UV followup possible

Secondary Eclipse: Albedo Upper Limit from MOST Microvariability and Oscillations of

STars Space-based photometer for stellar

seismology and exoplanet studies - ppm photometry

“Suitcase” in space 54 kg, 60x60x30 15-cm telescope Single broadband filter 380 ≤ λ ≤ 750 nm

Launch 30 June 2003 Russian Rockot = old ICBM

Cost Can$10M US$7M Euro$6M

PI Jaymie Matthews UBC

Secondary Eclipse: Albedo Upper Limit from MOST Microvariability and Oscillations of STars Space-based photometer for stellar

seismology and exoplanet studies - ppm photometry

“Suitcase” in space 54 kg, 60x60x30 15-cm telescope Single broadband filter 380 ≤ λ ≤ 750 nm

Launch 30 June 2003 Russian Rockot = old ICBM

Cost Can$10M US$7M Euro$6M

PI Jaymie Matthews UBC

MOST Albedo Upper Limit

HD209458 b albedo < 0.25 (1) in the MOST bandpass Jupiter’s albedo is 0.5 HD 209458 b is dark! MOST will reach 0.13 in current observing campaign

Rowe et al. 2005

Towards a Physical Characterization of Exoplanets

Transiting PlanetsModels DataHD209458bNear FutureEarths

HD209458b: Interpretation I

Basic picture is confirmed Thermal emission data

T24 = 1130 +/- 150 K The planet is hot! Implies heated from external

radiation

Transmission spectra data Presence of Na

A wide range of models fit the data

Seager et al. 2005

HD209458b: Interpretation II

Models are required to interpret 24 m data

H2O opacities shape spectrum

T24 is not the equilibrium T T24 = 1130 +/- 150 K A wide range of models match the

24 m flux/T

Teq is a global parameter of model Energy balance, albedo,

circulation regime E.g. Teq = 1700 K implies that AB is

low and absorbed energy is reradiated on the day side only

HD209458b: Interpretation II

Models are required to interpret 24 m data

H2O opacities shape spectrum

T24 is not the equilibrium T T24 = 1130 +/- 150 K A wide range of models match the

24 m flux/T

Teq is a global parameter of model Energy balance, albedo,

circulation regime E.g. Teq = 1700 K implies that AB is

low and absorbed energy is reradiated on the day side only

4/12/1** )]1([2/ AfDRTTeq

HD209458b: Interpretation III Models with strong H2O

absorption ruled out Hottest models are ruled out

Isothermal hot model is ruled out by T24 = 1130 +/- 150 K

Steep T gradient hot model would fit T24 but is ruled out by 2.2m constraint

Coldest models are ruled out High albedo required--very

unusual Cold isothermal model required to

fit T24--doesn’t cross cloud condensation curves

Confirmed by MOST

HD209458b: Interpretation III

Beyond the “standard models” Low H2O abundance would

fit the data C/O > 1 is one way to

reach this See Kuchner and Seager

2005 Solar System giant planets

have 3x solar metallicity Jupiter may have C/O >~ 1,

but spectra look similar to C/O=0.5

HD209458b C/O > 1

HD209458b Interpretation Summary Data for day side

Spitzer 24 microns IRTF 2.2 micron constraint MOST albedo upper limit

A wide range of models fit the data

Confirms our basic understanding of hot Jupiter atmospheric physics

Some models can be ruled out Hot end of temperature range Cold end of temperature range Any model with very strong H2O

absorption at 2.2 microns

Non standard models C/O > 1 could fit the data

Towards a Physical Characterization of Exoplanets

Transiting PlanetsModels DataHD209458bNear FutureEarths

Seager, in preparation

Hot Transiting PlanetsOrbiting Bright Stars

Transit [Rp/R*]2 ~ 10-2

Transit radius

Emission spectra Tp/T*(Rp/R*)2 ~10-3

Emitting atmosphere ~2/3 Temperature and T

Transmission spectra [atm/R*]2 ~10-4

Upper atmosphere Exosphere (0.05-0.15)

Reflection spectra p[Rp/a]2~10-5

Albedo, phase curve Scattering atmosphere

Pushing the limits of telescope instrumentation

Near Future Data

from Seager et al. 2005

Near Future Data

New transiting planets orbiting bright stars HD 209458 b

Spitzer thermal emission 3.6, 4.5, 8, 10 microns HST/STIS primary transit MOST albedo limit HST/NICMOS: H2O

Spitzer 3 transiting planets orbiting bright stars 6 non-transiting planets

SOFIA, Kepler, JWST

Cho et al. ApJL 2003

Tracer pvTemp

Hot Super Earths New Super Earths

M=7.5 ME, P=1.9d, Rivera et al. 2005

Msini =14 ME, P=9.5d, Santos et al. 2004

M=18ME, P=2.8d, 4-planet system,McArthur et al. 2004

Msini=21ME, P=2.6d, M star, Butler et al. 2004

Solar System planet masses Uranus: 17.2 ME

Neptune: 14.6 ME

Jupiter: 318 ME

Saturn 95 ME

What is the nature of these planets??

An Artist's depiction of the new planet orbiting Gliese 436. Credit: NASA/JPL.

Credit: NASA/JPL.

Towards a Physical Characterization of Exoplanets

Transiting PlanetsModels DataHD209458bNear FutureEarths

Are We Alone?

Are there Earth-like planets?

Are they common?

Do they harbor life?

Evolution of the planetary atmosphere is determined by many factors:• atmospheric escape• gas-surface reactions• spectral energy distribution of host star• geologic activity• initial volatile inventory• active biology• atmospheric circulation will drive climate

But, Venus and Earth look the same to Kepler and SIM

Terrestrial Planets

• Find and characterize Earth-like planets around nearby stars

• Need to null out parent star by 106 to 1010

• Look for biomarker gases• Launch date:

• 2014 TPF-C • 2019 TPF-I

mid-IR spectra

NASA’s Terrestrial Planet Finder

Woolf , Smith, Traub, Jucks, ApJ, 2002 Modeling 1D Earth spectra is made

easier by the right input data!

Earth as an Extrasolar Planet

• rotational period• weather• presence of oceans • reconstruct map?

Ford, Seager, & Turner, Nature 2001

High contrast between land and ocean causes changes in flux

Earth as an Extrasolar Planet

S. Seager

Institute for Advanced Study, Princeton, July 2002

Vegetation as a Surface Biomarker

S. Seager

S. Seager

Vegetation as a Surface Biomarker

Surface Biosignature Chlorophyll causes strong absorption

blueward of 0.7 m Light scattering in air gaps between water-

filled plant cells causes strong red reflectance

Plants absorb energy at short wavelengths for photosynthesis; reflect and transmit radiation at long wavelengths for thermal balance

Reflection favored over transmission? CO2 more accessible to plants with airgaps

Photosynthetic plants cause a global spectral signature even though Earth is not completely plant covered

Clark 1993; Seager et al. 2004

Woolf , Smith, Traub, Jucks, ApJ, 2002 Modeling 1D Earth spectra is made

easier by the right input data!

Earth as an Extrasolar Planet

Beyond Earth PaleoEarth

Large amount of CH4? Snowball Earth Pangea Early faint sun paradox

Sun was 30% cooler 4 billion years ago

CH4? NH3? CO2?

Varying orbital and physical planet parameters

Rotation rates, obliquities, eccentricities Surface temperatures? Cloud cover

fractions and patterns? Spectral signatures?

Kristine BryanPangea: 225 million years ago

Cho and Seager in prep

Towards a Physical Characterization ofExtrasolar Planets

Transiting planet atmospheres can be characterized without direct detection

Models are maturing, ideas beyond the solar abundance, chemical equilibrium models are being considered

A growing data set for HD209458b

Extrasolar Planet Discovery TimelinePast• 1992 pulsar planet• 09/1995 Doppler extrasolar planet discoveries take off• 11/1999 extrasolar planet transit• 11/2001 extrasolar planet atmosphere• 1/2003 planet discovered with transit method• 4/2004 planet discovered with microlensing method

Present• 2005 transit planet discoveries take off• 2005 transit planet day side temperature• 2005 hot Jupiter albedoFuture• 2008 hundreds of hot Jupiter illumination phase curves• 2011 Frequency of Earths and super earths• 2016 First directly detected Earth-like planet• 2025 Unthinkable diversity of planetary systems!