Lecture 4 2:Lecture 4.2: The Search for Another EarthThe ...

Lecture 4 2:Lecture 4 2:Lecture 4.2:Lecture 4.2:

The Search for Another EarthThe Search for Another Earth::

Space Projects: TPF Darwin etcSpace Projects: TPF Darwin etcSpace Projects: TPF, Darwin, etc.Space Projects: TPF, Darwin, etc.

В. Г. Турышев В. Г. Турышев Jet Propulsion Laboratory, California Institute of Technology

4800 Oak Grove Drive, Pasadena, CA 91009 USAГосударственный Астрономический Институт им. П.К. Штернберга

Университетский проспект, дом 13, Москва, 119991 Россия

Курс Лекций: «Современные Проблемы Астрономии»для студентов Государственного Астрономического Института им. П.К. Штернберга

7 февраля – 23 мая 2011

The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH

Overview

• Space projects searching for life:– NASA Navigator program:

• Experimental signatures of life

• Looking back at the EarthLooking back at the Earth

• Strategies for searching for life

T t i l Pl t Fi d• Terrestrial Planet Finder– science, various design options, next steps


Big Questions

• Are there Earth-like planets around nearby stars?

• Are there signs of life on these planets?

SIM: TPF ISIM:Space Interferometer

Mission TPF-C/O:Terrestrial Planet Finder Coronagraph

TPF-I:Terrestrial Planet Finder

Interferometerg p

Or Occulter


TPF CTPF C TPF C & TPF ITPF ITPF ISIM

Habitability of an Earth-like Planet

Molecularcolumn

Mass IR flux IR color Vis flux Vis & IR spectra

TPF-CTPF-C TPF-C & TPF-ITPF-ITPF-ISIMm

easu

red

Eff temp.

Radius Albedo Greenhouse

m

Radius Albedowarming

Density Surface Surface & cloud

deriv

ed

yof planet gravity

Surface & cloud reflectances

Lapse rateof atmos

Surfacepressure

Scale heightof atmos.

of atmos.SurfaceTemp.

Type ofplanet

Likelihood ofl t t t i & Presence

Cumulus,cirrus, ice,im

plie

d

planetplate tectonics &atmos retention

Presence of H2O

rock, sand,water

i


Variability On An Earth-like Planet

Visible spectra

Near-IRspectra

Infraredspectra

TPF-C TPF-ITPF-C

erve

d

Orbitaleccentricity

SIM

p p p

W

obse y

Vegetationvariation

deriv

ed

Water, oxygen,carbon dioxide

variations

Cloudvariation

Surfacespectrumvariation

Temperaturevariations

d

Cloud heightvariations

ObliquityThermal time constof atmosSeasonsLength

of day

impl

ied

Mass ofatmosphere

Large-scaleweatherpatterns

Continents,oceans, ice

areas patternsareas


But What is a Habitable Planet?

• Not too bigA oid accreting– Avoid accreting material to become gas giant

• Not too small

– Lose atmosphere

• Not too hot or too cold

– No liquid water

N t t l t t• Not too close to star

– Avoid tidal lock


Finding Terrestrial Planets

• Detecting light from planets beyond solar system is hard:

Earth is 10 billion/million times fainter than Sun

– Planet signal is weak but detectable (few photons/sec/m2)

10-10

visible

photons/sec/m

– Star emits million to billion more than planet

10-6

infrared– Planet within 1 AU of star

– Dust in target solar system 300 brighter than planet300 brighter than planet

• Finding a firefly next to a searchlight on a foggy night


Four Hard Things About TPF

• Sensitivity (relatively easy):– Detection in hours spectroscopy in days.

Integration time (distance/diameter)4– Integration time (distance/diameter)4

– Need 12 m2 of collecting area (>4 m) for star at ~10 pc• Angular resolution (hard):

– 100 mas is enough to see ~25 stars, but requires >4 m coronagraph or >20 m interferometer

– Baseline/aperture distance• Starlight suppression (hard to very hard):

– 10-4 to 10-6 in the mid-IR– 10-8 to 10-10 in the visible/near-IR

• Solar neighborhood is sparsely populated:– Fraction of stars with Earths (in habitable zone) unknown– Unknown how far we need to look to ensure successUnknown how far we need to look to ensure success– Surveying substantial number of stars means looking to ~15 pc


Signatures of Life

• Oxygen or its proxy ozone is most reliable biomarker

– Ozone easier to detect at low Oxygen concentrations but is a poor i di t f tit f Oindicator of quantity of Oxygen

• Liquid water on a planet’s surface is considered essential to life.

C b di id i di t t h d id ti t t t i l• Carbon dioxide indicates an atmosphere and oxidation state typical of terrestrial planet.

• Abundant Methane can have a biological sourceg

– Non-biological sources might be confusing

• Find an atmosphere out of equilibrium

• Expect the unexpected


Mars Odyssey Looks Back at Earth

9

10

Water

7

8Water

4

5

6

Intensity

Carbon

2

3

4

Ozone

CarbonDioxide

W t

0

1

0 5 10 15 20 25 30 35 40 45 50

Water

0 5 10 15 20 25 30 35 40 45 50

Wavelength ( m)


Lab demo, with planets added

Jupiter½ Jupiter

D Earth

500 D-shaped images of dark hole, Rotated to sample annulus on sky,

Planets added, Common speckles removedCommon speckles removed,

Planets pop out of noise.

Shows that Earth could have been detected.

Trauger & Traub, Nature, April 2007


Larger Telescope …. More Stars

2 m

mas

)

56 stars 4 m460 stars

8 m3100 stars

Z ce

nter

(m

3100 stars16 m

18,000 stars

HZ

Database: http://nsted.ipac.caltech.edu/NStED/

Assumes IWA = 2 /D,& planet in center of HZ


Earth Spectra through Geologic Time

Present Earth: age = 4.5 GyrPresent Earth: age 4.5 Gyr

Early Earth: age = 1 GyrEarly Earth: age = 1 Gyr

Labeled features can be detected with an 8-m diameter telescopeand coronagraph in less than one day.


Planets found so far…

Planets found so far (HUGE ones!)

Planets we hope to find (Earth-size!)Planets we hope to find (Earth-size!)


Main Idea Behind Life-Searching Projects

• We study Spectra!:• We study Spectra!:– If we can image a planet, we can measure its spectrum,

characterize its surface,and search for life.


Color Gives a First Impression of a Planet

Solar system planets have colors that label them by type.

Blue (0.4‐0.6 m), Green (0.6‐0.8 m), Red (0.8‐1.0 m)


Earth Spectra

Thermal infrared VisibleThermal infrared Visible


Visible Earthshine Spectrum

Chl h llChlorophyll720 nm edge

W H ORayleigh

Ozone O3 O O

Water H2O

Ozone O3 Oxygen O2

Marked features show habitability& signs of life

• Observed Earthshine, reflected from dark side of moon.

Woolf, Smith, Traub, & Jucks, ApJ 574, p.430, 2002


Near-IR Earthshine Spectrum

Integrated‐All features show

Earth spectrum habitability& signs of life:H O OH2O, O2,CO2, CH4,cirrus, cumulus

Individual gas species

Ref.: Turnbull et al., ApJ, June 2006


Far-infrared spectrum of Earth: validation

• Integrated light of Earth, seen by TES enroute to Mars.

• CO2, O3, H2O dominate.

• CH4 N2O ~hidden by 6-CH4, N2O hidden by 6micron water.


Candidate space missions for 2010-2020

• Terrestrial Planet Finder Coronagraph (TPF-C)– Candidate space mission in coming decade (2020s?)– Goal: Image Earth-like exoplanets and measure their visible

spectra

T t i l Pl t Fi d O lt (TPF O)• Terrestrial Planet Finder Occulter (TPF-O)– Candidate space mission in coming decade (2020s?)– Goal: Image Earth-like exoplanets and measure their visibleGoal: Image Earth like exoplanets and measure their visible

spectra

• Terrestrial Planet Finder Interferometer (TPF-I)– Candidate space mission in following decade (2030s?)– Goal: Image Earth-like exoplanets and measure their infrared

spectraspectra


TPF-Interferometer

1. star, planet, & zodi,seen as a single (not resolved)

blob by each telescope

2. four collector telescopes& one combiner plus delay lines& one combiner, plus delay lines,

all free-flying

4. arrayrotates

Single wavelength

3. transmission pattern, times sky image,seen as a single blob;

total amount of light received is noted

5. measured total light level,as array rotates a full turn

(bumps are the planet)

The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTHIR Interferometer

Goal Earth at 10 pc TimePlanet? R=3/SNR=5 2.0 hourAtmosphere? R=20/SNR=10 2.3 day

CO H OCO2, H2O Habitable? R=20/SNR=25 15.1 dayO3, CH4

• Interferometer with cooled two to four• Interferometer with cooled two to four 3~4 m mirrors – 30 m boom – 75-1000 m baseline using formation flying75 1000 m baseline using formation flying

• Operate at 1 AU for 5 years to survey 150 stars


Nulling Interferometry

B

/B

B

1.00

0.40

0.60

0.80

0.00

0.20

0 0.25 0.5 0.75 1


Interferometer Detects and Characterizes Planetary SystemsCharacterizes Planetary Systems

• TPF produces image of planetary system– Orbital location– Temperature and radius

• TPF produces spectrum to search for biomarkers• 1-2 m telescopes to find Jupiters nearest Earths• 1-2 m telescopes to find Jupiters, nearest Earths• 3-4 m telescopes for full TPF goals


Occulters

Planet

NWD Starshade JWSTTarget Star NWD StarshadeTarget Star

• Telescope big enough to collect enough light from planet

• Occulter big enough to block star– Want low transmission on axis and high

transmission off axis• Telescope far enough back to have a properly small

IWAIWA• No outer working angle: View entire system at once


Earth Over Geologic Time

CH COCH4 CO2

Oxygen appears in atmosphereOxygen-producing bacteria start

Methanogens startFi t lif CO O O

Vis. spectrumFirst life consumes CO2

High CO2 compensates for faint SunKaltenegger et al 2005 in prep.

O3 O2

Refs: Kasting, Scientific American; Kaltenegger et al, 2006; Holland 2006

TPF and Biosignaturesg

An Integrated Program of Planet Finding Science

KEPLER

Optical signs of habitable

Survey of distant stars for Earths

Navigator Program

KECK

TPF‐C

habitable worlds

Survey of nearby stars for dust and

1SIM

Mid‐infrared signs of habitable worlds

stars for dust and giant planets

LBTI

TPF‐IMasses and orbits of large terrestrial planets

ARE THERE OTHER HABITABLE WORLDS?

JWSTARE THERE OTHER SOLAR SYSTEMS LIKE OUR OWN?

PLANET DETECTION N b i l

PLANET CHARACTERIZATION • Planet chemistry in visible & infrared • Presence of water • Radius • Surface gravity and temperature • Atmospheric conditions

•Young Jupiters

•Transit Follow‐up

2005 2010 2015 2020 2025

• Nearby giant planets • Young, hot Jupiter's

• Atmospheric conditions • Biomarkers

•Debris Disks

An Integrated Program of Planet Finding Science

KEPLER

Optical signs of habitable

Survey of distant stars for Earths

Navigator Program

KECK

TPF‐C

habitable worlds

Survey of nearby stars for dust and

Cancelled

1SIM

Mid‐infrared signs of habitable worlds

stars for dust and giant planets

LBTI

TPF‐IMasses and orbits of large terrestrial planets

ARE THERE OTHER HABITABLE WORLDS?

Cancelled Cancelled

JWSTARE THERE OTHER SOLAR SYSTEMS LIKE OUR OWN?

PLANET DETECTION N b i l

PLANET CHARACTERIZATION • Planet chemistry in visible & infrared • Presence of water • Radius • Surface gravity and temperature • Atmospheric conditions

•Young Jupiters

•Transit Follow‐upOvercost

2005 2010 2015 2020 2025

• Nearby giant planets • Young, hot Jupiter's

• Atmospheric conditions • Biomarkers

•Debris Disks

Evolution of the TPF Flight Design Concepts

• The original idea was to fly a thermal infrared interferometer on a fixed 80‐m boom, similar to SIM, but bigger (and cooled)

• Disadvantages:• Disadvantages:– Vibrations

– Fixed baselineFixed baseline

Nulling interferometerNulling interferometer

• As you rotate the interferometer, the planet(s) pop into and out of the nullsp p

TPF-I (or Darwin): Free-flying IR interferometer

• This idea has now evolved into a free‐flying interferometer, similar to ESA’sd D i i iproposed Darwin mission

• Advantages: good contrast ratio, excellent spectroscopic biomarkers• Disadvantages: needs cooled, multiple spacecraft

“Emma” designfor for

TPF-I/Darwin

Peter Lawson TPF-I white paper

Peter Lawson TPF-I white paper

TPF-C: Visible/near-IR coronagraph

• It may be easier, however, to do TPF in the visible, using a singletelescope and spacecraft

• Advantages: single spacecraft and telescope• Disadvantages: high contrast ratio between planet and star

TPF-Coronagraph: 8 x 3.5 m

Off‐axissecondary

imirror

V shaped

8 x 3.5 m

V‐shaped thermal shields

8 x 3.5 moff‐axisprimary mirrorCoronagraph

sectionsection

Diffraction: Airy ringsDiffraction: Airy rings

• Diffraction from a circular aperture leads t Ai ito Airy rings

• 84% of the light is in the t l di k Th thcentral disk. The other

16% is in the rings. Need to get rid of this!eed to get d o t s

Possible pupil mask (and corresponding i d f i ) f Cpoint spread function) for TPF-C

Mask PSF

8 m

Mask PSF

Marc Kuchner, NASA Goddard

Classical and non-classical apodization techniques

Classical PIAA

Ref: O. Guyon, A&A 404, 379 (2003)

PIAA Point Spread FunctionPIAA Point Spread Function

Baseline TPF‐C design (nulling at 4/d)

Contrast needed for TPF‐C

(nulling at 4/d)

Ref: O. Guyon, A&A (2003)

Different (standard) pupil masksDifferent (standard) pupil masks


Telescope diameter for detectionp

Figure 9: Telescope diameter needed to detect (SNR=5) a companion 0.1”


from a mv=5 star with a flux ratio of 109. The central wavelength is 0.5 mand the bandwidth is 0.2 m.

New Worlds Observer: f i ibl l fi da 2-spacecraft visible planet finder

• One can also detect terrestrial planets in the visible by placing an occulting disk (or flower) between the telescope and the targetOcculter

• Advantages: preliminary concept study shows adequate starlight suppression capabilities~40,000 km suppression capabilities

• Disadvantages: Pointing this array at multiple targets and

i t i i i i ti l

Telescope

maintaining precise inertial alignment over 50,000 km is time‐and fuel‐ consuming and possibly

hibi i Th b f

Diagram from homepage of Webster Cash, Univ. of Colorado

prohibitive. The number of target stars is much smaller.

And How Will We Know That A Pl t S t Lif ?A Planet Supports Life?

Look for Look for liquid evidence of oxygen

qwater

Analyze the reflected light from the planet

Look for signs of biological

ti itfrom the planet to see if the planet has an

activity (methane)

atmosphereAnd Rule Out Other Explanations?

17

NIMS Data (from Galileo)( )

(‘A’ band)( )

Sagan et al. (1993)* *…but credit Toby Owen for pointing this out (1980)

NIMS data inthe near-IR

• Simultaneous presence of d d d (O2 and a reduced gas (CH4

or N2O) is the best evidence for life

*Credit Joshua Lederburgand James Lovelock for the id (1964)idea (1964)

Sagan et al. (1993)*

Galileo Spectral Imaging of EarthGalileo Spectral Imaging of Earth

‘True’ color(red green violet)

Longer wavelengths(red green 1 m)(red, green, violet) (red, green, 1 m)

Sagan et al. (1993)

The ‘Red Edge’ of ChlorophyllThe Red Edge of Chlorophyll

Spectral bands centeredon the ‘red edge’

Sagan et al. (1993)

Visible Spectrum of Earth

d l h f h fl d f d k d fIntegrated light of Earth, reflected from dark side of moon; Rayleigh, chlorophyll, O2, O3, H2O. Ref.: Woolf, Smith, Traub, & Jucks, ApJ

2002; also Arnold et al. 2002

Thermal IRspectraspectra

Source:R. Hanel, Goddard Space Flight CenterSpace Flight Center

Pre-TPF Study Will Span Wavelengths, Techniques, Years, Ground and Space,

Theory and Observation

Hale Bopp

ConclusionsConclusions

• NASA’s proposed TPF mission (or ESA’s Darwin mission) mayNASA s proposed TPF mission (or ESA s Darwin mission) may eventually be able to locate Earth-sized planets around other stars and take either visible or thermal-IR spectra of their atmospheresatmospheres

• O2 (or O3) and CH4 have absorption bands in both wavelength regions that may be used as potential indicators of extraterrestrial life

But,

• We need other bioindicators for reduced, early-Earth type atmospheres

• A lot more work is needed to design the ultimate mission!• A lot more work is needed to design the ultimate mission!


Signal, Noise, and Time

If every star has an Earth-like planet in its habitable zone,how many targets could we see,

how well could we characterize each planet, how large a telescope would we need,

and how long would it take?and how long would it take?


Earth spectrum


Earth & Zodi Model

* Assume Earth = 10-10 Sun.Then the Earth signal per day, at V, in 10% band, in A = 2, is

S = (227 elec )[(D/D )/(x/x )]2(e/e )(t/t )/(R/R )S = (227 elec.)[(D/D0)/(x/x0)]2(e/e0)(t/t0)/(R/R0)

* Assume zodi = 21.5 mag/arcsec2 (STDT, =10-7, local+exozodi).Then the noise per day, at V, in 10% band, (sqrt of zodi) is

N = (89 elec.)[(e/e0)(t/t0)(z/z0)/(R/R0)]1/2

Here D0 = 1 m telescope diameterx0 = 10 pc distance to stare = 0 5 electrons/photon (efficiency)e0 = 0.5 electrons/photon (efficiency)t0 = 1 day integration timeR0 = 10 / (spectral resolution)z0 = 1 local + 1 exo zodi (total zodi toward star)


Integration time

The SNR is Earth signal / zodi noise, which gives integration time:

t = (0.16 day) (S/N)2 [(x/x0)/(D/D0)]4 (R/R0)(z/z0)/(e/e0)

for both V and I wavelength regions.

If we want S/N = 5, and use R=R0, z=z0, e=e0, we get0 0 0 g

t = (4.0 day) [(x/x0)/(D/D0)]4

where x0 = 10 pc and D0 = 1 m, as before.


Number of stars & integration times (10% resol.)

From the NStED database, for all stars with a mid‐HZ greater than IWA = 2 /D, where here use = 1 micron.

Diameter # stars distance distance distance timeper star

timeper star

timeper star

close mid far close mid far

1 0 ‐ ‐ ‐ ‐ ‐

2 m 56 3 pc 22 pc 160 pc 0.003 d 5.9 d yrs

4 m 460 3 pc 37 pc 390 pc 0.0002 2.9 d yrsp p pd

y

8 m 3100 3 pc 62 pc 500 pc ~0 d 1.4 d yrs

16 m 18000 3 pc 89 pc 340 pc ~0 d 0.4 d months


Spectral Feature Detection

• A spectral feature has depth d with respect to the continuum, where 0<d<1.

• It has width , and resolution R = /.

• The integration time to detect this feature, with a given S/N, is

t(feature) = t(continuum) R / d2


Earth-like Spectral Feature Detection Times (days) (days)

Assume targets are at middle of nearest half of each sample, so representative of about 30, 200, 1500, and 8000 stars, respectively.

Dtel xpc

Rayleigh0.45m

H2O1.13m

H2O0.94m

O3

0.60mO2

0.76mCH4

0.89mCH4

1.00mH2O

0.82m

2 m 10 pc 1 2 4 7 8 13 21 1002 m 10 pc 1. 2. 4. 7. 8. 13. 21. 100.

4 m 20 pc 0.2 0.4 0.8 1. 2. 3. 5. 30.

8 m 30 pc 0.04 0.1 0.2 0.3 0.3 0.5 0.7 4

16 m 40 pc 0.003 0.008 0.02 0.03 0.03 0.05 0.08 0.3

t E th l E thall ages all agespresent Earth early Earth


Th k !Thank you !

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