lecture 4 2:lecture 4.2: the search for another earththe
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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
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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)
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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.
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
Planets found so far…
Planets found so far (HUGE ones!)
Planets we hope to find (Earth-size!)Planets we hope to find (Earth-size!)
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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.
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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)
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
Earth Spectra
Thermal infrared VisibleThermal infrared Visible
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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.
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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
Ref: O. Guyon, A&A (2003)
Telescope diameter for detectionp
Figure 9: Telescope diameter needed to detect (SNR=5) a companion 0.1”
Ref: O. Guyon, A&A (2003)
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!
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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?
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
Earth spectrum
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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)
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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.
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
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
The SEARCH for ANOTHER EARTHThe SEARCH for ANOTHER EARTH
Th k !Thank you !
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