Download Lecture 2: Exoplanets and life

Lecture 2: Exoplanets and life
David Catling,
[email protected]
Earth seen from 4 billion miles away, by the Voyager 1 spacecraft
Outline
2.0)HabitableZone
-guidetoexaminingthemostlikelycandidates
-butisitvalid?
2.1)Afewbasicspectralprinciples
2.2)Biosignatures
-Earthasacasestudy
-Disequilibrium=>largebiogenicfluxes
-VisibleversusthermalIR
Part 2.0:
The circumstellar
habitable zone
Definitions of habitable zone (HZ)
•  The idea goes back to William
Whewell (1853) and Harlow
Shapley (1950s)
•  Michael Hart (working on a NASA
project) calculated habitable zone
limits in 1978,1979; gave modern
definitions:
•  Habitable zone (HZ): the region
around a star in which an Earth-like
planet could maintain liquid water
on its surface at some instant in
time
•  Continuously habitable zone (CHZ)
-- the region in which a planet could
remain habitable for some period of
time (usually the main sequence
lifetime of the star)
Michael Hart’s calculations
produced a very narrow HZ
•  Hart’s 4.6-Gyr CHZ around our own Sun was
–  0.95 AU: Runaway greenhouse
–  1.01 AU: Runaway glaciation
•  Continuous Habitable Zones (CHZs) around
other stellar types were even narrower
–  Hart concluded: Earth may be the only habitable
planet in our galaxy
•  But Hart’s calculations were flawed: CO2
greenhouse underestimated and glaciated
planets could not deglaciate
Question
surface temperature
Given what you know about our Solar System,
what would you guesstimate for the limits for
the inner and outer edge of the HZ around
our Sun? In AU?
hot
cold
?
?
small
large
separation
What defines the boundaries of the
(conventional) habitable zone?
•  Inner edge: determined by loss of water when
too hot
•  Outer edge: the greenhouse warming limit of CO2
because of CO2 condensation into clouds.
•  BUT: Some regard these Hab Zone limits as
unduly restrictive:
•  Inner limit: a planet that starts out fairly dry, not
subject to a runaway greenhouse (Abe+, 2011).
•  Outer limit: Stuff with other greenhouse gases
besides CO (Stevenson, 1999)
Runaway Greenhouse Effect = Limit on outgoing thermal IR
Thought experiment: move Earth closer to the Sun
Atmosphere becomes so
H2O(g) rich that the 8-12 μm
window closes
Outgoing IR limit ~300 W m-2
because atmosphere becomes
opaque at all IR wavelengths.
Set by the physics of H2O
opacity
J. F. Kasting, Icarus (1988)
• Surface melts
1) 
Water loss limit: a hot ocean and “moist greenhouse” where
water escapes ~ 0.95 AU
2) 
Runaway greenhouse limit (could be reached rapidly if an
impactor turns an ocean to steam that persists) ~ 0.85 AU
Caveat: 3-D effects of clouds can shift these limits
Habitable
zone INNER
EDGE
What we don’t know: Actual trajectory of Venus
1. “Born” with a steam
atmosphere of accretion and a
runaway greenhouse?
Was Venus always dead?
2. Or started with an ocean and
the runaway happened ~0.5 Gyr
later?
Earth-like early Venus?
Outer edge: Wider than Hart’s original
idea because of carbonate-silicate cycle
Walker+ (1981) model
Earth:
Cycle
replenishes
CO2 in ~0.5
million years
•  This cycle regulates Earth’s atmospheric CO2 level
on million-year timescales, acting as a planetary thermostat
•  It ensures that the liquid water habitable zone
around the Sun and other stars is fairly wide
•  However, CO2 itself eventually condenses
into cloud particles as a planet gets farther
away from the Sun
=> outer edge (based on this limit) is ~1.4 AU
to 1.7 AU
(uncertainty because of uncertain cloud
physics)
The Hab Zone around the Sun
0.95-1.4 AU
0.85-1.7 AU
The Sun’s Luminosity Increases with
Time
Ts = Earth temp. with today’s atmosphere; Te with no atmosphere
The Continuously Habitable Zone (CHZ)
Bracketed
distance
shows the
CHZ since
4.6 Ga
Something to look forward to…
Earth’s sky 4 billion years from now
Hartmann (1986)
We can also calculate Habitable Zones
and Continuous Habitable Zones for
other types of stars…
HertzsprungRussell
(HR)
Diagram
People tend to
rule out O, B and
A stars
as targets for
searching for
habitable planets
-Short lifetime
-Intense UV
The habitable zone around
other stars
Mass of star relative to the Sun
Zero-Age Main Sequence
habitable zones
•  Gold strip indicates the habitable zone
Question
Would a a tidally locked
planet be uninhabitable?
No. Atmospheric transport of heat can warm the nightside,
especially if the atmosphere is thick (Joshi+, 1997; 2003).
(consider temperature distribution on Venus and Titan)
See Seager (2013) “Exoplanet Habitability”, Science.
But:ARevisedHabitableZone
Light blue = conventional HZ. Green= extended for dry planets
Dark blue = extended for reducing H2 greenhouses
Key points
•  Habitable zones around F, G, and early K stars are
relatively wide. Some argue they’re even wider.
•  Stars hotter than about F0 are bad candidates for
harboring habitable planets, primarily because of their
short main sequence lifetimes
•  M stars have good CHZs, in theory. Their habitable
zone planets have tidal locking and must rely on
atmospheric/oceanic heat transport to the night-side.
•  But there might be problems with HZ planets around
M-stars, e.g. early thermal escape (or impact erosion)
of atmospheres.
Part 2.1:
A few spectral principles
A planet viewed from a distance
Is it inhabited/ habitable? We just see a pale blue
color. Perhaps the pale blue is a mix of blue oceans
+air and white from clouds. Or is it just Uranus? Can
we do better? Yes, if we have spectra.
(In fact, it’s Earth from Voyager 1 in 1990, viewed from
6.1 billion km away, 32° above Earth’s orbital plane)
Emission and Absorption Spectra
•  Whether the spectrum is continuous, or in emission or
absorption tells us about the composition and
temperature structure of the object we are studying.
Thermal IR spectra from afar.
Absorption spectra: emission from surface through cool atmos.
relative energy intensity
SO2
Water
CO2
CO2
CO2
Water
VISIBLE
NEAR-INFRARED
CO2
H 2O
O2
H 2O
H 2O
H 2O
Iron Oxides
absorb
Graphics: T. Pyle
Terrestrial Planet Finder (TPF) concept
Visible or thermal-IR?
TPF-Coronograph
≈ 1010
TPF-Interferometer
≈ 107
From: Chas Beichman, JPL
• Contrast ratio:
1010 in the visible
107 in the thermal-IR
• But angular resolution
set by diffraction:
wavelength
θ ≥ λ = telescope
diameter
D
Required aperture:
~8 m in the visible
80 m in the IR
Can we deduce a rocky planet’s Tsurface
using spectra from afar?
Tequilibrium
Venus
Earth
Mars
-45C
-18C
-63C
Tactual-surface
462C (735K)
15C (288K)
-58C (215K)
Greenhouse effect
505C
33C
5C
The value if the planet were airless, all other things being equal
Surface (emission) temperature tells us if liquid water is stable.
But…
A greenhouse effect is at least as important in determining
that planet’s surface temperature as is distance from the star!
Earth’sgreenhouseeffectfromspace
Photons coming from air high up where the signal indicates 215 K
“window”: IR radiation is that
expected for the 320 K surface
LEFT:NoonoverN.Africa.Tsurface=320K.Dashedlinesshowblackbodycurvesfor
parWculartemperatures(Hanel+,1972)).Red=255K(globaleffecWvetemp.)
RIGHT:Howtointerpretpartsofthecurve.Arrowsindicatefromwhereblackbody
fluxesoriginate,accordingtotheStefan-Boltzmannlaw
Brightness temperature
On planets with very opaque atmospheres, a
spectrum may not reveal surface T
6
Wavelength (microns)
8
10
14
20
•  Venus spectrum: photons emanate from cold, high parts of the
(IR-optically thick) atmosphere. No photons here go from the
hot 735 K surface directly to space.
•  Exoplanets: we may need to infer the amount of greenhouse
gases from spectra and use models to estimate the surface
temperature if there is no “atmospheric window”.
Part 2.3:
Biosignatures
The same year as the Voyager 1 ‘Pale Blue Dot’, the
Galileo spacecraft (en route to Jupiter) took spectra of
Earth – a dry run for exoplanet searches
From the Galileo spacecraft
Red-to-near-infrared data
relative energy intensity
H2O lines
O2 absorption @760 nm
(‘A’ band)
=> Lots of water and atmospheric O2
Logflux(mW/cm2/μm/sr)
Ge(nges,matesofgas
abundances…e.g.,RadiaWvetransfer
H2O
model+retrieval/fit
H2O
H2O
CO2
CO2
CH4
H2O
N2O CO2
H2O
H2O
H2O
O3
Invertbandsofwellmixedgas(i.e.,CO2
@4.3μm;4.8μm)
N2O
CO2
CH4
wavelength(μm)
Temperaturepressureprofile
CH4,N2O,H2O,O3
abundances,albeit
GalileoNear-IRMappingSpectrometer(Drossart+,1993)
non-uniqueverWcal
NIMS’long-visible,[email protected]μm=>columnO2
distribuWon
IbelieveN2wasassumedbySagan+(1993).
Oxygen and methane: a more
robust biosignature
•  Green plants and algae (and
cyanobacteria) produce
oxgyen from photosynthesis:
O2
CO2 + H2O à CH2O + O2
•  Methanogens and
fermenters produce
methane from organics:
2CH2O à CH4 + CO2
•  CH4 and O2 are out of
thermodynamic equilibrium
by many orders of
magnitude! Hence, their
simultaneous presence is
strong evidence for life
CH4
Caveat..
1) ALL planetary atmospheres are in
disequilibrium
Geophysics competes with biology
2) Life feeds on disequilibrium so
sometimes some disequilibrium might
mean “no one home” i.e.,
uneaten free food=> no grad students
CO or H2 (if oxidant available) => no life
Equilibriumofeachgaswithfixedbulkair:
O2,N2,CO2,H2O(g)
Gas Abundance
CS2
10-11–10-10
OCS 10-10
Equilibrium Source
abundance
~0
>80%Biology;+volcanic
~0
Biology+photochemistry
SO2
10-11–10-10
~0
Volcanic+photochemistry
CH4
1.8×10-6
10-145
>90%biology;+
geothermal
10-60
Biology
3x10-30
Biology+photochemistry
2x10-19
Biology(+minorabioWc)
NH3 10-10–10-9
O3
10-8–10-7
N2O 3×10-7
Detectablein
GalileoNIR
spectraof
Earth?
Note:NumberofmoleculesinEarth’satmosphere~1044
Chemical free energy in a mole of air on
different planets
LIFE
LIVE
DEAD ZONE
ZONE
?
Hypothesis:
Planets are
inhabited when
| ΔG/RT | > 1
From: Catling & Bergsman
(2010,2014)
Hypothesize that biospheres are implicated when:
atmospheric chemical free energy > thermal energy
Caveat: this idea may not work for anoxic Archean Earth
Galileo Spectral Imaging of Earth
A false-color infrared
image reveals a redabsorbing pigment
(chlorophyll, which
appears orange-brown
here) on the
continents.
No such pigment
anywhere else in the
Solar System.
The ‘Red Edge’ of
Chlorophyll in South America
Area C (Amazon)
Andes
Patagonia
Spectral bands centered
on the ‘red edge’
Plants and algae absorb
below 0.7 µm (700 nm)
and reflect above that
wavelength
Summary: Detecting if a Planet Supports
Earth-like Life.
Look for
evidence of
biosignatures
(e.g., O2)
Analyze the
reflected light
from the planet
to see if the
planet has an
atmosphere
Look for liquid
water and
appropriate
surface
temperature
Rule Out Abiotic Explanations
17
Direct spectra: future concepts
•  The real payoff is from directly observing Earth-like
planets, i.e., separating their light from that of the
star, and taking spectra of their atmospheres
•  Future has big, space-based telescopes
•  Earth-sized planets could conceivably be
detected by future ~30 m-scale ground-based
telescopes with adaptive optics; but looking for
biosignature gases through Earth’s
atmosphere will be challenging
Primary transit transmission
(absorption) spectroscopy
(when exoplanet passes in front)
-  Whole spectrum gets dimmer during transit
-  But at absorbing gas wavelengths the dimming is bigger
-  Clouds and hazes can confound this technique
Secondary eclipse spectroscopy
(when the planet passes behind)
Isolate a planet’s thermal spectrum
Has been used by Spitzer (IR) Telescope to look at “hot Jupiters”
http://www.nasa.gov/mission_pages/spitzer/news/070221/index.html
Summary
“Conventional” Habitable Zone (HZ)
-provides a guide for Earth-like candidates
0.85-0.95 AU to 1.4-1.7 AU around a Sun.
-  but it is only a rough guide. The HZ could be wider.
Biosignatures
-  There are practical pros and cons in looking at different
parts of the spectrum, i.e., visible versus IR.
-  But the “take home” message is good news: It’s within
our technological capability (decade(s)) to discover
exoplanet alien biospheres if they exist
Further reading
Available
in late March in Europe
Catling & Kasting (2017)
Cambridge Univ. Press