Lecture 2: Exoplanets and Life

Earth seen from 4 billion miles away, by the Voyager 1 spacecraft Outline

2.0) Habitable Zone -guide to examining the most likely candidates - but is it valid?

2.1) A few basic spectral principles

2.2) Biosignatures - Earth as a case study - Disequilibrium => large biogenic ﬂuxes - Visible versus thermal IR

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 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 surfacetemperature ? ? 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 CO2 (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 Habitable 2) Runaway greenhouse limit (could be reached rapidly if an zone INNER impactor turns an ocean to steam that persists) ~ 0.85 AU Caveat: 3-D effects of clouds can shift these limits EDGE What we don’t know: Actual trajectory of Venus

1. “Born” with a steam 2. Or started with an ocean and atmosphere of accretion and a the runaway happened ~0.5 Gyr runaway greenhouse? later?

Was Venus always dead? 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… HR Diagram Hertzsprung- Russell (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 Zero-Age Main Sequence habitable zones Mass of star relative to the Sun

• Gold strip indicates the habitable zone Tidal Locking

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) But: A Revised Habitable Zone See Seager (2013) “Exoplanet Habitability”, Science.

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.

SO2 CO2

Water CO2 Water relative energy intensity

CO2 VISIBLE NEAR-INFRARED

CO2

H O 2 H2O H O H2O 2 O2

Iron Oxides absorb

Graphics: T. Pyle Terrestrial Planet Finder (TPF) concept

Visible or thermal-IR?

• Contrast ratio: 10 TPF-Coronograph 10 in the visible TPF-Interferometer 107 in the thermal-IR ≈ 1010 ≈ 107 • But angular resolution set by diffraction:

λ wavelength θ ≥ = D telescope diameter Required aperture: ~8 m in the visible 80 m in the IR From: Chas Beichman, JPL Can we deduce a rocky planet’s Tsurface using spectra from afar?

Tequilibrium Tactual-surface Greenhouse effect Venus -45C 462C (735K) 505C Earth -18C 15C (288K) 33C Mars -63C -58C (215K) 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’s greenhouse eﬀect from space Photons coming from air high up where the signal indicates 215 K “window”: IR radiation is that expected for the 320 K surface

LEFT: Noon over N. Africa. Tsurface = 320 K. Dashed lines show blackbody curves for parcular temperatures (Hanel+, 1972)). Red = 255 K (global eﬀecve temp.) RIGHT: How to interpret parts of the curve. Arrows indicate from where blackbody ﬂuxes originate, according to the Stefan-Boltzmann law On planets with very opaque atmospheres, a spectrum may not reveal surface T Wavelength (microns) 6 8 10 14 20 Brightness temperature • 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

H2O lines

O absorption @760 nm relative energy intensity 2 (‘A’ band)

=> Lots of water and atmospheric O2 Geng esmates of gas abundances…e.g., Radiave transfer )

sr model + retrieval/ﬁt H O 2 CO H2O 2 Invert bands of well-

/μm/ H2O CH4 H2O 2 N2O CO 2 mixed gas (i.e., CO2 CO 2 H2O H2O @4.3μm; 4.8μm) O3 H2O

CO2 N2O Temperature- CH4 pressure proﬁle

Log ﬂux (mW/cm wavelength (μm) CH4, N2O, H2O, O3 abundances, albeit Galileo Near-IR Mapping Spectrometer (Drossart+, 1993) non-unique vercal NIMS’ long-visible, O2 band @0.76 μm => column O2 I believe N2 was assumed by Sagan+ (1993). distribuon Oxygen and methane: a more robust biosignature • Green plants and algae (and cyanobacteria) produce O2 oxgyen from photosynthesis:

CO2 + H2O à CH2O + O2

• Methanogens and fermenters produce methane from organics:

2CH2O à CH4 + CO2 CH4 • CH4 and O2 are out of thermodynamic equilibrium by many orders of magnitude! Hence, their simultaneous presence is strong evidence for life 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 Equilibrium of each gas with ﬁxed bulk air:

O2, N2, CO2, H2O(g) Gas Abundance Equilibrium Source Detectable in abundance Galileo NIR spectra of Earth? -11 -10 CS2 10 – 10 ~0 >80% Biology; + volcanic OCS 10-10 ~0 Biology + photochemistry

-11 -10 SO2 10 – 10 ~0 Volcanic + photochemistry -6 -145 CH4 1.8 × 10 10 >90% biology; + geothermal -10 -9 -60 NH3 10 – 10 10 Biology -8 -7 -30 O3 10 – 10 3 x 10 Biology + photochemistry -7 -19 N2O 3 × 10 2 x 10 Biology (+minor abioc)

Note: Number of molecules in Earth’s atmosphere ~1044 Chemical free energy in a mole of air on different planets

LIFE

Hypothesis: LIVE DEAD ZONE ZONE 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 red- absorbing 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 Look for liquid evidence of water and biosignatures appropriate surface (e.g., O2) temperature

Analyze the reflected light from the planet to see if the planet has an atmosphere 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