Continuously Habitable Zones around Stars

Planetary Temperatures

L*4πdp

2

EnergyReachingPlanet

!"#

× πRp2

PlanetCross−section$

× (1− a)FractionAbsorbed

!"# where a = reflectivity "albedo"( ) 4πR*2σT*

4

4πdp2 πRp

2 (1− a)

4πRp2

PlanetSurfaceArea−m2!"#

× σTp4

RadiatedW /m2

$

Power absorbed by a planet:

Power radiated by a planet:

In thermal equilibrium:4πR*

2σT*4

4πdp2 πRp

2 (1− a) = 4πRp2σTp

4

or: Tp =12

R*dp(1− a)1/4T* or Tp =

12

1dp

L* 1− a( )σπ

⎛⎝⎜

⎞⎠⎟

14

Tp = 278 1dAU

L*Lsun

⎛⎝⎜

⎞⎠⎟

141− a( )14 Kelvinsor more simply:

dAU =278T

⎛⎝⎜

⎞⎠⎟2 L*

Lsun1− a

At what distance will water freeze & boil?

Set L* = Lsun and calculate d for T = 273 K (water freezes) and T = 373 K (water boils, std atm pressure).

With a greenhouse effect, need an additional term - ε

ε = 1 means no greenhouse effect. Otherwise ε < 1.

Tp = 278 1dAU

L*Lsun

⎛⎝⎜

⎞⎠⎟

14 1− a

ε⎛⎝⎜

⎞⎠⎟14 Kelvins

dAU =278T

⎛⎝⎜

⎞⎠⎟2 L*

Lsun

1− aε

If a = 0.39 and ε = 0.5 (add some greenhouse)

If a = 0.39 (Earth’s reflectivity)

but ε = 1 (no greenhouse)

If a = 0 and ε = 1

(blackbody planets)

The HZ (“ecoshell”) depends on the properties of the planet

As star’s L changes and planet’s atmosphere evolves, the HZ MOVES!! - Related to “Faint Sun Problem” - how was life on Earth possible when Lsun was 25% less???

Need to include the effects of the evolution of stars and of the planetary atmospheres. First addressed by Michael Hart in the 1970’s....

CONTINUOUSLY HABITABLE ZONES

First calculated for Earth-Sun by Hart (1978 Icarus, 33, 23-39)

Included the following processes:

Rate of outgassing of volatiles (H, C, N, O) from the interiorCondensation of H2O vapor into oceansSolution of atmospheric gases into oceansPhotodissociation of H2O in the upper atmosphereEscape of H from the uppermost atmosphere (exosphere)Chemical reactions in atmospheric gasesPresence of life and variations in biomassPhotosynthesis and burial of organic sedimentsUrey reaction (CaSiO3 + CO2 ⇔ CaCO3 + SiO2)Oxidation of surface minerals (2FeO+O ⇒ Fe2O3)Variations in the luminosity of the SunVariations in the albedo (reflectivity) of the EarthGreenhouse effect

The criteria he assumed for life to arise were:

Liquid water with T < 42 C for 0.8 ByrConcurrent presence of C and N in atmosphere and oceansAbsence of free O in atmosphere

His starting conditions:

No atmosphereAlbedo (reflectivity) = 0.15 (rock)Start 4.5 by ago

His process:

Using time steps of 2.5 Myr, vary the composition of juvenile volatiles until the best fit to present conditions is reached.

MAIN RESULTS:• His "best" initial gas composition was 84% H2O, 14%CO2, 1%CH4, 0.2%N2

• Most of the H2O vapor condensed promptly into oceans

• Early atmosphere was dominated by CO2

• CO2 was later removed by the Urey Reaction

• O released by photolysis of H2O vapor and later by photosynthesis. This O destroys the CH4 (the O being consumed in the process, of course). By 2 by ago, most of the CH4 was gone, leaving N2 as the dominant gas.

Since then, there has been a slow buildup of O2. By 420 Myr ago, enough O2 and O3 had built up to provide protection from solar UV, making life on land tolerable.

OTHER IMPORTANT RESULTS AND LIMITS

In these models, once CH4 was gone and the luminosity of the Sun reached its current value, if T(surface) < 278K, Runaway Glaciation occurs, and in none of the simulations is it ever reversed. This occurs 2 Byr ago if the Earth were located 1.01 AU from the Sun, a mere 1% further away!

If the earth were at 0.95 AU from the Sun, a Runaway Greenhouse Effect occurs 4 by ago, and in none of the simulations is it ever reversed!

These results, which include runaway effects, provide only a very narrow (0.06 AU) CHZ for the Earth. CHZ IS VERY NARROW!!

What are the CHZs like for other stars? - Hart (1979 Icarus, 37, 351-357)

Thickness goes to ZERO for masses less than 0.8 solar masses, and for masses greater than 1.2 solar masses.

Stellar Mass SpT Rin Rout Thickness

>1.20 Red Giant Too Soon 1.20 F7 1.616 1.668 0.054 1.15 F8 1.420 1.481 0.061 1.10 F9 1.240 1.310 0.069 1.05 G0 1.086 1.150 0.064 1.00 G2 0.958 1.004 0.046 0.95 G5 0.837 0.867 0.030 0.90 G8 0.728 0.743 0.015 0.85 K0 0.628 0.629 0.001 0.835 K1 0.598 0.598 0.000

In all cases Δd < 0.1 AU, suggesting that the average planetary system only had a ~1% chance for an Earth-like planet in the CHZ.

"It appears therefore, that there are probably fewer planets in our galaxy suitable for evolution of advanced life than had been previously thought." M. Hart (1979).

OVERALL PICTURE

The evolution of other terrestrial planets will be similar to that of the Earth if inside the CHZ

CHZs are widest around G0 main sequence stars, and shrink to zero at F7 at the hot end, and K1 at the cool end.

Some shortcomings of Hart models addressed by James Kasting & others:

Newer models are somewhat more “optimistic”

Not included in Hart’s models: recycling of carbon

A more recent version from Foley 2015, ApJ, 812, 36

Early Habitable Zone Later Habitable Zone

Continuously Habitable Zone - CHZ

Newly-appreciated role of METHANE

ceeded about eight times the present-dayvalue of around 380 parts per million(ppm), the mineral siderite (FeCO3) wouldhave formed in the top layers of the soil asiron reacted with CO2 in the oxygen-freeair. But when the investigators studiedsamples of ancient soils from between 2.8billion and 2.2 billion years ago, theyfound no trace of siderite. Its absence im-plied that the CO2 concentration musthave been far less than would have beenneeded to keep the planet’s surface fromfreezing.

Even before CO2 lost top billing as the

key greenhouse gas, researchers had be-gun to explore an alternative explanation.By the late 1980s, scientists had learnedthat methane traps more heat than anequivalent concentration of CO2 becauseit absorbs a wider range of wavelengthsof Earth’s outgoing radiation. But thoseearly studies underestimated methane’sinfluence. My group at Pennsylvania StateUniversity turned to methane because weknew that it would have had a muchlonger lifetime in the ancient atmosphere.

In today’s oxygen-rich atmosphere,the carbon in methane is much happier

teaming up with the oxygen in hydroxylradicals to produce CO2 and carbonmonoxide (CO), releasing water vapor inthe process. Consequently, methane re-mains in the atmosphere a mere 10 yearsand plays just a bit part in warming theplanet. Indeed, the gas exists in minusculeconcentrations of only about 1.7 ppm;CO2 is roughly 220 times as concentrat-ed at the planet’s surface and water vapor6,000 times.

To determine how much higher thosemethane concentrations must have beento warm the early Earth, my students and

w w w . s c i a m . c o m S C I E N T I F I C A M E R I C A N 81

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Methanococcales

Thermoplasmatales

Methanopyrales

Methanomicrobiales

Caldisphaerales

Cenarchaeales

Desulfurococcales

Sulfolobales

Thermoproteales

UNIVERSAL ANCESTOR

BACTERIAIncluding cyanobacteria,

proteobacteria and gram-positive bacteria

EUKARYAIncluding

plants, animals, protists and fungi

ARCHAEA

ThermococcalesCR

ENAR

CHAE

OTA

Methanosarcinales

Methanobacteriales

Archaeoglobi

HalobacterialesEU

RYAR

CHAE

OTA

KORARCHAEOTA NANOARCHAEOTA

Methane-producing microbes calledmethanogens (labeled in red) make upnearly half of all known Archaea, one ofthe three domains of living things—including Bacteria and Eukarya—thatarose separately from an unknownancestor. Methanogens exist in a varietyof shapes, including rods and spheres(photographs), and live exclusively inoxygen-free settings. Because theoldest of the five orders of methanogensoccupy low-lying branches of theArchaea domain, most biologists thinkthese microbes were among the firstorganisms to evolve. —J.F.K.

METHANE MAKERS ON THE TREE OF LIFEI collaborated with researchers from theNASA Ames Research Center to simulatethe ancient climate. When we assumedthat the sun was 80 percent as bright astoday, which is the value expected 2.8billion years ago, an atmosphere with nomethane at all would have had to containa whopping 20,000 ppm of CO2 to keepthe surface temperature above freezing.That concentration is 50 times as high asmodern values and seven times as high asthe upper limit on CO2 that the studies ofancient soils revealed. When the simula-tions calculated CO2 at its maximumpossible value, the atmosphere requiredthe help of 1,000 ppm of methane tokeep the mean surface temperature abovefreezing—in other words, 0.1 percent ofthe atmosphere needed to be methane.

Up to the Task?THE EARLY ATMOSPHERE could havemaintained such high concentrations onlyif methane was being produced at ratescomparable to today. Were primordialmethanogens up to the task? My col-leagues and I teamed up with microbiol-ogist Janet L. Siefert of Rice University totry to find out.

Biologists have several reasons to sus-pect that such high methane levels couldhave been maintained. Siefert and othersthink that methane-producing microbeswere some of the first microorganisms toevolve. They also suggest that methano-gens would have filled niches that oxygenproducers and sulfate reducers now oc-cupy, giving them a much more promi-nent biological and climatic role than theyhave in the modern world.

Methanogens would have thrived inan environment fueled by volcanic erup-tions. Many methanogens feed directly onhydrogen gas (H2) and CO2 and belchmethane as a waste product; others con-sume acetate and other compounds thatform as organic matter decays in the ab-sence of oxygen. That is why today’s meth-anogens can live only in oxygen-free en-vironments such as the stomachs of cowsand the mud under flooded rice paddies.On the early Earth, however, the entire at-mosphere was devoid of oxygen, volca-noes released significant amounts of H2.With no oxygen available to form water,

the hydrogen probably accumulated inthe atmosphere and oceans in concentra-tions high enough for methanogens to use.

Based on these and other considera-tions, some scientists have proposed thatmethanogens living on geologically de-rived hydrogen might form the base ofunderground microbial ecosystems onMars and on Jupiter’s ice-covered moon,Europa. Indeed, a recent report from theEuropean Space Agency’s Mars Expressspacecraft suggests that the present Mar-tian atmosphere may contain approxi-mately 10 parts per billion of methane. Ifverified, this finding would be consistent

with having methanogens living belowthe surface of Mars.

Geochemists estimate that on the ear-ly Earth H2 reached concentrations ofhundreds to thousands of parts per mil-lion—that is, until methanogens evolvedand converted most of it to methane.Thermodynamic calculations reveal thatif other essential nutrients, such as phos-phorus and nitrogen, were available,methanogens would have used most ofthe available H2 to make methane. (Mostscientists agree that sufficient phospho-rus would have come from the chemicalbreakdown of rocks and that various

w w w . s c i a m . c o m S C I E N T I F I C A M E R I C A N 83

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JAMES F. KASTING studies the origin and evolution of planetary atmospheres, especiallythose of Earth and its nearest neighbors, Venus and Mars. Since earning his Ph.D. in at-mospheric science at the University of Michigan at Ann Arbor in 1979, he has used theo-retical computer models to investigate atmospheric chemistry and to calculate the green-house effect of different gases and particles in both the present day and the distant past.Recently Kasting has begun exploring the question of whether Earth-like planets might ex-ist around other stars in our galaxy. He is working with several other scientists to designthe theoretical foundation for NASA’s Terrestrial Planet Finder, a space-based telescope de-signed to locate planets around other stars and scan their atmospheres for signs of life.

THE

AU

THO

R

Global ice ages

Carbon dioxide

Methane

Oxygen

Rela

tive

Conc

entr

atio

n

4.5 2.5 1.5 0.5 0

High carbon dioxide compensates for the faint, young sun

Oxygen begins to appear in the atmosphere

First microscopic life begins consuming carbon dioxide

Methanogens begin making major contributions to the atmosphere

Oxygen-producing bacteria get their start

3.5Time (billions of years ago)

RELATIVE CONCENTRATIONS of major atmospheric gases may explain why global ice ages (dashedlines) occurred in Earth’s distant past. Methane-producing microorganisms flourished initially, but asoxygen skyrocketed about 2.3 billion years ago, these microbes suddenly found few environmentswhere they could survive. The accompanying decrease in methane—a potent greenhouse gas—couldhave chilled the entire planet. The role of carbon dioxide, the most notable greenhouse gas in today’satmosphere, was probably much less dramatic.

Did drops in methane lead to ice ages? “Snowball Earth”?

See the complete 2004 Scientific American article by James Kasting:http://homepages.uc.edu/~sitkoml/AdvancedAstro/Kasting-

Scientific_American_04.pdf

Take away point:

“An earth-mass planet has just been detected in the habitable zone”

What’s being assumed about the planet’s atmosphere and evolution?