Continuously Habitable Zones around Stars Planetary Temperatures Power absorbed by a planet: Planet Cross− section $ L 2 4 * × π R2 × (1− a) where a = reflectivity ("albedo") 4π R σT 2 p !"# * * π R2 (1− a) 4πdp 2 p !"# Fraction 4πdp Energy Absorbed Reaching Planet Planet Surface Area−m2 !"# Power radiated by a planet: 2 4 4π Rp × σTp $ Radiated W /m2

4π R2σT 4 * * R2 (1 a) 4 R2 T 4 In thermal equilibrium: 2 π p − = π pσ p 4πdp 1 or: 1 R* 1/4 1 1 ⎛ L*(1− a)⎞ 4 Tp = (1− a) T* or Tp = ⎜ ⎟ 2 dp 2 dp ⎝ σπ ⎠

1 1 ⎛ L* ⎞ 4 1 or more simply: Tp = 278 ⎜ ⎟ (1− a) 4 Kelvins dAU ⎝ Lsun ⎠ At what distance will water freeze & boil?

2 ⎛ 278⎞ L* dAU = ⎜ ⎟ 1− a ⎝ T ⎠ Lsun

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.

1 1 1 ⎛ L* ⎞ 4 ⎛ 1− a⎞ 4 Tp = 278 ⎜ ⎟ ⎜ ⎟ Kelvins dAU ⎝ Lsun ⎠ ⎝ ε ⎠

2 ⎛ 278⎞ L* 1− a dAU = ⎜ ⎟ ⎝ T ⎠ Lsun ε If a = 0 and If a = 0.39 (Earth’s If a = 0.39 ε = 1 reflectivity) and ε = 0.5 (add some (blackbody but ε = 1 (no greenhouse) planets) greenhouse)

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)

The criteria he assumed for life to arise were: Included the following processes:

Liquid water with T < 42 C for 0.8 Byr Rate of outgassing of volatiles (H, C, N, O) from Concurrent presence of C and N in atmosphere and the interior oceans Condensation of H O vapor into oceans 2 Absence of free O in atmosphere Solution of atmospheric gases into oceans

Photodissociation of H2O in the upper atmosphere Escape of H from the uppermost atmosphere His starting conditions: (exosphere) Chemical reactions in atmospheric gases No atmosphere Presence of life and variations in biomass Albedo (reflectivity) = 0.15 (rock) Photosynthesis and burial of organic sediments Start 4.5 by ago Urey reaction (CaSiO3 + CO2 ⇔ CaCO3 + SiO2) Oxidation of surface minerals (2FeO+O ⇒ Fe2O3) His process: Variations in the luminosity of the Sun Variations in the albedo (reflectivity) of the Earth Greenhouse effect 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 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.

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). Not included in Hart’s models: recycling of carbon

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

Newer models are somewhat more “optimistic” 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

METHANE MAKERS ON THE TREEI collaborated OF LIFE with researchers from the NASA Ames Research Center to simulate Global ice ages the ancient climate. When we assumed Archaeoglobi that the sun was 80 percent as bright as today, which is the value expected 2.8 Carbon dioxide billion years ago, an atmosphere with no methane at all would have had to contain Methane aCal whoppingdisphaerales 20,000 ppm of CO2 to keep ncentration the surface temperature above freezing. That concentration is 50 times as high as Cenarchaeales

modern values and seven times as high as Relative Co the upper limit on CO2 that the studies of Oxygen ancient soils revealed. When the simula- tions calculated CO2 at its maximum 4.5 3.5 2.5 1.5 0.5 0 ARCHAEOTA possible value, the atmosphere required Time (billions of years ago) RY EU

the help of 1,000CRENARCHAEOTA ppmSulf ofolobales methane to Oxygen begins to appear in the atmosphere Thermococcaleskeep the mean surface temperature above freezing—in other words, 0.1 percent of Oxygen-producing bacteria get their start the atmosphere needed toThermoproteales be methane. Methanogens begin making major contributions to the atmosphere Methane-producing microbes called Up to the Task? methanogens (labeled in red) make up First microscopic life begins consuming carbon dioxide THE EARLY ATMOSPHERE could have nearly half of all known Archaea, one of the three domains of living things— maintained such high concentrations only High carbon dioxide compensates for the faint, young sun including Bacteria and Eukarya—that if methane was being produced at rates arose separately from an unknown BACTERIA EUKARYA Including ancestor. Methanogens exist in a variety Including cyanobacteria, comparable to today. Were primordial RELATIVE CONCENTRATIONS proteobacteria and plants, animals, of major atmospheric gases may explain why global ice ages (dashed of shapes, including rods and spheres gram-positive bacteria methanogens up prot to ists the and task? fungi My col- lines) occurred in Earth’s distant past. Methane-producing microorganisms flourished initially, but as (photographs), and live exclusively in leagues and I teamed up with microbiol- oxygen skyrocketed about 2.3 billion years ago, these microbes suddenly found few environments oxygen-free settings. Because the whereDid they could survive.drops The accompanying in decreasemethane in methane—a potent greenhouselead gas —tocould oldest of the five orders of methanogens ogist Janet L. Siefert of Rice University to have chilled the entire planet. The role of carbon dioxide, the most notable greenhouse gas in today’s occupy low-lying branches of the try to find out. Archaea , most biologists think atmosphere, was probably much less dramatic. these microbes were among the first Biologists have several reasons to sus- ice ages? “Snowball Earth”? organisms to evolve. —J.F.K. UNIVERSAL ANCESTOR pect that such high methane levels could the hydrogen probably accumulated in with having methanogens living below have been maintained. Siefert and others the atmosphere and oceans in concentra- the surface of Mars. ceeded about eight times the present-day key greenhouse gas, researchers hadthink be- thatteaming methane-producing up with the oxygen in microbeshydroxyl tions high enough for methanogens to use. Geochemists estimate that on the ear- value of around 380 parts per million gun to explore an alternative explanation. radicals to produce CO2 and carbon were some of the first microorganisms to Based on these and other considera- ly Earth H2 reached concentrations of (ppm), the mineral siderite (FeCO3) would By the late 1980s, scientists had learned monoxide (CO), releasing water vapor in have formed in the top layers of theSee soil as thatthe methane complete traps more heat than evolve.2004 an theThey process.Scientific also Consequently, suggest that methaneAmerican methano- re- tions, articlesome scientists by have James proposed thatKasting:hundreds to thousands of parts per mil- iron reacted with CO2 in the oxygen-free equivalent concentration of CO2 becausegens wouldmains have in the filledatmosphere niches a mere that 10 oxygen years methanogens living on geologically de- lion—that is, until methanogens evolved air. But when the investigators studied it absorbs a wider range of wavelengthsproducersand playsand sulfatejust a bit reducerspart in warming now the oc- rived hydrogen might form the base of and converted most of it to methane. samples of ancient soils from between 2.8 ofhttp://homepages.uc.edu/~sitkoml/AdvancedAstro/Kasting- Earth’s outgoing radiation. But those planet. Indeed, the gas exists in minuscule billion and 2.2 billion years ago, they early studies underestimated methane’scupy, givingconcentrations them aof much only about more 1.7 promi- ppm; underground microbial ecosystems on Thermodynamic calculations reveal that found no trace of siderite. Its absence im- influence. My group at Pennsylvanianent State Scientific_American_04.pdfbiologicalCO2 is roughly and climatic 220 times role as concentrat- than they Mars and on Jupiter’s ice-covered moon, if other essential nutrients, such as phos- plied that the CO2 concentration must University turned to methane becausehave we in edthe at themodern planet’s world. surface and water vapor Europa. Indeed, a recent report from the phorus and nitrogen, were available, have been far less than would have been knew that it would have had a muchMethanogens6,000 times. would have thrived in European Space Agency’s Mars Express methanogens would have used most of needed to keep the planet’s surface from longer lifetime in the ancient atmosphere. To determine how much higher those freezing. In today’s oxygen-rich atmosphere,an environmentmethane concentrations fueled by volcanicmust have beenerup- spacecraft suggests that the present Mar- the available H2 to make methane. (Most

JOHNNY JOHNSON JOHNNY Even before CO2 lost top billing as the the carbon in methane is much happiertions. Manyto warm methanogens the early Earth, feed my students directly and on tian atmosphere may contain approxi- scientists agree that sufficient phospho- hydrogen gas (H2) and CO2 and belch mately 10 parts per billion of methane. If rus would have come from the chemical www.sciam.com methane as a wasteSCIENTIFIC product; AMERICAN others con-81 verified, this finding would be consistent breakdown of rocks and that various sume acetate and other compounds that form as organic matter decays in the ab- JAMES F. KASTING studies the origin and evolution of planetary atmospheres, especially sence of oxygen. That is why today’s meth- those of Earth and its nearest neighbors, Venus and Mars. Since earning his Ph.D. in at- anogens can live only in oxygen-free en- mospheric science at the University of Michigan at Ann Arbor in 1979, he has used theo- vironments such as the stomachs of cows retical computer models to investigate atmospheric chemistry and to calculate the green- and the mud under flooded rice paddies. house effect of different gases and particles in both the present day and the distant past.

On the early Earth, however, the entire at- AUTHOR THE Recently Kasting has begun exploring the question of whether Earth-like planets might ex- mosphere was devoid of oxygen, volca- ist around other stars in our galaxy. He is working with several other scientists to design noes released significant amounts of H2. the theoretical foundation for NASA’s Terrestrial Planet Finder, a space-based telescope de-

JOHNNY JOHNSON JOHNNY With no oxygen available to form water, signed to locate planets around other stars and scan their atmospheres for signs of life.

www.sciam.com SCIENTIFIC AMERICAN 83

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?