Table of Contents

Introduction

Dreams of worlds, dreams we can generate

Dreams of people now

Why I’m writing this

What is habitability?

Habitability 1.0: habitability for at least one form of lie

Habitability 2.0: for complex life

Habitability 3.0: for us

Habitability dimensions: space, time, type of life form

A note about “we”

A very brief history of speculations about habitable planets

The search for habitable planets… and for sentient life

Mostly, Devil’s Advocate

What astrobiologists want first and foremost: a reasonable temperature

Getting the temperature right

Start with the star

Life-giving radiation, part I

The nature of electromagnetic radiation (long section)

Exploring details: How stars work, particularly the “nice” stars

Making stars

The nuclear reactions inside the Sun and similar “nice” stars

The first step: proton-proton fusion

Maxwell, Boltzmann, and quantum tunneling to the rescue

The weak interactions slows down p-p fusion enormously

Once fusion starts, a deuteron and a positron are formed

… and the final fusion

Summary of nuclear fusion in the Sun

From elementary processes to the whole Sun: structure and “function”

There is a nice stability to a low-mass star such as our Sun

A very close look at the Sun

Sidelight: how do we know what elements are in the Sun… and link to detecting chlorophyll action

Truly important: the mass of a star determines its temperature, lifetime, and and any chance for life

Scaling relations; the math, or a jump to a summary

Big stars live fast and die

Digression: an artificial star for our use – fusion reactor

Anomaly of the extremely high temperature of the corona

Where does the star put its energy? Wavelengths and intrinsic match to needs of organisms

Big, hot stars and cold, flaring stars – useless for life

How our Sun has lived out its lifetime

Conclusion: be near a nice, low-mass star, but we need a cataclysm nearby

Now let’s get to the planet around the star

Temperature on the planet – right for life? Difficulty of estimation

Basically, the energy balance is all radiative

From the star to the planet

Star size, distance

Radiation reaching the planet, to be absorbed

Radiation leaving Earth is thermal radiation

Calculating the radiative temperature of all the planets

Some problematic stars and problematic orbits for a planet

Big, hot stars

Not a red dwarf with flaring and tidal locking

And binary stars pose difficulties

Stability of the planet’s orbit

The planet has to “behave,” too

Albedo, greenhouse effect, orbital eccentricity, axial tilt, rotation

Good neighbors, or at least tolerable neighbors

Dramatic visitors

Jupiter’s role is documented

Life on Earth has made it through threats from within our own Solar System

Threats from outside a stellar system – explosions, sterilizing radiation

To the planet surface

How solar (stellar) energy is disposed

Along with oceans, the atmosphere transports heat between latitudes [redundant w/below the…?]

Importance of an atmosphere in radiative balance

The kinds of radiant energy from stars, planets, and, well, everything

Delving into electromagnetic radiation as it meets matter

The nature of atoms and molecules, and how they interact with radiation

Radiation moving through stars and atmospheres

Shortwave and thermal radiation

Three fates of shortwave radiation – transmission, absorption, scattering

Fate of absorbed radiant energy in molecules and in the whole planet

Most ends up as heat without doing notable chemistry

Absorption of radiation by chlorophyll illustrates the various processes internal to a molecule

Chlorophyll is an exceptional molecules

The flux of thermal infrared radiation is critical for the greenhouse effect

Absorption bands of molecules according to motions excited

How thermal infrared radiation moves through the atmosphere

Air has bigger things in it than simple molecules

More attributes of light – momentum, polarization, geometry of propagation

The greenhouse effect on a planet – every habitable planet

The simplified model

Does a planet’s greenhouse effect suffice for habitability? (No, of course)

A greenhouse effect can be too low, too high, just right, and stable, unstable, resettable

The roles of various greenhouse gases, particularly on Earth

The joint action of the various greenhouse gases

A greenhouse effect needs a starter

The Goldilocks level of the greenhouse effect on Earth

Far too low on Mars, far too high on Venus

Greenhouse gases and getting them on a planet – water, compounds of carbon and nitrogen, ozone

Delivery and processing of greenhouse gases on other planets

Impossibility of ammonia as the basis of life and of its greenhouse effect

Making do without a greenhouse effect on an exoplanet?

Keeping a greenhouse effect in bounds, or failure to do so: feedbacks in planetary climate

Negative feedbacks – rock weathering, dissolution into oceans, thermal radiation loss, plant growth

Negative feedbacks on exoplanets

Positive feedbacks that act to amplify changes in climate

Ice melting/freezing, soil organic matter decomposition, water vapor load in the atmosphere

Really long term: solar or stellar brightening

Loss of water from a planet: ballistic and aide by an electric field – fate of a small planet

On exoplanets, what can we expect for the stability of the greenhouse effect? A litany

Additional gleanings about Earth’s carbon cycle at the core of its greenhouse effect

What counts is what’s near the surface – deep and shallow cycles

The photosynthesis – respiration short cycle

The upward march of CO₂ on Earth (details in a later section)

The long-term picture

Agents of change in the GHE and climate: movers and destabilizers

Growth of stellar output over time; astronomical changes; Earth’s orbit and axial orientation, including

Ice ages; geological forces, and the needed scraping up of continents

Volcanism, tectonics, and a thermally active Earth

Life itself – the oxygen crisis for the GHE and for iron availability; carbon burial; methane and N₂O

Fire-using humanity changes the climate

And on the exoplanets? With or without technological civilizations?

Consequences of climate disruptions for living organisms

Acclimate, migrate, adapt (change) genetically

Genetic replacement

More to climate than temperature: Moving heat around on the surface

Planetary rotation (and fatal tidal locking); axial tilt limits

A couple more heat sources to dismiss: tidal flexing and geothermal energy

Beyond temperature: what else does a habitable planet need?

Chemistry as a constraint or a whole set of constraints

Getting the right chemical elements – at the surface

Water is our sine qua non, as the medium for living cells

Water physical properties; protons as energy currency; redox level

Where did we get our water? [redundant section?]

Hold onto water, not too much gas! Getting too big

Beyond water: other elements sorting out on Earth and other planets

Is life only carbon-bases? (Yes, says chemistry)

Condensable or non-gaseous elements; their recycling by geothermal heat (mantle convection)

Redox reactions – why oxygen? No problem retaining oxygen

The elements for life re not distributed evenly on the Earth’s surface

Those other essential elements – transition metal, halogens, iodine, boron….

Recycling of several of life’s key chemical elements

Nitrogen

Phosphorus

Transition metal elements for biochemical electron-transport reactions

What evolution has given Earth… and what life forms may have evolved on exoplanets

Living organisms are composed of discrete cells, with compartments

All life descends from a single common ancestor – universal genetic code, protein chirality

Photosynthesis with starlight will be or is the ultimate source of energy for life in the long run

Physiology with cellular communication

Viruses on exoplanets?

Organisms need highly competent mechanisms to repair damage

Preserving combinations of genetic traits – reproductive isolation into species

Recycling of “dead carcasses”

Expect a huge diversity

Habitability of the Earth – and can we keep it?

It’s been habitable (1.0) continuously, even if very tenuously at times

Past mass extinctions

Natural and anthropogenic hazards, not siloed

Our many intellectual and practical tools to understand our situation and to act

Aspects of habitability – regional and global

We evolved to deal well only with immediate, localized threats

Natural hazards, from local to global

Fallback – genetic engineering? No

Many anthropogenic hazards

Tipping points, and changeover times

Transients and bigger equilibrium effects

Our agricultural system is tuned to current climate

Biotic extensions of climate change – insect vectors, loss of biodiversity, …

Other hazards – nuclear war

Ways out of nightmare scenarios – population control; replacement of capitalism

Conclusion: I think we are alone

Is the search for extraterrestrial life worth the effort? What we may learn

The search for exoplanets – who, what, when, why, where

SETI, the search for extraterrestrial intelligence

Will we colonize other planets, why, and how?

The “What” question has multiple dimensions; a special physiology; slow rocket travel

What would a Mars colony look like? Keeping warm, growing crops

When? Is the technology ripe?

How would colonizers get there, and how would they survive?

Particulate radiation, adverse physiological changes, psychological challenges

How: keeping hale and hearty – oxygen, water

Crops and food

Recycling wastes

Cosmic radiation

Where on Mars?

Why? Escaping Earth’s long-term fate? Our species has a finite lifetime, anywhere

About the author

Appendices, alphabetically – more details about topics in-line in the text

Appendix. Blackbodies and blackbody radiation [needs to be written]

Appendix. Calculating_planet_temperature

Appendix. Energy balance of an organism

Appendix. The exponential_function

Appendix. How heat moves through soil and rock

Appendix. Human use of clothing expands tolerable climates

Appendix. Mass, energy, and all that

Appendix. Plate_tectonics

Appendix. Recycling of nitrogen on Earth

Appendix. Relaxation_times

Appendix. The rocket equation in_free space

Appendix. Stable isotope ratios as a tool [needs to be written]

Sidebars, alphabetically

Sidebar. The absolute temperature scale

Sidebar. Adiabatic lapse rate in the atmosphere

Sidebar. Decline of nutrient content in plants at high CO¬2, particularly nitrogen

Sidebar. The Earth’s crust is not quite fully oxidized.

Sidebar. The Ediacaran Period – multicellular life becomes evident (eventually)

Sidebar. Energy liberated in the fusion to two protons and an electron to a deuteron

Sidebar. Equable zones on Proxima Centauri b (not really)

Sidebar. Exponential notation

Sidebar. Heat that stays

Sidebar. The Hertzsprung-Russell diagram of star temperature and luminosity

Sidebar. How do we know the mass of a star.

Sidebar. How can we measure the size of a star, its radius

Sidebar. Integating the Planck equation over a waveband

Sidebar. The Maxwell-Boltzmann distribution of molecular velocities

Sidebar. The mixed blessing of plate tectonics

Sidebar. Molecules and moles

Sidebar .Much more energy to get to mars vs. the Moon

Sidebar. Multiplicity in star systems

Sidebar. Neutrinos

Sidebar. Nucleons and electrons. Chemical bonds and nuclear binding

Sidebar. The odd abundance of odd-nucleon atoms

Sidebar. Population bottlenecks, human and other

Sidebar. Power laws

Sidebar. Random walk

Sidebar. Spectroscopy - determining chemical elements in stars by absorption and emission of light at discrete wavelengths

Sidebar. Summary of_mass_extinctions

Sidebar. Why do photons take so long to escape the core of the Sun