Table of Contents



   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



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