The (Thermo)Dynamics of Life in SF

So you want to write hard science fiction.   You want to write stories that are consistent with science as we know it today, and perhaps you also want to locate your stories away from the earth—perhaps far from the earth.  If you know what that story is, and you know the science you need to write it, close your browser window and do it now.  You owe it to your readers, and you owe it to yourself.

If, however, you’re stuck in a rut, you may need to try something different to get inspired.  If, like me, you haven’t finished a story in four months (with or without a health concern to justify that situation), you may need to do some research.  In that frame of mind, let’s talk about what your off-world setting requires to support life.

PressurePhase diagram of water, derived from diagram at University of Arizona

The chemistry of life requires a liquid medium to transport chemicals within the living body.  In our neck of the woods, that means water[1].  The need for liquid water, however, puts a hard limit on the locations where water-based life could develop[2].

At any pressure lower than 6.117 millibars (the triple point of water), liquid water can’t exist.  Instead, it sublimates directly from a solid state to a gaseous one.  For comparison, one earth atmosphere is 1013 millibars.  Mars, with its surface pressure of 6.36 millibars, has just barely enough atmosphere to sustain liquid water.  The tiny Jovian moon Europa can sustain liquid water because its icy crust holds things down. Most small planets, however, especially small rocky ones, cannot support liquid water, and you’ll have to work hard to justify the presence of living organisms there.

What this means for world building is that you probably need a world with either underground seas or a mass large enough to keep your atmospheric pressure up above the triple point of water.  Size isn’t the only factor—Venus, which is smaller than Earth, has a surface pressure 92 times ours—but it’s something to consider.


A related question is the temperature range required for life.  Assuming the need for liquid water, biological processes need a local (internal) temperature between 0°C and 100°C at a “typical” earth atmospheric pressure[3]. Traditionally, this is interpreted to mean that your planet needs to be in the “Goldilocks” Habitability Zone, neither too hot nor too cold.  Earth is in this zone, mostly because it’s the right distance from our sun, but there are other factors, including geological heating or atmospheric collection and reflection of heat, which can modify this range.

If you’re designing a brand new world for your SF story, you probably want to give it goldilocks habitability.  If the star is red or orange, your planet will be close to its sun, and may even be tidally locked.  If you have a blue-white supergiant, the planet will be farther away, and the sun may perhaps appear smaller.  If other factors affect your world’s temperature, like insulation from thick clouds or tidal heating from the gas giant it happens to be orbiting, these factors will affect the descriptions in your story, and you’d best think them through in advance.  There is a lot of room for creativity here, but it’s a lot of work, too.



Pressure and temperature, however, are really just expressions of a bigger need for all living things, and that is energy. Life requires energy to overcome the limits imposed by the second law of thermodynamics, which states that entropy (disorder) in a system will always increase.  Living things are massively more ordered than the universe at large, so we can only survive by creating disorder somewhere else.  Generally this implies a transfer of energy from a state in which it is concentrated to one where it is dispersed.

Nearly all of the energy available to living things on earth comes from stars, and I don’t simply mean solar energy.  Coal, gas, and oil come from the bodies of plants and animals, which themselves can trace their source of energy back to the sun.  Wind energy comes from the sun, and even geothermal energy comes from the decay of radioactive isotopes forged in the core of a long-past supernova.  The only energy source I can think of that isn’t indirectly solar is tidal energy, and tides get their energy from the same force of gravity that drives fusion in our sun.

When writing SF about life on other planets, it may be useful to ask where the energy comes from, and how it travels through your world to enable the processes of life.  It isn’t enough to have a static, warm world for life to exist; we need a dynamic one with an external source of energy that life can tap to survive.  This may sound hard, but we’re talking about hard SF here, and I think the most interesting story ideas can be born when a creative mind tries to wrap itself around a difficult issue.

[1] There may be life based on liquids other than water, but its chemistry would be far different from ours.  As a polar molecule, water readily dissolves ions that non-polar solvents like liquid methane or nitrogen could not.
[2] Yes, there are bacteria at the South Pole, where the temperature peaks around -17°C. Even if these bacteria are biologically active (and the American Society for Microbiology asserts that they can’t be), these bacteria were imports from warmer climes.
[3] At higher pressures, water stays liquid longer, so a hot super-earth might conceivably have liquid oceans. Unfortunately, the energy that makes it hotter might also cause water vapor to escape, leading to a water-deprived atmosphere like the one on Venus.
Photo credits:  Triple point diagram derived from a lecture at the University of Arizona chemistry department.  Thermometer by User:Gringer [Public domain], via Wikimedia Commons. False-color image of the sun from NASA via Wikimedia Commons.

Scientific World Building

In hard science fiction, many authors choose to describe characters who come from (or journey to) worlds surrounding distant stars.  In the early days of science fiction, authors often chose stars whose names were well known:  Sirius[1], Alpha Centauri[2], Arcturus[3], Betelgeuse[4], Altair[5], and the like.  As scientific knowledge has expanded, however, writers have needed to increase the level of detail with which they research stars for their planets.  Fortunately, we have the internet to help us with this research.  Here a a few things to consider (and links you can use) when siting your next habitable planet.

Starting Point

There are billions of stars in our galaxy, but most people only know of a few. How can we learn about some of the others from which we might select? One source of data is Hipparcos, a satellite launched in 1989 by the European Space Agency. During its five years in orbit, Hipparcos collected data on more than one hundred thousand stars. This data is freely availalble online, and some sites provide the ability to search the Hipparcos database for stars in a particular region of the sky.

If you would rather start with known planets, that data is available online, too.  The University of Puerto Rico, Arecibo has an interesting database of known exoplanets, including some information on potentially habitable planets.  The SIMBAD catalog is another useful tool for researching known stars — once you know the name of a star from Hipparcos, you can get its name in other catalogs, to do further research.

Spectral Class

stars by spectral class

The most readily visible attribute of a distant star is its color. Decades ago, this information was used to group stars into spectral classes. Although golden-age SF writers seem to favor the brightest stars in the sky, writers in the 70’s and 80’s more frequently selected yellow (G-class) stars like our own sun. Alpha Centauri A, and Tau Ceti have been popular choices, though both are problematic for other reasons (see below).

For those with a background in chemistry, spectral class raises some interesting questions about the impact on life.  M-class stars produce less ultraviolet (UV) light than our sun does.  How would this effect photosynthesis?  A, B, and O-class stars produce far more energetic X-rays and gamma rays than our sun.  How would this affect mutation rates?  This sort of thing could be fodder for some interesting stories, I think.

Multiple Star Systems

While Alpha Centari A is a nice yellow star, it still has problems due to the proximity of its partner star, Alpha Centauri B. Centauri B is a K1-class star, which makes it somewhat smaller than our sun, and it orbits at a distance between 11 and 30AU from Centauri A. When the stars are close to one another, there is a significant chance that B’s gravity might disrupt the orbit of any planets in the habitable range of A. Even if this didn’t happen, there is also the concern that the periodic approach of B might heat up a planet in the habitable zone, making life unpleasant.

Personally, I try to avoid star systems with multiple stars — the “safe” ones probably have the partner so far away that they look like nothing more than a bright star — but some really great stories have been born from smart people thinking about life on a planet with more than one sun.


Even if you want to choose a solitary, ordinary yellow (G) or orange (K) star, there are a lot of choices available.  How do you decide?  A good place to start might be HabCat, a list of stars on the PHL site listing the stars they consider most likely to harbor habitable planets. One of the key characteristics in this decision-making process is a star’s variability: our sun is pretty stable, but some stars grow brighter or dimmer on an irregular basis. Planets near a variable star are at high risk of freezing or burning up.  Once again, it could make for an interesting setting for your story.


One assumption behind the HabCat list is that a star needs to have a certain amount of heavy, metallic elements nearby for rocky planets like earth to form.  We can’t necessarily see these planets yet, but we can measure the amount of metals in a star by looking at its spectral lines.  Tau Ceti and another nearby star, Epsilon Eridani, both seem to be metal-deficient.

More recent studies of Epsilon Eridani seem to indicate that it has at least one large rocky planet orbiting there, so the lack of metallicity for dim stars may not be as prohibitive as once was thought.  If you’re designing a world, however, you might want to consider what it would mean for life if (for example) potassium, calcium, or iron were hard to come by.


If your story includes space travel to a distant star, you will either need to imagine faster-than-light travel or take into account the massive distances involved.  As it happens, parallax data from the Hipparcos database can be used to calculate how far away a given star might be.  For example, a star like HD-113576, with a parallax of 112.8 milliarcseconds, is 28.9 light years distant.  HD-17511, however, has a parallax of only 10, which places it 326 light years away.  If you’re travelling at “only” 9/10ths the speed of light, that can make a difference.


If there is one thing I learned from browsing Hipparcos, there are a lot of stars out there that might support life. With a little bit of work, we can go beyond the traditional choices and place our planets around real stars where no writer has gone before.

[1] Voltaire, in his early SF tale Micromegas, describes his protagonist as having come from Sirius.  Larry Niven’s planet Jinx is also located there.
[2] Philip K. Dick (Clans of the Alphane Moon), Arthur C. Clarke (The Songs of Distant Earth), and Larry Niven (Wunderland in his Known Space stories) all placed habitable planets around Alpha Centauri.
[3] Arcturus is featured as a planet name in Asimov’s Foundation trilogy, and David Lindsay placed his planet Tormance there in his novel A Voyage to Arcturus.  You may debate whether or not Asimov intended Arcturus the planet to orbit the star of the same name, however.
[4] Much of Planet of the Apes takes place in a planet near Betelgeuse.
[5] SF classic Forbidden Planet takes place on Altair IV, and colonies near Altair feature in several Star Trek episodes.