Just what does it take to make a habitable world? Keith Cooper is editor of Astronomy Now, the British monthly whose first editor was the fabled Patrick Moore. An accomplished writer on astronautics and astronomy as well as a Centauri Dreams regular, Keith has recently become editor of Principium, the newsletter of the Institute for Interstellar Studies, whose third issue has just appeared. In this essay, Keith looks at our changing views of habitable zones in light of recent work, and takes us to two famous science fictional worlds where extreme climates challenge life but do not preclude it. How such worlds emerge and how life might cope on them are questions as timely as the latest exoplanet findings.

by Keith Cooper


Literally overnight, two habitable planets – tau Ceti f and HD 85512b – were rendered barren and lifeless. What was the cause of this cataclysm? A nearby supernova? Asteroid impacts? On the contrary, it was something far more mundane.

A dozen light years away, scientists at Penn State University were re-analysing the extent to which habitable zones penetrate the space around stars; in other words, at what distance liquid water could potentially exist on a planetary surface assuming an Earth-like atmosphere. The basics for habitable zone theory had been worked out in part by, among others, Penn State’s James Kasting in decades previous. Building on his work, Ravi Kumar Kopparapu and Ramses Ramirez discovered that habitable zones are found further from their stars than had been envisaged (see Habitable Zones: A Moving Target for more).

The result was bad news for our two exoplanets. Suddenly, as the habitable zone shifted imperceptibly around them, they found themselves on the wrong side of the inner habitable zone boundary, too close to their respective stars. Consequently the Planetary Habitability Laboratory at the University of Puerto Rico, Arecibo, declared them uninhabitable. Too bad for any life-forms living there.

Despite only knowing the scarcest of details about these worlds – mass, radius, density, the amount of heating from their stars – these two worlds have been cast into the obsolescence in a manner that seems shockingly final. We know so little about these planets, how can we possibly say whether they are habitable or not, especially when the only standard we are holding them to is habitability for human beings?

Key Factors for Habitability

Determination of habitability is based on worlds not necessarily having exactly the same atmosphere as Earth, but at least having water and carbon dioxide, which are abundant and vital for life, Dr Abel Mendez of the Planetary Habitability Laboratory at the University of Puerto Rico, Arecibo, tells me. “The problem of the inner edge is that once you evaporate more water you get into a runaway greenhouse effect that will make the planet lose all its water,” he says.

There are other factors that play a part though. Just because a planet is inside a habitable zone doesn’t mean it is automatically habitable. The presence of an atmosphere, water, a global magnetic field, plate tectonics and a not too heavy impact rate are all factors. For those worlds close to the edges of the habitable zone, the margins are even narrower.

For example, habitability of planets on the edge could be largely dependent upon cloud cover, says Mendez, which can increase a planet’s albedo, or reflectivity, preventing heat from reaching the surface, but if there’s no way to see clouds on a planet many light years away, how can we just write off worlds like tau Ceti f and HD 85512b? Mendez admits nothing is for certain. “The intention of the habitable zone is to determine the limits [at which habitable planets can exist from their stars], but I will not call them hard limits yet due to uncertainties such as the effects of clouds.”

A constrained, limited view of habitability that says only Earth-like conditions will do limits the number of worlds we think would look friendly. And there’s nothing wrong with this approach – we know that a planet like Earth is suitable for life, so that is what we look for, whereas we don’t know yet whether life could exist on worlds like Europa or Titan, for example. It’s not that planetary scientists are ignoring other kinds of worlds, either. “Many groups are considering the more exotic possibilities, such as tidal habitable zones, habitable planets around white dwarfs, etc,” says Mendez. “The problem is that the habitability of such conditions are harder to observe or interpret than known biosignatures, and observational astronomers need to measure things, but we will get there.”

Science Fiction at the Boundaries

Until we do, however, we’re left to speculate with our imaginations and where is that not done best but in science fiction? So let’s take a look at a few imaginary worlds that are different to Earth but which could exist on the boundaries of the habitable zone and see how they stack up in comparison. Could reality really be as strange as fiction?

One common science fiction trope is the planet with the same climatic conditions over its entire globe, for example the desert planet, the ice planet, the jungle planet. In reality things are more complex – you can have what seems like all four seasons in one day on parts of the Earth. We don’t expect the same climate at the equator as at the poles. Meanwhile the change of seasons see cycles of weather, not just on our planet but on Mars, Saturn and Titan to name but three. What then do we make of our first two science fiction choices, the desert world Arrakis from Frank Herbert’s Dune, and the ice planet Hoth from The Empire Strikes Back?

Arrakis first. Dry as a bone, it has no surface water and no precipitation. What little atmospheric moisture there is is harvested by wind-traps and the water then ferried by canal to underground reservoirs in anticipation of using it as part of the terraforming of the planet. In the novels, however, the planet is mostly sand dunes, inhabited of course by the fearsome sandworms, except for at the pole where a large slab of bedrock ringed by mountains provides a more habitable zone.


Image: A sandworm rears up out of the desert of Arrakis on the March, 1965 cover of Analog. I can never resist the chance to display the artwork of the remarkable John Schoenherr. What memories…

So how did Arrakis end up like this? In Dune, Frank Herbert described the world as having salt flats, indicating that it once had lakes. The introduction of the non-indigenous sandworms, in their protoform as ‘sand trout’, saw them sequester all of the water. Arrakis, described as orbiting the star Canopus, changed from a fertile world to a desert planet.

We have our own desert planet in the Solar System in the form of Mars, skirting the outer edge of the habitable zone. While there are no sandworms on the red planet, liquid water dried up on the surface long ago and today only exists in frozen ice caps, sub-surface ice or possibly in aquifers deep underground. Indeed, what happened to Mars’ water, and the truth behind the climatic history of the planet, are still something of mystery, but we can hazard a best guess.

We know that Mars once had running water on its surface, in the form of rivers, lakes and even a northern sea. They existed billions of years ago. Today we see only their long-lasting consequences on the Martian terrain: river channels, floodplains, a surface chemistry forever altered by the presence of liquid water. The problem with Mars is that it is small, which results in a double whammy for the planet: its diminutive size means not only a smaller gravitational field but also a greater loss of heat from its core. As Mars’ interior began to cool, its molten core began to stiffen and the magnetic dynamo contained within began to stall. These two things conspired to allow the solar wind to strip Mars’ atmosphere, including its water vapour. (The European Space Agency’s Mars Express spacecraft has actually witnessed this stripping in action, watching Mars’ atmosphere lose oxygen at a rate ten times faster that Earth’s atmosphere; see Earth’s Magnetic Field Provides Vital Protection.

Move Mars closer in to the Sun and you could easily have a warmer Arrakis-type world. So desert worlds are feasible and you don’t require sandworms to create them either. But what about the other extreme, an ice planet like Hoth?

Life on the Outer Edge

Twice in Earth’s history – 2.5 billion years ago, and about 700 million years ago – our planet completely froze over [PG note: I must have had a typo here before; see the comments below re the 700 million year figure]. Even the oceans were covered with a thick layer of ice, right down to the equator. Dubbed ‘Snowball Earth’, what causes such events is uncertain, but a significant reduction in atmospheric carbon dioxide (possibly as a result of increased silicate weathering in the warm and wet tropics as continents gathered there) or methane (destroyed through oxidisation, as a result of an oxygen influx into the atmosphere from the first oxygen-exhaling life-forms) would do the trick. Both carbon dioxide and methane are potent greenhouse gases; without them the planet cooled and must have teetered close to the edge of an abyss from which it would never recover.

Of course, it did recover. The freezing of the planet brought the carbon-silicate cycle to a halt. Water vapour froze out of the atmosphere, which meant that precipitation ground to a halt. Ice covered the land so there could be no weathering and ice topped the oceans, preventing carbonates from reaching the sea floor. The way out of this predicament for the planet was that there was still an input into the carbon-silicate system, namely carbon dioxide belched out by volcanoes. Gradually the atmosphere accumulated carbon dioxide, with no rain to wash it out. Temperatures rose and the Earth thawed, but the point is that ice planets can very easily happen, particularly if a world lacks plate tectonics to provide that carbon dioxide input that acts as part of a thermal blanket for the world. If there were a ‘slushy’ belt around the equator, which doesn’t quite freeze over, then some life may be able to survive, although it’s hard to imagine what ecology could flourish on a planet like Hoth to permit a food chain with the monstrous yeti-like wampas at the top. Ironically, if methane was the primary greenhouse force in early Earth’s atmosphere, and was destroyed by oxygen, then the discovery of another snowball planet around another star could potentially be a biosignature indicating the presence of oxygen-exhaling life on that world.

Hoth was a world covered in ice. What about planets covered in water, such as Solaris in Stanislaw Lem’s novel of the same name (ignoring the fact that this fictional planet’s global ocean was actually a living entity)? According to the United States Geological Survey seventy percent of Earth’s surface is covered in water and simulations depicting planet formation suggest that planets could easily acquire much more water than Earth did; indeed, Earth is actually quite dry. Perhaps water is delivered to planets by comets and asteroids, or perhaps these water-worlds are born further out, beyond the ‘snow line’ where water-ice is prevalent, before migrating inwards to hotter climes where their ice melts. There’s even observational evidence for water-worlds – in February 2012 Hubble Space Telescope observations of the 6.5 Earth-mass world GJ 1214b, some forty light years distant, show that starlight passing through its atmosphere is being absorbed at the characteristic wavelength of water vapour, enough to contribute a large fraction of the planet’s mass.

All of these worlds – desert, ice and ocean planets – could potentially be habitable to a point; even in Earth’s own snowball periods, life persevered. However their occurrence was before the arrival of complex life and it is doubtful such life would have survived the onset of such a catastrophic change in climate. More to the point, Mars was once wet and warm with a thicker atmosphere, even if it was only for a short while, while still existing outside of the habitable zone. Now it is a barren. On the other hand Earth was once a frozen wilderness despite being in the habitable zone, but is now resplendent with life.

While habitable zones are a starting point, it is clear they are not necessarily the final word on habitability and locating planets within their limits does not guarantee that they are going to be Earth-like, nor does it automatically correspond that planets outside of the habitable zone will be inhospitable. Furthermore, astronomers also suspect that life could exist in such exotic locales as planets in ten hour-orbits around white dwarfs, on tidally locked worlds around red dwarfs, on exomoons orbiting gas giants and even on rogue planets that wander interstellar space, kept warm by their own innate radioactivity. Surely if any of these types of planet are discovered to be habitable it will prove that reality can be far stranger than fiction.