The mantra in the search for extraterrestrial life is “follow the water.” In fact, a prime goal for NASA’s Kepler mission is to provide a statistical estimate of Earth-sized planets in stellar habitable zones where surface oceans of water could exist.
But the imaginations of science fiction writers have gone beyond the imaginations of astronomers.
In Frank Herbert’s classic 1965 novel “Dune,” the arid planet Arrakis resembles a bigger, warmer Mars with small ice caps and subsurface aquifers. In the “Star Wars” film series, Luke Sykwalker’s home planet Tatooine is a dry world parched by twin suns.
Now, a team of researchers lead by Yutaka Abe of the University of Tokyo are proposing that desert-like terrestrial planets may be great places to buy real estate. And, there are probably lots of them to choose from in our galaxy.
Desert worlds could have started out dry because of a lack of water and ices in the circumstellar disk they formed out of. In particular, planets in wide binary systems, like nearby Alpha Centauri, may not have an outlying ice cloud of comets to irrigate the planets.
The researchers did computer simulations of global climate changes on a dry Earth-sized planet. They kept the model planet simple: it didn’t have an axial tilt like Earth and had a circular orbit — hence no seasons.
The model shows that such worlds could be remarkably resilient for the survival of life. Of course, there would have to be at least some water around; it would be found at the cold polar regions, pooled into lakes.
Most importantly, the researchers find that desert planets are more resistant to the kind of global mass-extinction traumas that water planets could undergo.
For example, at least 650 million years ago Earth was a “snowball planet” entrapped in ice. It had suffered from runaway glaciation because all the continents were piled up in the Southern Hemisphere, restricting the polar flow of warming ocean currents. Almost all life had been wiped out.
Thanks to relentless volcanism, Earth’s ice-entombed surface eventually broke out of its frozen shell. A biological arms race followed. Multicelled creatures emerged on Earth in a flurry of activity called the Cambrian Explosion about 550 million years ago.
A desert planet doesn’t have cloud cover, snow, or ice that would trigger a global ice age by increasing the planet’s reflectivity, so most of the warming sunshine bounces back into space.
Likewise, a dry planet does not face the fate of having its oceans evaporate away as its sun starts to grow brighter with age. Evaporating oceans would saturate a planet’s atmosphere with water vapor that traps infrared radiation. The planet then undergoes a runaway greenhouse effect that can raise surface temperatures as high as that of an electric oven on self-clean, as is the case with Earth’s sister planet Venus.
What’s sobering is that Earth will become a desert planet in roughly one billion years as the sun’s steadily increasing luminosity evaporates our oceans. In fact, Earth will look a lot like the giant Saturnian moon Titan. Great sand dunes girdle the moon and lakes (containing methane and ethane) pockmark the polar regions, resembling the scattering of lakes in northern Minnesota. One is called Ontario Lacus, after the Great Lake that straddles Ontario, Canada and northern New York State.
The researchers speculate that the aging Earth might go through a phase where it is a habitable desert planet before the greenhouse effect takes over and smothers everything.
This may have happened on Venus, too, the team reports. If Venus started out with oceans it might have gone through a transitory desert planet period as the oceans evaporated. The team says that Venus may have been habitable as recently a billion years ago before the greenhouse heat essentially sterilized the planet (or did it?).
In the Discovery Channel program “Alien Planet” a pair of robots visit an imaginary world called Darwin IV that had its ocean evaporate away long ago. All that’s left is a dried up ocean floor and a Texas-sized sea. The residual water has been tuned into the consistency of Jell-O by microorganisms intent on bio-engineering their last holdout.
The team’s computer models show that a habitable zone for dry planets is three times wider that for water planets. This means the Kepler metric for habitable zones may be misleading if it applied to only water worlds. Because of the wider range of habitability, life-bearing desert worlds may be more common in our galaxy than Earth-like worlds.
Image credit: NASA