In Frank Herbert’s science fiction novel Dune, planet Arrakis is a vast desert world, inhabited by sandworms and Bedouin-like humans called the Fremen.
Could such hot desert planets be home to certain extraterrestrial life-forms?
Studies suggest that desert planets could be more common type of habitable planet in the galaxy, rather than watery planets.
When scientists search for alien worlds that could support life, water is an important criteria. Nearly everywhere there is water on Earth, there is life.
Artistic impression of what planet Arrakis could look like.
As such, the search for life elsewhere in the universe has largely focused on “aqua planets” with a lot of liquid water on their surfaces, either terrestrial planets largely covered with oceans, such as Earth, or theoretical “ocean planets” completely covered by a layer of water hundreds of miles deep, somewhat like thawed versions of Jupiter’s moon Ganymede.
To be habitable, aqua planets must orbit their stars in a so-called “Goldilocks zone” where they are neither too hot nor too cold. .
If aqua planets are too far from the Sun, they freeze; if they are too close, steam builds up in their atmospheres, trapping heat that vaporizes still more water, leading to a runaway greenhouse effect that boils all the oceans off the planet, as apparently happened on Venus.
Eventually, such planets get so hot, they force water vapor high enough into the atmosphere for it to get split into hydrogen and oxygen by ultraviolet light.
The hydrogen then escapes into space, the oxygen likely reacts with the molten surface and gets incorporated into the mantle, and the planet’s atmosphere loses all its water over time.
Instead of aqua planets with abundant water on their surfaces, researchers investigated what “land planets” might be like, ones with no oceans and vast dry deserts, but perhaps oases here and there.
Cover art from Frank Herbert’s novel, “Heretics of Dune.” In Herbert’s Dune series, sandworms live on the desert planet Arrakis. Image Credit: Ace Books
The planet Arrakis depicted in science fiction classic “Dune” is one exceptionally well-developed example of a habitable land planet, said planetologist Kevin Zahnle at NASA Ames Research Center.
Arrakis is essentially a bigger, warmer, sparsely inhabited version of Mars with a breathable oxygen atmosphere and Polar Regions cool and moist enough to have small water icecaps and morning dew.
The scientists reasoned the scarcity of water on a land planet might actually help it have a larger habitable zone around its star.
For one thing, a land planet has less water for snow and ice that can reflect sunlight back into space. As such, it can in principle absorb more heat to better resist global freezing, enlarging the cold outer limits of its habitable zone.
In addition, the dearth of water in a land planet’s dry atmosphere makes it trap less heat than an aqua planet, helping it avoid a runaway greenhouse effect and expanding the inner, hotter edge of its habitable zone.
Sunrise on an alien desert planet.
Also, the less water there is in the atmosphere, the less there is for ultraviolet radiation to break up into hydrogen and oxygen.
Researcher Yutaka Abe at the University of Tokyo with Zahnle and their colleagues experimented with a number of simple three-dimensional global climate models for Earth-sized planets.
For their simulations of land planets, they left the rotation rates, atmospheric pressures and carbon dioxide levels unchanged but removed oceans and vegetation, leaving behind groundwater locked underneath the surface.
The scientists discovered that a land planet’s habitable zone was three times bigger than an aqua planet’s.
“A pale blue dot is not the only model for an Earth-like habitable planet.”
“The first habitable planet is more likely to be a member of the land planet class than the aqua class,” they wrote in their science paper.
When analyzing what the cold outer limits were for these worlds, Abe and his colleagues found that complete freezing of an aqua planet occurred when the amount of sunlight dipped below 72 to 90 percent of what Earth receives, depending on how its axis of the rotation was tilted toward the Sun.
Morning greets a desert world