Alien life more likely on 'Dune' planets

Sep 02, 2011 By Charles Q. Choi
Alien life more likely on ‘Dune’ planets
Mars is a desert planet. Credit: NASA/JPL/Malin Space Science Systems

Desert planets strikingly like the world depicted in the science fiction classic "Dune" might be the more common type of habitable planet in the galaxy, rather than watery planets such as Earth, researchers suggest.

Their findings also hint that Venus might have been a habitable desert world as recently as 1 billion years ago.

Nearly everywhere there is water on Earth, there is life. As such, the search for life elsewhere in the universe has largely focused on "aqua planets" with a lot of 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 they 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. 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 habitable zone in our solar system, compared to the habitable zone for the red dwarf star Gleise 581. Credit: ESO

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. 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 ," they report in their paper, which was recently published in the journal Astrobiology. "The first habitable planet is more likely to be a member of the land planet class than the aqua class."

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. On the other hand, land planets were better at resisting global freezing, with sunlight dipping below 58 to 77 percent before the planet completely iced over. This means land planets could be farther away from their stars and remain potentially habitable.

When it came to the hot inner limits of these planets, the researchers calculated that liquid water could remain stable at the poles of an aqua planet — its coldest areas — only until the amount of sunlight it received exceeded 135 percent that of modern Earth. In comparison, liquid water could remain stable at the poles of a land planet until it received 170 percent of Earth's sunshine, meaning it could orbit closer to its star and still be habitable.

Such a land planet might be much like the fictional planet of Arrakis, "although I don't think the sandworms sound possible to me," Zahnle said. "The picture of the equatorial zone being just too hot to live at is there, as well as the poles being habitable. I would actually think that the poles would be a good deal wetter than in 'Dune' — there would be more open water at the poles, maybe even small streams and lakes and such."

Morocco dune at sunrise. Are desert worlds the more common type of habitable planet in the galaxy? Credit: wikicommons / Matanya

Planetary scientist Jim Kasting at Pennsylvania State University, who did not take part in this study, said this is clever research. However, Kasting was uncertain if these findings actually will help find new habitable planets, whether they be land or aqua. In order to tell if any worlds are habitable by our standards, they need to exhibit signs of water, and "it is not clear that there is enough water on these 'Dune' planets to be observed [by our telescopes]. So I don't think this will change our strategy for looking remotely for life."

Zahnle disagreed. "These planets might not exhibit signs of water that we can see, but they would of oxygen," he said. "Also, we're finding that water is so ubiquitous, it cannot be regarded as a signature of a planet's habitability."

If anything, since land planets can get closer to stars than aqua planets and still be habitable, Zahnle expected habitable land planets to be discovered before habitable aqua ones. The closer a planet is to its star, the faster it orbits and the more often it dims its star's light, making it easier for our telescopes to spot.

Kasting also was not convinced that small amounts of water are stable on a planetary surface. He suspected that the little water on a land planet would get sucked up by rocks or get pulled down into the mantle or both.

Zahnle agreed, "but we're not looking for planets that are habitable permanently, just ones that might be habitable long enough for life. No planet is habitable permanently, not even Earth."

Earth itself may one day become a desert world, researchers added. As our sun ages, it is brightening at a rate of 9 percent every billion years, radiation that will eventually deplete our planet's liquid water by breaking it down into hydrogen and oxygen. However, they calculated that might still remain habitable in the billions of years before the Sun begins dying — it might avoid the runaway greenhouse effect that made Venus unlivably hot -- and only lose about a third of its oceans before the Sun's death.

One interesting question the habitability of land raises is whether or not Venus, the hottest planet in the solar system, ever could have fostered life. Assuming that Venus once had oceans of liquid , the researchers' calculations suggest "it is possible that Venus went through a period where it was a dry but habitable planet," Zahnle said.

Indeed, Venus could have persisted as a habitable land planet until as recently as roughly 1 billion years ago. Zahnle said that Venus back then would have been "very hot in the tropics, cooler and wetter at the poles. Sort of Earth-like, not a lot of carbon dioxide."

Future research can investigate precisely how habitable Venus might once have been, Zahnle added.

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antialias_physorg
4.2 / 5 (10) Sep 02, 2011
To be habitable, aqua planets must orbit their stars in a so-called "Goldilocks zone"

There are other sources of heat than the sun which can cause planets (or moons) to have water. For starters one just needs to look down: we're living on a thin crust over thousands of kilometers of molten rock.

Heat can come from radioactive sources, residual heat from planetary formation, tidal forces, ...

Ganymede, Io and Europa (none of which are in the goldilocks zone) probably have water and/or ice. That we find another planet with the right oxygen mix and pressure is also pretty unrealistic: given that the oxygen in our atmosphere isn't a product of inorganic processes but the waste product of anaerobic bacteria over billions of years.
CapitalismPrevails
3.8 / 5 (10) Sep 02, 2011
So we should keep an eye out for giant monster worms with Spice in them?
Isaacsname
5 / 5 (8) Sep 02, 2011
So we should keep an eye out for giant monster worms with Spice in them?


Naw, just keep a lookout for Tusken Raiders when we land, they're fiesty lil' bastards.
FrankHerbert
Sep 02, 2011
This comment has been removed by a moderator.
that_guy
5 / 5 (4) Sep 02, 2011
Mars and Venus basically both fit into these categories, so apparently, the extended zones will be outliers rather than the rule for desert planets. The mix of conditions would have to be just right.

I think, as antialias touched on, we'll probably find most of our interesting planets in interesting places. My money says that we'll find most habitable planets and cradles of life will seem to be outside our conventional view, and improbable compared to our current conjecture.

That said, I'm still waiting for them to build a big enough telescope to take a good peek at the gleise 581 system.
ArtflDgr
1 / 5 (14) Sep 02, 2011
there is no runaway global warming on venus...

and i wont mention that their ideas of what makes water leave a planet will fit a land planet MORE Than a water planet. ie, a land planet is what you have left afther their ficticious process (ignoring real process that is not runaway global warming sans CO2)), is a land planet...

ie... if the water burned off the earth for being too warm.. ie outside the habitable zone... it would become a land planet

duh

chosenbygrace
1 / 5 (13) Sep 02, 2011
Mainstream scientists are so delusional.
chosenbygrace
1 / 5 (1) Sep 02, 2011
So we should keep an eye out for giant monster worms with Spice in them?


No: feta cheese. Just ate one.
Malyuta_Skuratov
3.7 / 5 (6) Sep 02, 2011
Sure, one should never trust a scientist over a right-wing talk show. After all the talk shows are funded by real people (ah, corporations) while scientists are payed by the government whose sole purpose is to tax and spend. Makes perfect sense :-)
Sean_W
1.8 / 5 (5) Sep 02, 2011
Venus is hotter than it should be just from its proximity to the sun because of the huge pressure of it's atmosphere. Since Venus is missing the Earth's best greenhouse gas--water vapor--it has a much weaker greenhouse effect and would actually be cooler than Earth if it had the same atmosphereic density.
Telekinetic
1.7 / 5 (11) Sep 02, 2011
About 5 years ago, I witnessed a UFO of the silver cigar tube type at approximately 500 feet in the air. It was dusk but there was still plenty of light to see it clearly. It was making a slow trajectory above a very heavily wooded area and with binoculars, I could see that there were no wings, tail assembly, ailerons, markings, or sound. There was a narrow slit in the front that I assume was for visibility. Its movement was completely straight and was definitely not a gas filled blimp. This is absolutely not fiction, and I asked a flight instructor with 20 years as an Air Force mechanic if a craft can fly straight without wings of some kind to which he replied "No". I asked then how would he explain what I saw and he said that the area has had numerous UFO sightings. I think our universe is old enough and hospitable enough to have had many extraterrestrial civilizations for over millions of years.
sherriffwoody
5 / 5 (1) Sep 03, 2011
About 5 years ago, I witnessed a UFO of the silver cigar tube type at approximately 500 feet in the air. It was dusk but there was still plenty of light to see it clearly. It was making a slow trajectory above a very heavily wooded area and with binoculars, I could see that there were no wings, tail assembly, ailerons, markings, or sound. There was a narrow slit in the front that I assume was for visibility. Its movement was completely straight and was definitely not a gas filled blimp. This is absolutely not fiction, and I asked a flight instructor with 20 years as an Air Force mechanic if a craft can fly straight without wings of some kind to which he replied "No". I asked then how would he explain what I saw and he said that the area has had numerous UFO sightings. I think our universe is old enough and hospitable enough to have had many extraterrestrial civilizations for over millions of years.

The most advanced races don't use craft at all.
Telekinetic
1 / 5 (3) Sep 03, 2011
DanM
4 / 5 (1) Sep 04, 2011
Such a planet might have primitive life, but nothing advanced enough to need oxygen. Oxygen is produced from water by photosynthetic micro-organisms. And because the oxygen in the atmosphere drifts all around the planet, the water has to be enough to reach all around the planet, too, or else conversion of water into oxygen will bleed most of the water away until all that's left is barely what is used by the bacteria to split the water and free the oxygen. But how will the water be replenished? There can be no recycling system, because the oxygen constantly drifts away from the poles to establish a very low density in the atmosphere all around the planet. If there is not enough water to form oceans once something like the late heavy bombardment is over, then the amount added by the occasional comet will hardly be enough to sustain life.
DanM
4 / 5 (1) Sep 04, 2011
Gravity and low enough temperatures can be counted on to keep any liquid water in place, but gaseous oxygen is not constrained by surface contours, only by gravity, so it diffuses.

And remember that the photosynthetic bacteria need to be near enough to the surface to receive adequate sunlight, so the water must also be on the surface, making it likely to be evaporated by that same sunlight. Sunlight doesn't penetrate more than a few hundred feet into water, so how deeply can it penetrate land?

And because the water must be near enough to the surface to support photosynthetic bacteria if oxygen is to be available, there must be enough surface water to maintain a rough local equilibrium with atmospheric water vapor -- and that means water across the entire surface, not just the poles. (And because of rotation, the density of the atmosphere is lowest at the poles.)
DanM
4 / 5 (1) Sep 04, 2011
But that's only for advanced life. Primitive life can get by deep beneath the surface, extracting energy from volcanic chemicals and not needing starlight. But that life is EXTREMELY primitive -- single-celled at best. And such a planet, not having enough water to sustain a life-produced oxygen-rich atmosphere, would not be particularly habitable for humans in anything approaching the density we had on this planet several millennia ago. It might be an interesting location for an outpost, but hardly a second home. And forget native intelligent life.
DanM
4 / 5 (1) Sep 04, 2011
On the other hand, a planet with so much water that there is no land surface would also not sustain intelligent life, though it would support multi-cellular organisms and perhaps even something approaching fish. Perhaps a planet with something on the order of 40% ocean coverage (as long as there is ocean at the equator, to provide for a sustainable planet-wide hydrologic cycle) would be adequate, with an upper limit of 90% or 95% water coverage.
Marcos_Toledo
2 / 5 (4) Sep 04, 2011
The problem with Venus is not a runaway greenhouse effect but a rotation rate it's day is longer than it year 247 days as against orbit of 225 days. Things would be different if it day was between twenty and thirty hours that would allow for lower temperture on Venus and it might be habitable,
ED__269_
not rated yet Sep 05, 2011
@MarcusToledo.
The problem is still a runaway greenhouse gas effect. sure, increased rotation will have baring on the rate of localised energy absorption and may result in lower temperature, but it still must impact on the greenhouse effects.

I say "may result in lower temperature" because irradiance incident on the surface is a measurement of the intensity of that irradiance, which is a function of the radial length from the sun.

what does have baring on radial length is momentum, and losses of H mass from the atmosphere most likely impact on the momentum - contributing to the runaway effect.
Humpty
1 / 5 (6) Sep 05, 2011
Jesus lives on all planets - so all planets are habitable.
antialias_physorg
5 / 5 (4) Sep 05, 2011
On the other hand, a planet with so much water that there is no land surface would also not sustain intelligent life

Why not? An octopus can do some pretty neat tricks which can be classified as intelligent problem solving. Fast forward a few dozen million years and that could actually lead to some intelligence given the proper incentive (i.e. versatile predators and/or competition from each other).
ColleenLudgate
not rated yet Sep 08, 2011
I think we're a little over our heads here ... I don't understand why we've limited ourselves to looking for sustainable life forms when we don't even know what it actually is. Why are we so convinced that for a planet to have life, it needs to have conditions similar to those here on earth? Maybe ... just maybe ... lifeforms have adapted to their own worlds as we have done with ours.
turboguppy
not rated yet Sep 27, 2011
there is no runaway global warming on venus...

and i wont mention that their ideas of what makes water leave a planet will fit a land planet MORE Than a water planet. ie, a land planet is what you have left afther their ficticious process (ignoring real process that is not runaway global warming sans CO2)), is a land planet...

ie... if the water burned off the earth for being too warm.. ie outside the habitable zone... it would become a land planet

duh



Ummm.... We're seeing the after effects of runaway greenhouse effect on Venus, not a currently happening runaway greenhouse effect. Also, CO2 isn't the only greenhouse gas, and water is well accounted for in the current theory of how Venus' atmosphere got the way it did.

And their 'ideas of what makes water leave a planet' is a process called photo-dissociation. It happens way up in an atmosphere, so there first has to be a lot of water up there for it to get 'burned off,' which wouldn't be the case on these planets.

turboguppy
not rated yet Sep 27, 2011
I think we're a little over our heads here ... I don't understand why we've limited ourselves to looking for sustainable life forms when we don't even know what it actually is. Why are we so convinced that for a planet to have life, it needs to have conditions similar to those here on earth? Maybe ... just maybe ... lifeforms have adapted to their own worlds as we have done with ours.


Well, we have only life on Earth as a guide, so why doesn't it make sense to search for other life with similar processes? talking chemistry, we know silicon isn't a very viable alternative to carbon, and that oxygen alternatives like fluorine and chlorine are just too rare, and ammonia may look good, but there are many problems with it as a thalassogen (not the least of which is the ease by which it photodissociates).

The biggest problem is that carbon, oxygen, and water are so dang HANDY--highly abundant and versatile. Just consider the easy by which amino acids and lipids form...
turboguppy
not rated yet Sep 27, 2011
I for one am very interested in the fact that Venus' entire crust seems to have melted about half a billion years ago, that Venus has a retrograde spin, and an extremely thick atmosphere. To me, this points to a giant impact, planet killer grade, that melted the surface of Venus, changed its spin, and fried its atmosphere.

The sudden, very high level of volcanism would explain how all that CO2 and SO2 got into the atmosphere. Any water on the surface would have been vaporized, and sent into the upper atmosphere to have its hydrogen neatly stripped away by photodissociation, which wouldn't have taken long.

Venus may have been a desert type planet, but could easily have been a watery planet like Earth. Also, consider Venus' slightly lower density than Earth--could this be due to a Theia-like impact where moon formation was NOT the result? I love the mystery of Venus--just sayin'.
turboguppy
not rated yet Sep 27, 2011
Such a planet might have primitive life, but nothing advanced enough to need oxygen. Oxygen is produced from water by photosynthetic micro-organisms. And because the oxygen in the atmosphere drifts all around the planet, the water has to be enough to reach all around the planet, too, or else conversion of water into oxygen will bleed most of the water away until all that's left is barely what is used by the bacteria to split the water and free the oxygen. But how will the water be replenished? There can be no recycling system, because the oxygen constantly drifts away from the poles to establish a very low density in the atmosphere all around the planet. If there is not enough water to form oceans once something like the late heavy bombardment is over, then the amount added by the occasional comet will hardly be enough to sustain life.


There is Coriolis effect to consider. Atmospheres are much more complicated than that. Also, bacteria free oxygen from CO2, not water.
turboguppy
not rated yet Sep 27, 2011
Venus is hotter than it should be just from its proximity to the sun because of the huge pressure of it's atmosphere. Since Venus is missing the Earth's best greenhouse gas--water vapor--it has a much weaker greenhouse effect and would actually be cooler than Earth if it had the same atmosphereic density.


If Venus had the same atmospheric density as the Earth, it would still have its water, and the argument would be moot. The theory goes: Massive Volcanism = tons of CO2 and SO2 released = greenhouse effect = heating = water vaporizes out of oceans = more greenhouse effect = no way for water to freeze out = photodissociation of water in upper atmosphere.

Meanwhile, the CO2 and SO2 keep bubbling out of the rocks, increasing the pressure. With no water on the surface there is no sink for CO2 or SO2, so they just stay there, forming acid rain and such. Eventually the Volcanism stops, but the damage is too great--Venus is post-runaway greenhouse; a hothouse planet.