Humankind has speculated about the existence of life on other planets for millennia. In the last half-century, the actual search for extraterrestrials took on a more high-tech approach with radio telescopes, space telescopes, and a host of various detection devices. And as the number of discovered extra-solar planets increases, so do the odds of encountering a species of alien life on one of those worlds. Knowing just where to look in the vastness of space helps in the search and new information from researchers at the NASA Ames Research Center and Tokyo University suggests that looking for planets that resemble the fictional arid world of Arrakis in Frank Herbert's science fiction classic Dune might be the best place to look.
Traditionally, scientists and researchers have sought extra-solar planets where water might be plentiful within a habitable region around stars known as the "Goldilocks Zone" (per the children's fable: the planet would be "just right" to support life). The researchers from NASA and Tokyo University, according to NASA planetologist Kevin Zahnle, decided to test the viability of "land planets" to see if there would be much difference in the range of the habitable zone. Reasoning that these land planets would have a larger "Goldilocks Zone" due to less surface water (extending colder limits due to less freezing and better heat retention and hotter limits due to less overall heat than waterworlds), researchers ran models like the Dune scenario, which posits a planet like Mars with polar icecaps and a breathable atmosphere.
What they discovered was that, according to computer simulations of three-dimensional global climate models, the habitable zone of such land planets and desert worlds increased to three times the size of a zone for an aqua planet.
"A pale blue dot is not the only model for an Earth-like habitable planet," the researchers report in the journal Astrobiology. "The first habitable planet is more likely to be a member of the land planet class than the aqua class."
Although it is overall good news for planetologists, astrobiologists, and astronomers on the hunt for extraterrestrial life, some do not believe that the findings will have a large impact on the search for habitable worlds. Penn State planetary scientist Jim Kasting, who was not part of the project, admitted that the research was clever, it probably would not affect the way scientists conducted searches. The standard search incorporates looking for water, Kasting said, and notes that "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."
But Zahnle defends the research, allowing that the search for alien life and habitability would be enhanced. "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."
Aqua class planets, or "waterworlds" as they are more commonly referred to, are thought to have the highest chance to support alien life. The most likely candidate discovered thus far is the planet GJ 1214b, which orbits the star GJ 1214 in the constellation Ophiuchus. Discovered in December 2009, the waterworld is relatively near -- 40 light years distant -- and is three times the size of the Earth.
The discovery of a more massive waterworld, Gliese 581d, was announced in May. Located in the constellation Libra, the planet, which is seven times the size of Earth, is even closer than GJ 1214b -- only 20 light years away.
The study also presented the likely scenario of the Earth as a future desert planet, albeit in some distant future. The researchers noted that the Earth could remain habitable during the billions of years between the present and the sun's dying. It could possibly even maintain a good percentage of its oceans all the way up to the moment of the sun's death. Research also suggested that the planet Venus may have once been a habitable desert planet -- "sort of Earth-like," Zahne says -- until a runaway greenhouse effect rendered it the hottest planet in the solar system about a billion years ago.
(photo credit: Luca Galuzzi, Creative Commons)