In this weekly series, Life's Little Mysteries rates the plausibility of popular science fiction concepts. Warning: Some spoilers ahead!
Narrators of nature shows often speak of Earth as a "living planet." To an extent, the metaphor is true: Biological beings do indeed swim, crawl and fly through our world's uppermost layers of ocean, land and sky. Plant life covers much of the Earth, and bacteria and viruses suffuse its soils, waters and even atmosphere.
But all that is still a far cry from the literally living, conscious planets that make appearances in many sci-fi and fantasy stories. Take Mogo in the "Green Lantern" DC Comics series, for instance. This planetary entity can change its climate and grow foliage in desired patterns on its surface at will.
Or consider Pandora from the 2009 film "Avatar." The flora and fauna on this lush moon have evolved tentaclelike organs that enable them to neurally interlink with each other. A globe-spanning consciousness exists, with Pandora's trillion interconnected trees acting like cells in a colossal brain, dwarfing our mind's 100 billion neurons.
Could anything like these far-out scenarios come to be? Creative writers certainly think so – just check out the long list on Wikipedia of "fictional living planets."
Yet the development of a planet-scale being looks to be an extreme long shot. Based on the chemistries and behaviors of life and nonlife, don't bet on Mogo or Pandora, scientists say.
"The way evolution works, I can't see it happening," said Peter Ward, a professor of paleontology at the University of Washington.
Planets come together bit by bit from the gas and dust that surround newly forming stars. Based on what we have learned from looking at our solar system and beyond, huge agglomerations of rocks, liquids and gases held together by gravity will not spring forth into self-aware consciousness. [A Field Guide to Alien Planets]
In one planetary case we know of — Earth — complex surface chemistry did eventually led to self-replicating, information-carrying molecules that we deem "life."
That microscopic life over the course of 4 billion years of evolution has grown to gigantic proportions, such as the blue whale and sequoia trees. The biggest single organism ever known is a giant mass of Armillaria ostoyae fungus sprawling across three square miles under a forest in Oregon.
Still, these biological feats are pipsqueaks compared to even a small moon.
"We have to use Earth's history as a guide," said Ward, wherein the idea of planet-canvassing organisms, let alone sentient ones, "falls apart to scientific testing."
A key reason is physiology. Though vast, that Oregonian fungus lacks neural tissue, which is the wellspring of the perceptual processing of information. "Intelligence comes from nerve cells," Ward pointed out.
Creatures lacking even simple nervous systems, such as amoebas, can still display a wide range of stimulus-response behaviors. But it takes big brains and highly developed nervous systems to begin exhibiting intelligence.
Those smarts come at a price, as nerve cells require enormous amounts of energy. Our brain makes up just 3 percent of our body, yet consumes 20 percent of our energy. Given this cost, life forms become only as crafty as evolutionary pressures require. "Very few creatures evolve any more intelligence than they need," said Ward. [What If the First Animals to Crawl Out of the Ocean Had Six Legs Instead of Four?]
In primates, complex social alliances are thought to have fueled human memory and language skills. Modern carnivores such as dogs and cats do demonstrate high degrees of social intelligence as well, but only as much as needed. "Dogs don't need to be any more intelligent," said Ward.
Supposing a fungus or a web of bacteria grew to cover a landmass – the latter of which did in Isaac Asimov's novel "Nemesis" – there is not much of a motive for it to develop sentience, Ward said.
Tooth and nail
Competition between species also makes a sentient planet seem like quite a stretch.
Picture any ecosystem on Earth, such as a pond, a forest, a desert. In these environments, critters (and plants) compete for limited resources of food, water and territory in order to survive and make more of themselves.
"If you think about what life is, it has three directives," explained Ward. "It has got to metabolize in order to get energy, it has to reproduce and it has to evolve, otherwise it's a crystal and it's not life."
Not only do species compete against others, but the individuals within a species usually try to outmuscle their peers. Consider the fights between fiddler crabs for real estate on the beach, or rival wolf packs over prey.
All in all, creatures are not programmed to begin cooperating together like the cells in an individual's body. "With natural selection, someone lives and someone dies," said Ward. "How do you go from many organisms competing to one great thing which doesn't compete?"
There are a few exceptions to that rule, however. Insect colonies, for one, are composed of many thousands of individuals, and these "superorganisms" act together altruistically to ensure a queen reproduces. "If you look at bees or ants, those societies are probably as altruistic as one is going to get," said Ward.
Although individual ants are not particularly bright, the colony as a whole comes together to achieve amazing results. An emergent intelligence called a "hive mind" leads to nest construction, foraging, young rearing, attack coordination and even agriculture in ant colonies.
So it's not totally out of the question that a single enormous ant colony could conquer a world and act as a single mind for the colony's (and the planet's) interests, until resources ran dry.
Keeping in contact
That notion, however, brings up yet another issue that would confront a global consciousness — internal communication.
Ants use chemicals called pheromones to exchange information; our bodies use nerves. These methods transmit information quickly across short distances, but getting a message around a colossal being would take quite a long time.
Even if a planet-size entity were a robot, like Unicron in "Transformers," the regions in its body could not digitally communicate faster than the speed of light. At planetary scales, "in a giant, connected web" – such as a mega-computer – "if the thinking is really sort of distributed, it's going to be slower than anything you have in your house," said Seth Shostak, senior astronomer at the SETI Institute in Mountain View, Calif.
Room for Gaia?
All those considerations aside, some scientists have advanced the argument that the Earth itself should be thought of as a giant organism.
Conceived in the 1970s, the "Gaia Hypothesis" proposes that life on Earth works in concert to preserve a mutually sustainable habitat. For example, the salinity and pH of the oceans are ideal for life because life has made it that way.
The Gaia Hypothesis is very controversial, and Ward has disputed it with the fact of several massive extinction events occurring throughout Earth's history. At any rate, self-regulating feedback loops still fall short of planetary sentience in the sense of Earth's biota taking deliberate actions with a group benefit in mind. "You never get that far," said Ward. "Life screws itself long before you can get to networked intelligence."
Shostak is dubious on the matter as well. "After billions of years here, we don't see a lot of this approach by flora and fauna," he told Life's Little Mysteries. "They're all interdependent, but they haven’t tried to make a one world organism kind of thing."
Ultimately, our definition of life might be too limited to encompass something like Gaia. But for now, Gaia joins Mogo in the fiction department.
Plausibility score: Virtually no conceivable mechanism, nor motive, would allow for the development of planet-size intelligent, biological beings. Unicron and Pandora are great for the movies, but when it comes to real life, they earn just one out of a possible four Rocketboys.
Live Science newsletter
Stay up to date on the latest science news by signing up for our Essentials newsletter.