Maybe Earth isn't so special. "We could have easily been standing on Venus having this conversation," says Mark Jellinek, a geologist who studies how planets form.
I envision standing under Venus' thick atmosphere of greenhouse gases, broiling on the dusty planet's surface. Temperatures there soar high enough to melt lead. Sure, Venus and Earth are often said to be the most comparable planets—similar in size, makeup, and distance from the sun—but "easily" seems like a stretch.
Or maybe not. According to recent research, if we restart the "experiment" of our solar system, seemingly insignificant early changes—like internal heat, climate, or water content—could completely reroute planetary history, says Adrian Lenardic, a planetary scientist at Rice University.
Maybe a rewind would lead to life on Venus instead of Earth; or maybe life on neither. After all, scientists are now learning, there might be much more to our balmy climate than Earth's perfectly-sized bulk circling our fiery sun at just the right distance. We cannot ignore a planet's history: "How it got to be where it is; how it started out; how it evolved over time," Lenardic explains.
On Earth, that tipping point could have been a period of intense meteorite impacts, argues Jellinek, of the University of British Columbia, in a recent study in the journal Nature Geoscience. These impacts would have helped exfoliate heat-producing radioactive elements on Earth's surface, allowing the planet to cool, and meanwhile jumpstarting a driver of Earth's internal thermostat, known as plate tectonics.
Habitability is also not permanent.
On Mars, the river channels and dried lake beds discovered there suggest the now-dusty planet had an ancient watery past. Perhaps life once creeped in those waters. And who knows? "Venus could have been a very habitable planet for quite some time," says Lenardic. (Learn fun facts about Venus.)
So what is the latest recipe for a habitable planet? The ingredients might surprise you.
To host an Earth-like menagerie and luscious vegetation, scientists have long said that planets need to fall within the so-called habitable zone. This swath of space exists at a "Goldilocks" distance from a star, where a planet is warmed just enough for liquid water to flow.
"At some level, no one argues with that," says Lenardic. The idea is rather intuitive: Approach a sizzling star too closely, and the planet incinerates; move too far away and the world freezes over.
But it's more complicated than that. For instance: how far from a star is the habitable zone? That depends how hot the star is.
And then there's the size of the planet. Too small and the planet's atmosphere escapes its gravity and is lost to space. Too large and the atmosphere becomes thick and "puffy," says Cowan, with potential to become a freezing giant like Neptune and Uranus.
Discoveries from NASA's Kepler telescope—launched in 2009 to seek out worlds ripe for life—show that a planet up to 1.5 Earth radii may be habitable, says Nick Cowan, a planetary scientist at McGill University.
Of the 1,030 planets (and counting) Kepler has identified, a handful meet the Goldilocks standards for both size and distance, with Kepler 452b perhaps the most similar to Earth.
But these qualities alone do not make a habitable place. And a growing cohort of scientists believe the recipe is much more complex.
A Perfect Crust
Earth's outer surface is flexible: It pulls and pushes, driven by the churning inner Earth, known as mantle convection. Sometime in geologic history—it is much debated when—this tug-of-war cracked the surface into a series of creeping plates.
But why do they matter? The plates are part of Earth's thermostat. Their collisions stoke volcanic eruptions, which "burp" the greenhouse gasses necessary for atmosphere. And as Earth gets "hot under the collar," the collisions pull extra gas back down into its depths, says Cowan.
Earth is the only planet known with a system of actively creeping plates, a process called tectonics, and according to Jellinek, it is what truly sets us apart.
Recent computer simulations show that even on Mars, with the perfect mix of gases in the sky, an active tectonic system would leave the surface of the planet quite livable, says James Kasting, a planetary scientist at Penn State, who helped shape the modern idea of a habitable zone.
All The Right Stuff
But what causes the plates to move in the first place? It's all in the ingredients.
When planets first form from a condensing cloud of dust, they sizzle with heat from the molten inner core, and radiate heat from radioactive surface elements. When hot, the surface bends and flexes. But like a candy bar in the freezer, the planet toughens as it cools, and when frozen threatens to break your teeth before it yields.
Jellinek argues that the Earth eventually arrived at a happy temperature, cool enough for the crust to break, thanks to an intense meteorite shower that rained down on our infant planet for its first 20-30 million years, shaving some heat-producing radioactive elements from the planet's surface.
Venus may be an example of what happens when temperatures run high, he says. Instead of Earth's minutely creeping plates, Venus' surface is too hot and "runny" to break. Eventually heat builds up until "the whole surface caves in on itself," says Jellinek. Catastrophic volcanism ensues, forcing the planet into a hothouse state.
But if Venus had been the target of the early meteorite shower instead of Earth, would that have changed the history of life as we know it? According to Jellinek this may be the case. But not all scientists agree. Kasting argues that our neighbor Venus is just too close to the sun to sustain liquid water.
Tiny and freezing, Mars is the other extreme: its surface never broke.
When looking at ingredients, we also can't stop skin-deep. The roiling convection in the inner earth drives the shifting plates. So if the planet's minerals are too dense, they'll "gum up your mantle convection and slow it down," says Cayman Unterborn, a graduate student at Ohio State University.
Remember the diamond planet? That dense, carbon filling prevents convection from ever starting, says Unterborn.
Testing the Recipe
Which of these factors is most important for habitability? It's tough to say.
The most crippling problem in creating a recipe for life is the lack of Earth-twins to study. "You're testing against this solar system and assuming it is universal," says Lenardic.
Until NASA's Kepler spacecraft launched in 2009 to seek out habitable worlds, scientists assumed that our solar system was a blueprint for other worlds.
What we found was that "our solar system is a freak," says Cowan. "Eventually we might discover that some of the stuff we found based on Earth is just not true," he says.
For instance, take Earth's magnetic field, which is widely thought to help hold onto our atmosphere. Though it probably helps shield us from solar winds and flares, is it truly important for habitability? The evidence is lacking, according to Jellinek and Lenardic.
The answers lie out there, somewhere, in the star-studded galaxy.
A world of new possibilities is on the horizon. NASA's James Webb telescope will launch in 2018—but even this advanced scope will be limited to searching in our backyard, says Cowen, about ten light-years from Earth.
To really get to the nitty-gritty details, we need to turn to the "next, next generation" of telescopes, says Cowen: HDST, LUVOIR, and ATLAST to name a few. These mega-scopes—as wide as five buses end to end—could squint at Earth-twin candidates much farther away, even roughly mapping clouds, continents, and oceans on their surface.
Though a launch before the 2030s is unlikely, the possibilities are tantalizing.
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