What does it take to make a planet like Earth?
The hope of one day discovering a planet just like ours that can support life is pretty much the Holy Grail of exoplanet hunting. But it’s not enough to just find a planet in the habitable zone, where temperatures might be appropriate for liquid water to exist on its surface. To maintain these temperatures, an approximately Earth-like atmosphere is implicit.
Our exoplanet-detection methods, be they radial velocity, transits, microlensing or astrometry, can tell us the mass, diameter and orbit of a planet. We can infer some properties of a planet’s atmosphere by cleverly mapping its mass and diameter to models of different kinds of exoplanets, while transit spectroscopy allows astronomers to tease out some details of the atmospheres of some planets. Soon, NASA’s James Webb Space Telescope will be able to examine in detail the composition of the atmosphere of the nearest exoplanets.
However, there’s one planetary property underlying all this that is seldom mentioned, would be difficult to detect from a distance (but not necessarily impossible), and which is vital for a planet to become habitable, and that’s plate tectonics. This relates to how Earth’s crust, which we call the lithosphere, is like a jigsaw, rendered from individual pieces, or plates, that float on an ocean of magma that we call the mantle.
Now, hang on. Plate tectonics are to blame for earthquakes and volcanoes as the plates butt up against each other, or pull apart from one another. How can these natural hazards be good for life? Certainly, if an earthquake hits where you live, or you find yourself in the path of a pyroclastic flow, it ain’t gonna be much fun. But while plate tectonics can be deadly in the short term, in the long term we probably wouldn’t even be here without them.
It’s all to do with the atmosphere. As we alluded to above, a habitable planet requires an atmosphere that can retain heat. Without our atmosphere, Earth’s average surface temperature would be –18 degrees Celsius, and the only water on the surface would be in the form of ice. The Earth reflects much of the radiation (note, we’re using the word ‘radiation’ here to mean electromagnetic energy, as opposed to the radioactive decay of elements) that it receives from the Sun back into space in the form of thermal infrared radiation. However, our atmosphere contains certain gases, such as carbon dioxide, water vapour and methane, that can intercept that reflected heat and absorb it before it can leak back into space, just like a garden greenhouse captures heat. Hence, our atmosphere forms a comforting blanket filled with greenhouse gases.
You will have undoubtedly heard of greenhouse gases in association with human-caused global warming and the climate catastrophe. These are greenhouse gases pumped into our atmosphere by human industry, motor vehicles and so on, therefore adding to the already naturally-produced greenhouse gases and greatly exacerbating the greenhouse effect.
Where do the natural greenhouse gases come from? Water is obviously plentiful on Earth. Methane can be produced biologically or geologically. Ozone is derived from oxygen in the atmosphere. However, by far the most potent natural greenhouse gas is carbon dioxide, and this is where our discussion circles back around to plate tectonics.
The carbon dioxide in Earth’s atmosphere is a product of the carbon–silicate cycle that regulates our planet’s climate over long time frames. It’s like a built-in thermostat; when Earth grows too cold it switches on the flow of carbon dioxide, and once Earth is cosily warm, it takes that carbon dioxide back, reducing the warming effect.
Volcanoes belch out huge quantities of carbon dioxide, which enters the atmosphere and acts to warm it. Higher temperatures means more rain over the long term, which acts to wash out carbon dioxide from the atmosphere, falling as rain to the surface where the carbon dioxide reacts with silicate rocks to produce bicarbonate, calcium and silica, which runs-off, via streams and then rivers, into the sea. Ocean-dwelling microbes incorporate this mix of materials into their shells of calcium carbonate, which inevitably sinks to the sea floor when the lifeforms die.
With large quantities of carbon dioxide washed out of the atmosphere, global temperatures reduce. Be aware, we’re talking tens of thousands, even hundreds of thousands of years for this process to take place (which highlights why human-caused global warming is so catastrophic, as it is accelerating the warming process to be on a scale of decades).
The carbon dioxide in Earth’s atmosphere is a product of the carbon–silicate cycle that regulates our planet’s climate over long time frames
Natural hazards aside, plate tectonics are really about the churning up of the Earth’s crust, with material being gradually subsumed at subduction zones where continental plates are crashing into one another, with one of the plates buckling and slipping underneath the other one and into the molten mantle. Along with this material is all the sequestered carbon dioxide that wound up being rained out of the atmosphere and incorporated into life forms in the ocean.
(Plants are another way of removing carbon dioxide from the air when they ‘breathe’ it as part of photosynthesis. However, the plants will eventually die, the carbon will be trapped in the ground – coal and oil are simply decayed, dead plant matter buried and compressed over millions of year – and eventually this too will be subducted back into the mantle as part of the carbon-silicate cycle.)
The carbon doesn’t remain in the mantle forever, and is gradually outgassed as carbon dioxide by volcanoes. The outgassing warms the atmosphere, and the cycle comes full circle, to begin again.
It doesn’t mean that Earth’s climate never naturally turns hostile, or never experiences ice ages, since these are predicated by a number of factors that are all rolled up into what we call the Milankovitch cycles. These factors include periodic changes in the ellipticity of Earths orbit, the change in Earth’s angle of rotation, and the precession of Earth’s rotational axis, which force changes in Earth’s climate over tens and hundreds of thousands of years. The point is that a well-functioning carbon-silicate cycle is able to prevent the planet reaching a point of no return from these cycles, and drag it back out of a snowball state.
The trouble for the long-term habitability of exoplanets is that, if our Solar System is anything to go by, then plate tectonics are rare. Earth is the only planet orbiting our Sun that definitely has them. We know that Venus has had volcanism in the past, and may still do, but Venusian volcanism does not seem to have been related to plate tectonics, instead mostly coming in a huge burst half a billion years ago. Nor does Mars appear to have plate tectonics, although in the past year, NASA’s InSight lander on Mars has reported three low-frequency marsquakes that seem to have a wave-pattern distinctive of earthquakes resulting from the movement of tectonic plates. Did Mars have plate tectonics in the past (that could have helped make the planet habitable billions of years ago? There may still be some remnant tectonic activity, but it’s clearly not enough to help warm Mars to habitable levels.
Things look equally disheartening when we explore further afield, to the planets we find orbiting stars far, far away. Many of the rocky planets that we have thus far discovered are so-called ‘super-earths’, with masses greater than Earth, and possibly up to ten times the mass of our planet. Theoretical models of the interiors of these worlds – for which we have no example of in our Solar System – disagree about their proficiency to produce plate tectonics.
The energy to drive plate tectonics comes from within. Convection currents – waves of heat – flow through the mantle, warmed at the boundary between the planet’s outer core and cooled from the outermost layers of the mantle. This temperature gradient results in heat flowing through the mantle, much like winds blow through the air as a consequence of an atmospheric pressure gradient. The flow of the mantle carries the plates floating above it along for the ride.
However, this relies on both the mantle and lithosphere not being too stiff. Though molten, the mantle is still rock, not liquid, and if it is too firm then it won’t flow. It will remain stagnant, and planets with no plate tectonics are referred to as ‘stagnant lid’ worlds. This is potentially a problem for super-earths. Jun Korenaga, Professor of Earth and Planetary Sciences at Yale University, has shown that the interiors of super-earths can potentially be so hot that they cause continental plates formed at mid-oceanic ridges to effectively lose much of their water, resulting in the plates becoming so stiff that they become locked in place rather than flowing above or under each other, meaning that no subduction can take place.
Look to the older planets
Age could also be a factor, but not in the way that you might think. Around half of the heating of Earth’s interior comes from its iron core, while the other half comes from heat-producing, radioactive elements such as uranium and thorium. If Earth had a larger iron core than it does (Earth’s core is 2,440 kilometres across) then it would produce a greater percentage of interior heating. Younger planets in the Milky Way Galaxy will have larger iron cores than older planets, simply because the more time that has passed, the more iron has been made available in the cosmos thanks to the profligacy of iron-producing Type Ia supernovae over that time.
However, too much of a good thing could actually be bad for the chances of younger planets being habitable. Recent research produced by a team of Australian scientists led by Craig O’Neill of Macquarie University suggests that the larger iron cores of these younger planets will make the mantle too hot. When that happens, the mantles lose their viscosity, like a particularly runny egg. This limits the stress placed on the lithosphere by the mantle, resulting in the lithosphere not breaking into plates, or even if it does, the mantle will flow beneath the plates without the friction to drag them along.
Instead, O’Neill thinks older planets with small iron cores strike just the right balance between being warm enough to create a temperature gradient to drive convection, but not too warm that the mantle becomes viscous or the plate too dry, and not too cold that the mantle becomes stiff. O’Neill reckons that the rocky exoplanets most likely to have plate tectonics will be older even than Earth, which is 4.54 billion years old (although the fact that we are indeed here and that Earth does have plate tectonics suggests that it’s not impossible for planets the age of Earth to have them). It’s even possible that the first generation of rocky planets, which are perhaps 12 billion years old, could have plate tectonics, whereas planets forming today may be too hot inside to enable them.
These factors potentially limit the number of habitable worlds in the Milky Way Galaxy. They also strengthen the conceptual idea that life in the Universe is likely to be much older than we are, which could have potential ramifications for SETI, the Fermi Paradox and the consequences of making contact.
Are plate tectonics really needed?
However, we are working on the assumption here that plate tectonics are absolutely necessary for habitability, and that a planet without them must be uninhabitable. Yet perhaps there are other ways for the planetary thermostat to operate.
Brad Foley and Andrew Smye at Penn State University have come up with an alternative. Their models have shown that stagnant lid planets could in principle retain enough heat from their formation, coupled with heat from radioactive elements, such that in the right temperature and pressure conditions, carbon dioxide can escape from rocks in the mantle and lithosphere and make its way to the surface, where it can leak out into the atmosphere. Foley and Smye found that there was a sweet spot, whereby a planet can release enough carbon dioxide at just the right rate to warm the atmosphere and maintain liquid oceans for up to four billion years, but not so fast that weathering can’t pull carbon dioxide out of the atmosphere to prevent a runaway greenhouse effect of the type that may have befallen Venus.
It’s worth noting that it’s taken 4.5 billion years for human life to evolve on Earth. If the Penn State scientists’ model is accurate, then had Earth been a stagnant lid planet it may have grown too hot for life to exist before we came on the scene.
A stagnant lid planet would also miss out on all the other things that plate tectonics do, such as churning up and recycling certain materials needed for life, such as water and methane, while the actual physical movement of the continental plates can transport life around the planet, increasing the opportunity for life to diversify and evolve in new and different ways.
The natural hazards that plate tectonics bring with them are no joking matter. People die in earthquakes and volcanic eruptions. They are a constant reminder of the fearsome forces of geology, but remember that they are also a necessary evil for the continued survival of life on our planet.