There is something strange about Earth. A few billion years ago, a process started here that we have never seen anywhere else. It completely reshaped the planet’s surface and its carbon cycle, sculpted new landscapes and has kept our home temperate and habitable for billions of years.
That process is plate tectonics, in which Earth continuously subsumes and reforms the slabs of its rocky outer shell. It is thought to be inextricably linked to habitability and perhaps an essential prerequisite for life itself. Without it, our lakes and rivers might have frozen or evaporated, the oceans could have been starved of nutrients and Earth’s climate would probably have veered into unlivable territory long ago. Life would have been in for a rough ride.
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At least, that’s the idea. But it is tough to know whether plate tectonics is really crucial to Earth’s verdant ecology, given that we have nothing to compare it with. We know of no other planet that exhibits plate tectonics: among the four rocky planets in our solar system, Earth is the only one to recycle its crust in this way, and we haven’t spotted definitive signs of it beyond our solar system either.
Until recently, this was more or less where the story ended. But now, with the help of the James Webb Space Telescope (JWST), scientists are beginning to explore the geology of rocky worlds beyond our solar system. Finding one with plate tectonics will be a huge ask. But if we succeed, it could be the key to figuring out how life began here on Earth – and where to search for it out in the cosmos.
While it might be feasible for life on another world to get by without plate tectonics, “we do know that on Earth, it definitely made it more likely that life could develop”, says at the University of Oxford. If Earth is a spaceship, plate tectonics is its life-support system: a recycling process and thermostat in one. It describes how our planet’s rocky outer skin is fractured into a jigsaw of shards – tectonic plates – that drift about over the gooey, molasses-like mantle. In areas where hot material from the mantle wells upwards and the plates pull apart, lava spews forth from volcanoes along the rift, forming new land. When plates crash together, one can end up slipping below the other, driving cool, dense material into the mantle.
Ocean life depends on nutrients that erode out of the continents and terrestrial life, in turn, depends on the fertility of the oceans. But without plate tectonics to build continental crust and constantly thrust up fresh mountain ranges, some scientists think erosion would slow as the land got flatter and flatter, starving the oceans of key nutrients.
Then again, without plate tectonics we might not have liquid water at all. It controls Earth’s “natural thermostat”, the carbonate-silicate cycle, which acts over millions of years to keep the planet’s temperature in check. In this cycle, atmospheric carbon dioxide reacts with water and freshly exposed rocks and is eventually swallowed into the planet’s mantle through plate tectonics, before being belched up again by volcanoes. It is crucial for regulating the amount of CO2 in the atmosphere and thus the temperature of our planet.
The carbonate-silicate cycle has been Earth’s saving grace in the past. It is believed to have rescued the planet from climate catastrophe over the aeons, including at least two “Snowball Earth” episodes, in which the entire world froze over, and multiple hothouse periods that killed off nearly all life. If human-caused climate change culminates in disaster, it could be this cycle that, over millions of years, allows our planet to heal.
Distant worlds
Perhaps other, more distant worlds have similar life-support systems – and now, the door to studying geologic processes such as plate tectonics on exoplanets is creaking open. Over the past two years, for example, JWST has begun revealing a small handful of rocky worlds in unprecedented detail.
Realistically, we are probably still a very long way from discovering plate tectonics on another world, just as we are far away from detecting a definitive signature of alien life. But thanks to new data and techniques, scientists are taking their first bold steps into an entirely new frontier: exoplanet geology.
If we want to find plate tectonics – or at least a strange alien equivalent – we will have to look beyond our solar system. The first planet to reveal its tectonic character just might be a world called LHS 3844 b, which orbits a small, dim star about 49 light years away. “I’m just really excited,” says at the Harvard-Smithsonian Center for Astrophysics in Massachusetts, who is leading an effort to examine the exoplanet’s rocky surface. “It’s really cool that this actually works, especially because these planets are so far away.”
Discovered in 2018, LHS 3844 b is a rocky world a bit bigger than Earth that clings so closely to its star that its years last only 12 hours. This tight orbit has also led the planet to become tidally locked: one hemisphere always faces the sun while the other never sees daylight, resulting in a between the day side and night side of about 770°C (1386°F). It is hard to imagine a more alien place. But planets like this seem to be quite common in our galaxy, says Meier. And unlike worlds reminiscent of Earth, orbiting sun-like stars, these kinds of “ultra-short-period” exoplanets – worlds with zippy orbits – are ideal targets for JWST because of their relative brightness and how frequently they can be observed passing in front of their stars.
“So, I’m really interested in figuring out: could they have plate tectonics? Or what kind of tectonic regime could they be in?” says Meier. In 2021, he and his team used computer models based on observations to show that LHS 3844 b could sport a new kind of driven by the enormous temperature contrast between the day and night sides of the planet: hemispheric tectonics.
“We basically mean upwelling – hot material rising – on one side, and cold material sinking into the mantle on the other side,” says Meier. Hemispheric tectonics might be widespread on other tidally locked worlds too, he says. Meier’s team has found at least that could recycle its crust in a similar way. While LHS 3844 b’s tectonics would be more lopsided than Earth’s, a fragmented, mobile crust broken into plates would at least be a form of plate tectonics, more than can be said for Venus, Mercury or Mars.
Volcanic patterns
We can’t yet detect tectonic plates directly on other worlds, but we may still be able to look for them using indirect clues. One possible sign comes in the form of particular patterns of volcanism. This is because a planet’s tectonic regime dictates how material moves through its interior, which can have a huge impact on volcanic activity. On Earth, for instance, volcanoes tend to pop up along plate boundaries and only rarely within plates. So, while volcanism itself isn’t a smoking gun for tectonics, certain patterns in volcanic activity could be. If LHS 3844 b does have hemispheric tectonics, Meier predicts that one side of the planet would be far more volcanically active than the other.
“[The models] make testable predictions of how volcanism should depend on the day-side temperature and the night-side temperature,” says at the University of Groningen in the Netherlands, who notes it would also be influenced by the planet’s size and composition. “The idea is, in the long term, that we can test some fundamental principles of how tectonics works on exoplanets.”
Alas, detecting volcanism on exoplanets isn’t as easy as pointing a telescope at a rocky world and looking for glowing lava fields. Rocky exoplanets are too small and dim compared with their stars for even JWST to glimpse directly. To learn about what is happening on these worlds, scientists need to use clever workarounds.
Generally speaking, we learn about exoplanets by sifting through the light of their stars. One way to do this is to watch as a planet passes behind its star, causing the star to apparently dim as the glow from the planet’s warmth gets blocked out. By isolating that smouldering light, scientists can take the planet’s temperature and look for patterns across its surface that may point to volcanic activity.
However, “we are talking about the precision limit of JWST”, warns Lichtenberg. “It’s very tough to make these measurements.” He and other researchers are still attempting it, but in the meantime there are other clues to chase. That’s because volcanoes do more than give off heat. They also spew gases into planets’ atmospheres – which we might be able to detect.
Our first glimpse of alien volcanic gases could come from the cool “exo-Earth” LP 791-18 d. at the University of Montreal, Canada, is leading to determine whether the planet is wreathed in a CO2-rich volcanic atmosphere. LP 791-18 d is caught up in a gravitational tug-of-war between its star and a massive neighbouring planet, and the resulting tidal forces should heat LP 791-18 d from the inside out, fuelling intense volcanism, says Benneke.
It is a bit like what is happening to Jupiter’s moon Io, which is riddled with hundreds of active volcanoes. “Io is the most volcanically active object in the solar system, and this planet, LP 791-18 d, is in a very similar situation,” he says. Actually detecting this strange world’s volcanism would confirm that we can find volcanoes that, on other planets, could be a sign of tectonics.
Finding another planet with plate tectonics is a challenge comparable to finding alien life
With the right tools, scientists could eventually start hunting for the chemical fingerprints of specific volcanic gases in the atmospheres of rocky worlds. by at the University of California, Riverside, and his colleagues investigated whether it would be possible to detect the atmospheric signatures of volcanism on an Earth-like exoplanet using a telescope like the Habitable Worlds Observatory, a proposed successor to JWST that NASA is tentatively aiming to launch in the 2040s.
At one point, the most promising potential signature of volcanism seemed to be sulphur dioxide, which would be bulletproof evidence of recent volcanic eruptions. But the researchers found that its signal would be swamped by that of ozone gas in an atmosphere. However, sulphur dioxide eats away at this gas over time, so fluctuations in ozone levels could point to volcanic activity instead.
It might also be possible to detect volcanic hazes, which would temporarily wipe out other atmospheric features. “If you were to take a series of observations across the span of a year, then you might be able to infer volcanism,” says Ostberg.
The ground truth
There is one more way we might be able to get a hint of a planet’s tectonic regime: figuring out what it is made of. When JWST ruled out atmospheres on a handful of planets, including the promising LHS 3844 b, “a lot of people were disappointed”, says Meier. “But we really need to see this as an opportunity. If these planets don’t have an atmosphere, that basically gives us a way of probing the surface directly.”
“When life gives you lemons, make lemonade – or when nature gives you airless planets, figure out what you can do with them,” says at the Max Planck Institute for Astronomy in Heidelberg, Germany. In this case, we can look at surface rocks. Wherever they are, rocks tell a story. Basalt, a blackish rock that dominates the landscapes of Hawaii and Iceland, forms when lava cools on Earth’s surface. Green-hued peridotites make up most of our planet’s mantle. And granite – which comprises the bulk of Earth’s continents but is practically absent from the rest of the solar system – forms when rock is melted down and recycled over and over, as it is in plate tectonics.
Because of the promise that rocks could tell us the history of distant worlds, researchers are already trying to determine what kind of rock dominates the surfaces of certain airless planets. LHS 3844 b is particularly enticing because it is very hot, but not hot enough to be molten – so its thermal glow is about as easy to spot as is possible for a rocky world. The composition of this glow can tell researchers what kind of rock is present, and even hint at its texture.
Remarkably, this might even let us get a first hint at the contours of LHS 3844 b’s landscape. Zieba is JWST data to figure out whether the planet’s surface is smooth, suggesting recent renewal by volcanoes, or made of crumbly, rubble-like regolith. Regolith forms in our solar system when rocky bodies are exposed to space for a long time and pummelled by impacts and the solar wind. If LHS 3844 b’s surface is made of regolith, that would be a good indication that its surface is old – a bad sign for plate tectonics.
Studies like this are our first steps into the entirely new field of exoplanet geology, striving to understand the interiors and surfaces of alien rocky worlds. “We have just barely scratched the surface of what there is to learn about rocky planets,” says Kreidberg.
But if we are going to understand why Earth is habitable, we will almost certainly need to hunt down plate tectonics elsewhere. It seems to be a crucial part of the habitability equation on our planet and it could be important elsewhere, too. But we can’t know for sure until we have more than just our world to examine. “I don’t know if we’ll ever learn what started tectonics on Earth, just from studying Earth,” says Ostberg. “And I think that’s why exoplanets are really valuable.”
Finding another planet with plate tectonics will be a challenge comparable to finding alien life. It might even be harder, says Benneke, because the signatures of geologic processes are often more ambiguous than those of life. In both cases, we are looking for alien instances of phenomena we can’t yet fully explain on Earth. Despite decades of research, neither the origins of life nor the onset of plate tectonics is settled science. Finding the latter might just help us understand the former.
As far as we know, the slow churn of Earth’s tectonic plates might be just as special as the tenuous biological veneer growing over them – one of a kind. Or not. Like the search for life, the search for truly Earth-like planets with tectonics will be a long one, a task for future generations of telescopes, and perhaps future generations of researchers. But it’s a journey we have finally begun.
Article amended on 12 December 2024
We have corrected the Fahrenheit temperature difference between the day and night side of LHS 3844 b
