For Earth to be able to support life, a lot of things needed to be just right. The Sun had to be the right size and brightness, and just at the right distance from the Earth; the atmosphere that protects the planetary surface from harmful radiation; the chemistry needed for water and the seeds of life; and a crust and plate tectonics. We don't often think about plate tectonics as a key ingredient for life, but it is. Without a crust, plate tectonics couldn't exist, and without plate tectonics, life as we know it couldn't exist.
The crust is where dry, hot rock from the deeper parts of the Earth interact with the water and air on the surface, producing new minerals and rocks. New crust is constantly being produced and destroyed, and if this didn't happen, the seafloor could become rigid and much more unfriendly to life. New research is suggesting that plate tectonics is essential to life as we know it.
There are also two types of crust on Earth: basaltic and granitic. The basaltic crust is dark and heavy, and sometimes called oceanic crust. Meanwhile, granite crust is light and accumulates into continent-sized rafts that move in this "sea of basalt." It's sometimes called continental crust. When an oceanic crust moves against the continental crust, the heavier and denser oceanic one subducts (or goes under the other one).
But our planet's crust may be a rarity, at least in our corner of the universe.
Keith Putirka from California State University, Fresno, and Siyi Xu of the Gemini Observatory analyzed the atmospheres of 23 nearby white dwarfs, looking for signs of so-called "pollution" -- chemical traces that stars can pick up from nearby planets as they explode into red giants.
"White dwarfs start out like our Sun, and late in life expand to become a red giant, and then collapse to a very small size – about the size of Earth," Putirka told ZME Science. "As the star collapses, planets orbiting the star (at least those not obliterated during the red giant phase) can orbit close enough to the star that they are destroyed by tidal forces. The debris that results can fall into the star’s atmosphere. This infalling debris is the “pollution” that is measured by astronomers, and records the composition of the formerly orbiting planetary objects."
The researchers found that some contain high amounts of calcium (Ca), but all have very low silicon (Si) and high magnesium (Mg) and iron (Fe) amounts. This suggests a composition closer to the mantle of exoplanets, and not at all what you'd expect to see from a planetary crust.
Roughly speaking, the Earth consists of a crust, a mantle, and a core. Although the movement of plate tectonics is driven by movement from the mantle, it's the crust that is fragmented into rigid plates (hence the "plate" tectonics). But there seems to be no sign of a granitic crust -- or even other crust types. So plate tectonics may have not existed on these planets, or if it did, it was very different from what we see on Earth.
"It’s hard to say whether granitic crusts might exist on other planets, or not," Putirka explained to ZME Science. "In our Solar System, granitic crust only exists in any great abundance on Earth, and its abundance is probably related to our abundant surface water and plate tectonics, which are also unique to Earth. The exoplanets that once orbited white dwarfs have silicate mantles (all the rocky material between the iron core and the crust) that are very different from Earth - so different that plate tectonics and crust formation might occur very differently."
"Some exoplanets may have mantle compositions that might yield very thick granitic crusts and more abundant continental material than on Earth. Others have mantle compositions that might not produce any continental crust at all. Many of these planets may look totally unlike anything we see in our inner Solar System."
The findings have important implications for potential life on other planets -- but it's hard to interpret just what this means. But what is clear is that we need to have a broader view when we consider exoplanet environments.
"I think it's fair to say that the trajectory of biological evolution is dependent upon geologic history. For example, if a planet has abundant water, but no granitic crust, then nothing like the terrestrial life as we see on Earth could possibly evolve – because there would be no terra firma for such evolution to take place. But we don't yet know how such odd exoplanets (in the white dwarf database) might evolve from a geologic standpoint because, up to now, we have focused our laboratory experiments (on how rocks melt or deform) based on questions about how Earth-like (or Mars-like or Moon-like) planets might evolve. But the compositions we see in the white dwarf data indicate planets that are mineralogically very different, and so require new experimental studies."
Ultimately, this study still shows that there's plenty we don't know about the Earth -- if we did, we'd have a better idea of what makes it so special (if anything). There are still plenty of questions we need to answer about our planet, and only once we do (and once we study our solar system neighborhood more closely) will we be able to understand planets outside of our solar system as well.
"My take on our findings is that it reflects back on what we still don’t know about Earth and our rocky planetary neighbors.," Putirka concludes "Earth not only has abundant water and life, but two very distinct kinds of crust - one of which (granitic, continental crust) is effectively unique in our solar system, and is essential for human evolution. Earth is also the only planet that has a long history of plate tectonics."
"How and why did these features appear on Earth, and what inhibited their development elsewhere in our own Solar System? Tectonics and crust formation are surely sensitive to planet size, orbital radius, and planet composition. To what extent can we change any of these parameters and still end up with a habitable planet – and/or something that, geologically, looks roughly like Earth? We'll have better answers to these questions when we conduct new experiments, and when we start exploring Venus, and when we shift our focus from trying to find life on Mars to instead better understanding the geological conditions that limited evolution there in the first place."
The study "Polluted white dwarfs reveal exotic mantle rock types on exoplanets in our solar neighborhood" was published in Nature Communications.