Forbidden Planet, Forbidden Chemistry
Bizarre chemical structures are helping us better understand how our planet formed and, perhaps, where to find life elsewhere in the universe.
Some chemical formulas just don’t look right. Any students who wrote H4O or Na3Cl on an exam would be well on their way toward failing because under Earthlike conditions those compounds would disintegrate instantly. But a new study suggests that some “impossible” combinations of elements might actually be common on planets in other solar systems.
Astronomers hunt for distant planets, called exoplanets, by searching for subtle changes in the light from stars: the light might dim slightly as a planet passes in front of a star, for instance. It’s not easy work, and the precision involved boggles the imagination; it’s like standing in Maine and searching for a flea on a light bulb in San Diego. Despite these handicaps astronomers have discovered almost 3,200 exoplanets since the first one was spotted in the 1990s.
Surprisingly, many exoplanets proved to be “super-Earths,” rocky planets several times more massive than Earth. One, called Kepler-10c, weighs 17 times more—roughly equal to Neptune’s weight but much more compact. Astronomers once thought such planets would be impossible because their mass should have sent them down a different evolutionary path: unlike Earth they should have retained hydrogen gas during their formations long ago and become gas-giant planets. Somehow, though, they lost their hydrogen and became giant space rocks instead.
Despite their mass and size, super-Earths probably have elemental compositions similar to Earth’s. Elements are made inside stars, in a process called stellar nucleosynthesis, and this process inevitably makes larger amounts of certain elements—such as oxygen, magnesium, and silicon—than others. So when stars explode and give birth to new solar systems, that trio ends up forming the bulk of rocky planets, Earths and super-Earths alike.
That said, those elements would experience far higher pressures inside super-Earths, and here’s where things get interesting. A group of American, Russian, and Chinese chemists recently ran a computer simulation that subjected virtual compounds of magnesium, silicon, and oxygen to pressures up to 30 million times greater than atmospheric pressure. The simulation also let researchers fiddle with the virtual compounds’ molecular geometry, among other properties. From those calculations the scientists determined which Mg-Si-O configurations had the lowest energy and would therefore be most stable.
Deep inside Earth, magnesium, oxygen, and silicon tend to form minerals like enstatite, MgSiO3. Inside super-Earths exotic compounds like MgSi3O12 and MgSiO6 would likely form instead. Even more exciting, MgSi3O12 seems to act like a metal. Considering that silicon and oxygen dominate the composition of MgSi3O12, this would be akin to discovering that silica sand (SiO2) can conduct electricity like copper or iron.
Beyond its novel chemistry this metallic sand could have big consequences in the hunt for alien life. So-called solar winds—streams of particles that emerge from stars—tend to erode a young planet’s atmosphere by knocking gas molecules into space. This chain of events in turn exposes nascent life to harsh conditions. But high concentrations of metallic MgSi3O12 inside a planet would boost the magnetic field. That’s important because planets’ magnetic fields can act as windbreaks and deflect solar winds, preventing atmospheric erosion. We can see the benefits of a magnetic field by comparing the richness of life on Earth, which has both a strong magnetic field and a robust atmosphere, with the barrenness of Mars, which basically lacks both. When searching for life on exoplanets, then, the chemistry of magnesium and silicon could be as important as that of carbon.
To be sure, this work on MgSi3O12 is still merely a computer simulation and needs verification. But the scientists running the project have a good track record. Previous simulations they’ve run predicted the existence of other funny-looking compounds, such as Na3Cl and NaCl3. When the team crushed sodium and chlorine together with high-pressure diamond anvils, these “forbidden” compounds did indeed form.
Beyond the implication it has for the existence of life on other planets, this work can illuminate how planets form and evolve in other solar systems, which will help us understand the origin and evolution of our own planet. Equally important, this work expands our chemical imagination and forces us to envision new realms of science. The only reason Na3Cl and MgSi3O12 look funny to us is that we’re used to terrestrial chemistry, which takes place under a narrow range of pressures and temperatures. But those conditions don’t hold everywhere in the universe. For all we know, Earth-sized planets might be rare in the galaxy, and our chemical intuitions might be nothing more than provincial prejudice. If so, these super-Earths and their super-chemistries could open up whole new worlds in the wider chemical galaxy.