The intense pressures at the heart of rocky planets larger than Earth will cause elements to combine in ways they never do on Earth. Modeling suggests this could result in exceptionally powerful magnetic fields and much more rapid geologic activity at the planet's surface.
Heavy planets still small enough to be rocky, rather than gas giants, are known as super-Earths. Our search for worlds beyond the Solar System has turned up plenty of them in the last few years. Their frequency has spurred a quest to learn more about their nature, so far depending on models, since the information we can glean from observations is almost negligible.
In Scientific Reports, Professor Artem Oganov of the Skolkovo Institute of Science and Technology, Russia explores what happens to silicon, oxygen, and magnesium at the immense pressures that would exist at the core of a planet with several times the mass of Earth.
"Earth-like planets consist of a thin silicate crust, a silicate-oxide mantle – which makes up approximately 7/8 of the Earth's volume and consists more than 90 percent of silicates and magnesium oxide – and an iron core. We can say that magnesium, oxygen, and silicon form the basis of chemistry on Earth and on Earth-like planets," Oganov said in a statement.
When exposed to forces 5 to 30 million times atmospheric pressure, Oganov found that compounds often end up with more oxygen than occurs on Earth. For example, silicon trioxide (SiO3) joins silicon dioxide (SiO2) as a possible combination of the two common elements. The largest super-Earths, with masses greater than 20 times that of our own world, allow the existence of MgSi3O12 and MgSiO6.
MgSiO12 has fundamentally different properties to other combinations of these three elements, being an electrical conductor rather than a dielectric or semiconductor. The presence of so much conductive material around an iron core could lead to much more powerful magnetic fields even than Earth's. It would also increase heat transference from the core to the surface of the planet, which Oganov argues could lead to more rapid movement of tectonic plates at the surface.
No super-Earth weighing 20 Earth masses has yet been discovered, and until recently such objects were thought impossible. It was anticipated that planets with so much gravity would become gas giants. However, the discovery of Kepler-10c, 17 times the mass of Earth, has rocked assumptions about where the maximum lies.
The enormous gravity on such a planet might challenge complex life, but an enhanced magnetic field would be a useful defense against the stellar storms thought to threaten planets orbiting red dwarfs.
Kepler 10-c is far too hot for life, but the recent detection of a planet with a mass at least 4.3 times that of Earth's in the habitable zone of a nearby star supports the view super-Earths are likely to be among the closest places to find life.