The steady stream of terrestrial exoplanet discoveries that have come from NASA’s Kepler mission, as well as other ground- and space-based observatories in recent years, have greatly expanded our knowledge and understanding regarding the variety of our galaxy’s planetary population. Furthermore, the discovery of so-called “Earth-analog” worlds (many of which have been characterised as potentially habitable, like Kepler-452b) have created the impression that they could more or less be crude copies of our home planet. Despite the promising prospects that this might be the case for some of them, there are many unknowns still remaining about several of the basic properties of these exoplanets, like their masses, densities, and overall chemical compositions. A new study comes to shed more light to these unknowns, by showing that rocky exoplanets could have internal compositions that are greatly different from Earth’s, which could in turn lead them to exhibit entirely different planetary environments than that of Earth.
The internal structures and overall properties of planets are influenced by a set of many different factors, but are mainly determined by the type of material that were present during their formation inside the circumstellar disks of their host star and their orbital distance from the latter. In our own Solar System, for instance, the temperatures in the inner parts of the protoplanetary disk that eventually gave rise to the planets 4.5 billion years ago were such that permitted only the heavier rocky and metallic particles to condense, leading to the formation of the terrestrial planets Mercury, Venus, Earth, and Mars. Further out where temperatures were cooler, allowing lighter volatile elements like hydrogen and helium to solidify, which formed the icy particles that eventually gave rise to the gas and ice giant planets Jupiter, Saturn, Uranus, and Neptune.
Our home planet has a differentiated interior which is mainly characterised by the presence of metals like iron, magnesium, and sulfur, as well as silicate rock and other high-pressure minerals like magnesium oxide (MgO). The latter in particular is the dominant constituent of the Earth’s mantle, due to the fact that it remains stable and non-reactive under the very high temperatures and pressures that exist there. This has led many planetary scientists to theorise that magnesium oxide could also play a dominant role in the interiors of other terrestrial exoplanets as well. Yet one defining factor that has helped to shape the compositions of the planets in our Solar System has been the metallicity of the Sun, which is the measure of the amount of “heavier elements” like oxygen, carbon, and iron in its core relative to hydrogen and helium. As different types of stars have different metallicities, it can be expected that they in turn would give rise to rocky planets whose interiors and compositions could differ greatly from Earth’s.
A research team, led by Sergey Lobanov, a geologist and geochemist at the Carnegie Mellon University in Washington, D.C., set out to study this concept further, by exploring the possibility of exoplanets exhibiting interiors that could be rich in magnesium peroxide (MgO2) instead. One key characteristic of magnesium peroxide is that it becomes highly unstable when it is heated, making it an unlikely contributor to an exoplanet’s elemental composition at first glance. Nevertheless, past theoretical studies had suggested that magnesium peroxide could indeed become stable at very high temperatures as well, in the range of a few thousand Kelvins. With the goal of testing this hypothesis, Lobanov’s team used synchrotron x-ray diffraction on small samples of diamond-anvil cells in order to simulate the high-pressure and temperature conditions that are found on planetary interiors. As part of the researchers’ experiment, the diamond-anvil cells were subjected to temperatures above 2,000 Kelvins and pressures as high as 1.6 million times that of ambient atmospheric pressure. The results of the experiment showed that when the diamond-anvil cells had reached a temperature of 2,150 Kelvins and a pressure approximately 950,000 times that of ambient atmospheric pressure, magnesium oxide chemically reacted with free oxygen to form magnesium peroxide, which remained stable under these extreme conditions. That, according to the researchers, suggested that magnesium peroxide could indeed take the place of magnesium oxide as a constituent of planetary interiors. “Our findings suggest that magnesium peroxide may be abundant in extremely oxidized mantles and cores of rocky planets outside our Solar System,” says Lobanov. “When we develop theories about distant planets, it’s important that we don’t assume their chemistry and mineralogy is Earth-like.”
Since many stars are more rich in oxygen than the Sun, this could mean that any potential rocky planets that they might harbor could have a different internal chemistry than that of the terrestrial planets in our Solar System, as evidenced by the results by Lobanov’s team’s study, which was published at the Scientific Reports journal. “These findings provide yet another example of the ways that high-pressure laboratory experiments can teach us about not only our own planet, but potentially about distant ones as well,” says Alexander Goncharov, a researcher from the Carnegie Institution for Science and member of Lobanov’s team.
Taking a cue from the Star Trek franchise, our galaxy’s population of alien worlds seems to be characterised by an infinite diversity in infinite combinations, as evidenced by the findings of exoplanetary research to date. Hopefully, the next generation of space-based observatories, like NASA’s James Webb Space Telescope, that are scheduled to be launched in the next couple of years will turn much of the current speculation regarding the physical characteristics of these distant worlds into hard facts, allowing us to study not only the chemical compositions of their atmospheres and surfaces, but of their interiors as well.