Deep inside planet Earth there is a fluid outer core and a solid inner core that rotate in opposite directions. This creates the dynamo effect, which is responsible for creating the earth’s planetary magnetic field. This field, also known as the magnetosphere, keeps our climate stable by preventing the Earth’s atmosphere from being lost into space. Of course, when scientists study rocky exoplanets, they wonder if they too have magnetospheres.
Unfortunately, we are forced to infer their existence from the evidence available until we can measure the magnetic fields of an exoplanet. That’s exactly what researchers at Sandia National Laboratories did with their Z Pulsed Power Facility (PPF). Together with their partners at the Carnegie Institution for Science, they were able to recreate the gravitational pressure of “super-earths” to see if they could generate magnetic fields.
The research team was led by Yingwei Fei, a geochemist from Carnegie Earth and Planets Laboratory (EPL), and Christopher T. Seagle, a postdoctoral fellow and manager of Sandia National Laboratories (SNL). Several researchers from the EPL and the SNL joined him. Their results were presented in a study recently published in Nature Communications.
Artist’s impression of the moon in the earth’s magnetosphere with “earth wind” made of flowing oxygen ions (gray) and hydrogen ions (light blue). Photo credit: E. Masongsong / UCLA / EPSS / NASA / GSFC SVS
When it comes to measuring the habitability of planets, scientists are forced to take what is known as the “low hanging fruit” approach. This means looking for Earth-like planets, which essentially means rocky planets with dense atmospheres made up of nitrogen, oxygen, carbon dioxide, methane, and other gases. Another important consideration is whether or not a planet is orbiting in the habitable zone (HZ) of its parent star.
Planets orbiting in this area experience temperatures warm enough to keep liquid water on their surfaces. As scientists have noted in recent years, geological activity is also an important factor in maintaining the habitability of the earth. As Richard Carlson, director of the Earth and Planets Laboratory, stated in a Carnegie Science press release:
“Although observing the atmospheric composition of an exoplanet is the first way to look for signatures of life beyond Earth, many aspects of a planet’s surface habitability are affected by what happens beneath the planet’s surface, and here is the long-term Carnegie researchers’ expertise on the properties of rocky materials under extreme temperatures and pressures comes in. “
Over the past few years, exoplanet surveys have resulted in no fewer than 4,341 exoplanets being detected in 3,216 systems (an additional 5,742 candidates are awaiting confirmation). Of those confirmed, 1340 rugged planets were identified as rocky planets many times the mass of Earth and up to 8 times the size – hence the name “Super-Earth”.
The Z machine from Sandia National Laboratories. Credits: SNL / Randy Montoya
“The ability to make these measurements is critical to developing reliable models of the internal structure of super-earths that range up to eight times the mass of our planet,” added Fei. “These results will have a profound impact on our ability to interpret observational data.”
“The question before us is whether any of these superplanets are actually Earth-like with active geological processes, atmospheres and magnetic fields,” said Joshua Townsend, Sandia physicist and co-author of the paper in a recently published SNL press release. In other words, are these exotic and massive rocky planets capable of supporting life as we know it?
The Z PPF is located in the heart of Sandia National Laboratories in Albuquerque, New Mexico. It is based on special instruments – like the multi-anvil apparatus, the piston cylinder, and the diamond anvil cell – to simulate the high pressure and temperature conditions in a the interior of the planet. This allows them to measure the physical properties of exoplanets and mimic their gravitational environment.
For their study, the Carnegie / SNL team replicated the gravitational pressure of “Super-Earths” by applying the equivalent of a giant gravitational pressure to bridgmanite (also known as magnesium silicate) almost instantly. This mineral is the most abundant material found inside rocky planets and has been used to simulate the mantle material of a super-earth.
Artist’s impression of the inner structure of the earth. Photo credit: Argonne National Labs
By exposing the Bridgmanit to hypervelocity shock waves generated by the Z-machine, the team was able to generate pressures representative of a super-earth interior. Under these conditions, the team found that bridgmanite has a very high melting point, which could have serious effects on the internal dynamics of super-earths.
As they suggested in their study, under certain thermal evolution scenarios, massive rocky planets could develop a thermally driven geodynamo at the beginning of their evolution. However, this dynamo effect can go away as the cooling of the planet’s interior slows down and starts again due to the movement of lighter elements and the crystallization of the inner core.
The experiments also made it possible to create a data table showing how the state of the interior of a planet (solid, liquid or gas) changes depending on pressure, temperature and density conditions (and for how long). As Fei explained about SNL’s release:
“In order to create models that will enable us to understand the internal dynamics and structure of super-earths, we must be able to extract data from samples that approximate the conditions there, which are 14 million times atmospheric pressure could exceed. However, we repeatedly encountered limitations when it came to creating these conditions in the laboratory. “
Artist’s impression of a super-earth planet orbiting a sun-like star. Photo credit: ESO / M. Kornmesser
“Z has provided our collaboration with a unique tool that no other technology can match to study the extreme conditions inside the super-earths,” he added of Carnegie Science’s publication. “The machine’s unprecedented high quality data was critical to expanding our knowledge of super-earths.”
Based on their analysis of the state of the super-earth interiors, the team also compiled a list of seven planets that may be worth investigating further. These include 55 Cancri e, Kepler-10 b, Kepler-36 b, Kepler-80 e, Kepler-93 b, CoRoT-7 b and HD-219134 b. As Seagle, who originally suggested these experiments with Fei, said:
“These planets, which we have found to be the most likely to support life, were selected for further study because their iron, silicate and volatile gas ratios are similar to that of the earth, plus internal temperatures to maintain magnetic fields for protection Solar wind are suitable. ”
Super-earths have become a focus of interest because of their larger size and mass, exerting great gravitational pressure. As a result, these planets are likely to hold onto their atmosphere longer to ensure that life has a better chance of evolving and evolving to a more complex state.
Artistic design of the super-earth GJ 625 b and its star GJ625 (Gliese 625). Photo credit: Gabriel Pérez / SMM (IAC)
Their considerable mass also means that pressure and temperature conditions in their interiors are more likely to result in a geodynamo. As Townsend explained, the contrast between Earth and Mars shows how this works. “Because Mars was smaller, it initially had a weaker gravitational field,” he said. “Then, when its core cooled rapidly, it lost its magnetic field and its atmosphere was subsequently removed.”
It’s no secret that the field of exoplanet research has grown by leaps and bounds over the past few decades. In the coming years, next-generation instruments will fly into space or be ready for use here on earth. In anticipation of this, scientists are working diligently to develop models, methods and frameworks that enable faster characterization.
Not only do these tell astronomers where to look, but they also help astronomers spot the tell-tale signatures that could indicate the presence of life (also known as biosignatures). The search for life beyond Earth has been tough and ongoing, and probably always will be. But it’s also getting a lot more exciting and soon!
Further reading: SNL, Carnegie Science, Nature Communications