Exoplanets: the interior of super-Earths is reproduced on Earth

This March 3, 2022, almost 5,000 exoplanets were discovered by the eyes of the noosphere, many of which are on orbit like the satellite Tess, successor to Kepler. We are waiting for revelations about some of them when the gaze of the James Webb telescope will focus on them and their atmospheres looking for potential biosignatures.

But in the meantime, exobiologists can try to assess the chances that some exoplanets rocks in the habitable zone may be favorable to the appearance and then the evolution of life, as we know it, by trying to model the interior of these exoplanets and in particular in the case of super-earths.

We know well that, in the case of our Blue Planet, its mantle intervenes in the geodynamics of the tectonic plateswhich has impacted the evolution of life on Earth with continental drift, or colossal eruptions like those of traps Siberian, and that its nucleus produces a magnetic field protector against cosmic rays via its geodynamo, the operation of which was explored in the lab with the VKS experience. The question therefore naturally arises as to whether these phenomena can be reproduced on other telluric planets outside of Solar system.

It is a problem of comparative planetology transposing to exoplanets the data of geophysics and geochemistry on Earth within the framework of cosmochemistry andastrophysicswhich we have been doing for years with numerical simulations on computers or experiments in high pressure physics, notably with diamond anvil cells.

The diamond-tipped mantle of the super-earths

As Futura had already explained in the previous article below, a team made up of several researchers, some of whom were from Carnegie Institution for Science (Washington, DC) had already explored the physics and chemistry from inside the super-earths with the Z-machine. Today, it is once again members of this institution who have conducted new experiments on this subject with anvil cells and who have combined the results obtained with numerical simulations as explained in an article published in Proceedings of the National Academy of Sciences but which can be viewed at arXiv.

Rocks in the Earth’s mantle are silicates and the data of cosmochemistry, the observations of protoplanetary discs tell us that silicates are present in these discs and that the rocky exoplanets must also contain silicates even if we also consider carbonaceous exoplanets.

Super-Earths are more massive than our Blue Planet so it is a priori even more difficult to probe the physics and mineralogy of their interiors than in the case of the Earth or the giants of the Solar System. But the team led by researchers from the Carnegie’s Earth and Planets Laboratory used a trick. The physico-chemical properties of theatom of germanium are close to those of the atom of silicon and calculations suggested that analogues of silicates, germanates, could behave similarly.

Experiments in diamond anvil cells with germanate of magnesium of Mg formula₂Geo₄ should make it possible to verify predictions of phase transition between two crystalline phases of silicates in the mantle of super-earths at lower pressures and temperatures.

It turned out that, under a pressure of two million atmospheres, a new phase emerged with a distinct crystal structure that involves a germanium atom bonded to eight germanium atoms.oxygen whereas, under ordinary conditions, most silicates and germanates are organized in what is called a tetrahedral structure, a central silicon or germanium bonded to four other atoms. However, under extreme conditions, this can therefore change and it was already known that under extreme pressures, silicates could take on a structure oriented around six bonds, instead of four, which had completely changed the situation in terms of understanding deep Earth dynamics.

One might therefore think that the discovery of the appearance of an eightfold bond could have similarly revolutionary implications for the way we think about the dynamics of the interiors of rocky exoplanets.

Scientists reproduce the interior of superterres on Earth with the Z-machine

Article of Laurent Sacco published on 02/11/2021

Super-Earths are larger in size than Earth and if they have atmospheres, some may be habitable. We are trying to determine under what conditions their coats can melt giving a nucleus liquid metal generating a protective magnetic field, by reproducing on Earth the state of these rocky coats.

A team of physicists just published in NatureCommunications the results of work that would no doubt have interested Percy Williams Bridgman (1882-1961), one of the pioneers of high pressure physics. By developing a technique for submitting samples of matter at pressures exceeding 100,000 atmospheres, the physicist made discoveries that earned him the 1946 Nobel Prize in Physics. We can cite, for example, that of the existence of new phases of ice.

We owe him above all the idea of ​​anvil cells, which made it possible to carry out experiments concerning the state of rocks inside the Earth or of matter in the heart of giant planets like Jupiter. Bridgman’s anvil cells were originally composed of carbide of tungsten. They paved the way for diamond anvil cellswhich is routinely used today for high-pressure physics experiments.

From Earth to Super-Earths

Today, high pressure specialists and planetary scientists have even broader ambitions. They want to understand the interiors of exoplanets and especially those that are rocky. This understanding is important to clarify the conditions of habitability of telluric exoplanets and thus contribute to advancing theexobiology.

The study of the Earth has taught us that its habitability is linked to plate tectonics, which plays a regulator with regard to runaway greenhouse effect or, on the contrary, entry into a glaciation almost global with the carbon dioxide. The presence of a significant magnetic shield represents a protective factor against the erosion of the atmosphere which was fatal in the case of Mars.

If we transpose these considerations to exoplanets and in particular to super-Earths, it therefore seems normal to model the interior of super-Earths to obtain additional knowledge, allowing us to assess to what extent they can allow the appearance of life and its evolution towards complex forms. We cannot separate the atmosphere of a telluric exoplanet from its interior and from the exchanges existing between them. But to achieve as much as possible a modelization scientist of the interior of the superterres, it is necessary not only to make analytical calculations and numerical simulations but to be guided by experiments. Experiments also used to check the validity of hypotheses and proposed theories.

Magnetic necking to probe the physics of high pressures

But to reproduce on Earth the pressure and temperature conditions prevailing in the heart of the super-Earths, it is good to have spectacular instruments, because the performance is by no means obvious. Fortunately, a team of American researchers from the laboratories sandia in Albuquerque (New Mexico) and the Carnegie Institution for Science (Washington, DC) had the famous Z-machine.

She made a name for herself when she unexpectedly reached temperatures of two billion degrees in 2006 using a material compression technique called constriction. axial (or also called Z pinch, which explains the name of the machine). Just as in the case of inertial fusion by laserthis makes it possible to compress a capsule of combustibleso a cylinder axially symmetric, using magnetic pulses. We are talking about magnetized liner inertial fusion (Maglif).

Transformed into exogeophysicists, the researchers carried out their work using a magnesium silicate which is known to be the most abundant in the terrestrial mantle and which received the name bridgmanite in honor of the pioneer of high-pressure physics.

Super-Earths protected by magnetic fields?

The bridgmanite samples were sandwiched between plates of copper and D’aluminum the size of a credit card. These plates were propelled at the speed of a rifle bullet under the action of magnetic fields generated by currents of 26 millionamps.

The impact of the plates on the rock samples caused the equivalent of seismic waves. The propagation speeds of these waves and their other characteristics being related to thestate of matter crossed, it was possible to deduce if one was in the presence of a liquid, a gas or a solid and therefore to draw the curves – the charts describing changes in state of bridgmanite in response to pressures generated.

On the strength of these results, the planetologists were able to specify under which conditions of size and composition of the super-earths would have a liquid metallic core resulting from the fusion of their mantle and capable of generating a magnetosphere protective. They deduced that several already known super-Earths could be promising for more detailed observations in the near future, because likely to have preserved a atmosphere thanks to their fields of gravity and a magnetic shield.

In this case, it is 55 Cancri e ; Kepler 10b, 36b, 80e, and 93b; CoRoT-7b; and HD-219134b.