AB Initio Simulations for Super-Earths

Since the first detection in 1995, awarded a Nobel Prize very recently, more than 4000 exoplanets have been discovered outside of our Solar System, around nearby stars. The size range goes from planets about the size of Mars to gigantic objects several times the size of Jupiter. Among the most common objects we found, are planets between 1 and 2 times as big as the Earth. We strongly suspect these objects to be rocky (made mostly of oxygen, silicon, magnesium, iron and nickel) that is why we nickname them: Super-Earth. On average almost every single star has a Super-Earth which means they are very common and we might find life on one/some of them. However little is known about these exoplanets and the ABISSE project aims at better understanding these objects. One of the key question that ABISSE is looking at answering is: is it possible for a Super-Earth to have a magnetic field? For a planet to produce a magnetic field, it needs to have a conducting fluid in a convective motion (the convection is what happens when you heat up water in a pan). Inside Earth the magnetic field is produced by a portion of the iron core in a liquid state and where a dynamo process occurs. On Super-Earths the iron core may very well be fully crystallized making a dynamo process impossible there. ABISSE is dedicated to finding where such a process could occur. Yet, a magnetic field would be an important feature to characterize the habitability of a planet. A magnetic field helps protecting the surface from deadly radiations coming from the host star and it is also likely to protect the atmosphere from being blown away by the stellar winds. Thus, determining the conditions for the existence of a magnetic field on large rocky objects is a step further in finding habitable planets and life outside of Earth.

The ultimate goal of ABISSE is to characterize the materials inside a Super-Earth and to determine if their properties offer the possibility for magnetic fields to be produced. The Earth is made basically of three areas: a thin crust of the order of 10 km, a mantle of 2,900 km and a core of 3,300 km in thickness. The highest pressure in the mantle is ~140 GPa which is 1,400,000 times the atmospheric pressure. In the core it goes up to 350 GPa. For Super-Earths it is anticipated that it could go higher than 500 GPa, just shy of 1000 GPa. Under these conditions, matter is highly compressed and its properties change. Using sophisticated numerical simulations, ABISSE characterizes matter under such conditions and answers questions such as: Is iron liquid? Solid? What happens to the mantle, do we have a magma ocean? Is the planet differentiated as in Earth? Are the conditions compatible with a dynamo process? All these pieces of information are necessary to determine if a magnetic field can exist and, consequently, help constraining the habitability of a Super-Earth.

In the coming decade, many space programs as well as ground-based observation campaigns are going to be dedicated to the study of exoplanets and the search for life outside the Earth and the Solar System. It is thus very timely to perform such studies allowing a better understanding of these new worlds.

We made great progress on two topics. The first one concerns the mantle. It had been suggested that molten silicates are electrically conducting and that an enrichment in iron could also increase the electrical conductivity even more. But one needs to first determine if the mantle if fully molten or not. As a first step, we studied the phase diagram of magnesium oxide which is one of the main end members of the expected composition of rocky planets mantle (MgO, SiO2 and FeO are the three end members and they can be mixed together in various proportions). With improved numerical capabilities, we were able to determine the solid-solid phase transition at pressure of about 500 GPa and were able to determine the melting line of this material. This is of critical importance to understand how a rocky planet cools down over time and how its structure evolves. Indeed, the difference in melting temperature can lead to a non-congruential crystallization meaning that some crystallizing materials may segregate from the remaining liquid materials. We also determined that a Super-Earth only made of MgO would crystallize very quickly. Only more complexe materials are likely to stay molten over a long period of time.

To explore the conductivity of silicates, we also teamed up with two experimental teams and provided numerical support to analyze their data. One experiment was dedicated to the conductivity of pure quartz under high pressure and the numerical simulations helped explaining the diagnostics of the experiments. The results are expected shortly.
We also studied the influence of iron on the electrical conductivity of periclase (MgO). We teamed up with an experimental team and showed numerically and experimentally that iron induces an important increase in the conductivity even in the solid state. We can thus anticipate a relatively high conductivity of the whole mantle for large super-Earth especially if they are enriched in iron and/or hot enough so that their differentiation and core formation is not complete.

The second set of key findings are raising even more questions. Indeed, we discovered that nickel and iron are unhappy when they are mixed together at high pressure. In our calculations we see that they tend to separate at low temperature mostly because of magnetic effects (iron and nickel are magnetic but with different characteristics). We do not know how the temperature is going to influence the behavior but it may potentially means that the cores of Super-Earths would be differentiated in iron-rich and nickel-rich sections. This is a whole new idea that needs to be thoroughly investigated.

All of this work is currently being reported in manuscripts submitted to different journals. We also presented the work in different scientific congresses and workshops. The PI also took part to different general public events where he presented the research on exoplanets, what they tell us about our origin but also all the questions they bring us.

We are still working on characterizing the effects of temperature on the iron-nickel alloys. This will help us determining if the differentiation within the core is happening in Super-Earth with iron-rich and iron-depleted regions. This would change the evolution of the core and thus the cooling processes within the planet.

In parallel we are currently working with modelers to implement the results of our calculations in their models. We should be able to determine the key parameters to characterize these exoplanets and find the conditions for which they may be habitable. This is a second step and it needs more complete data sets than just what we have right now. But with ABISSE we made a great start to this long haul project.

The hope is that by looking at a few chosen parameters (such as the size of the planet, its location, its host star type,...) we can give a likelihood for a given planet to be habitable. This would be a great way of helping us targeting objects that are the most likely to host life. It is also important in constraining the formation scenarios for exoplanets.

Periodic Reporting for period 1 - ABISSE (AB Initio Simulations for Super-Earths)

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