Exoplanets and habitability

Interior of rocky exoplanets:

So far, more than 4000 exoplanets have been discovered, the majority of which are super-Earths (rocky planets larger than Earth). Since our Solar System does not contain such planet, most of our understanding of their properties relies on the combination of laboratory experiments and numerical modeling. The interior of exoplanets is important to understand as it controls the geologic history of the planet. Astronomical observations provide information on the size and radius of exoplanets, which are used to infer their bulk density.

For rocky planets, the size of their Fe core relative to their mantle controls the density of the planet. The top figure shows astronomical data (small circles with their error bars) compared with modeled mass-radius relationships for super-Earths with different core mass fractions (CMF): similar to Mercury (having a large core), Earth and Mars, and planets made with pure iron (Fe) and pure silicate mantle (MgSiO3) (Boujibar et al. 2020, JGR Planets). The second figure shows the surface gravity relative to Earth, for super-Earths with varying core mass fraction. Since super-Earths are both massive and denser than icy and gaseous planets, their gravity rapidly increases with their mass. Super-Earths 1.4 to 1.9 times larger than Earth have a surface gravity close to Jupiter (Boujibar et al. 2020, JGR Planets).

State of the core in super-Earths:

When the iron (Fe) core of a planet cools down, while crystallizing, impurities that are contained in the Fe alloy are released to the liquid core. This crystallization causes convection of the liquid core, which originates the magnetic field. On Earth, this dynamo is essential for protecting us from harmful cosmic energetic particles. A planet with a crystallizing core will therefore more likely to have a dynamo acting as a shield. This process may be important for making a planet habitable. Considering a core mass fraction similar to Earth for super-Earths with different masses, I investigated the temperature at the top of the core (Tcmb) for which the core is crystallizing. This specific range of temperature is shown in white. Above that range (orange field), the core is fully liquid and below (blue) the core is all solid. More calculations on the initial heat of super-Earths show that depending on how violent was the accretion of these planets (energetic impacts), super-Earths are likely to contain a magnetic field (Boujibar et al. 2020, JGR Planets).