Abundance of iron and formation of terrestrial exoplanets


Title: A compositional link between rocky exoplanets and their host stars

Authors: Vardan Adibekyan, Caroline Dorn, Sérgio G. Sousa, Nuno C. Santos, Bertram Bitsch, Garik Israelian, Christoph Mordasini, Susana CC Barros, Elisa Delgado Mena, Olivier DS Demangeon, João P. Faria, Pedro Figuiera, Artur A. Hakobyan, Mahmoudreza Oshagh, Bárbara MTB Soares, Masanobu Kunitomo, Yoichi Takeda, Emiliano Jofré, Romina Petrucci and Eder Martioli

Institution of the first author: Instituto de Astrofísica e Ciências do Espaço & Departamento de Física e Astronomia, Universidade do Porto, Porto, Portugal

Status: Posted in Science [closed access]


Although we don’t yet have a complete picture of planetary formation – especially for the types of planets we don’t find in the solar system – we do have a good idea of ​​the overall process. It starts with a cloud of gas and dust somewhere in space. The turbulence will then lead to the formation of mass clusters in the cloud. When there is enough mass, the cluster collapses on itself due to gravity, heats up and forming a protostar. Over time, a disk forms around the protostar from the remaining gas and dust – the protoplanetary disc. Inside this disc, other collisions occur and the dust grains clump together, become protoplanets. Mass will continue to build up on protostars and protoplanets, which basically means that gas and dust “fall” on mass clusters, making them larger. Eventually, the protostar transforms into a main sequence star, and the protoplanets are destroyed or become full-fledged planets.

Figure 1: The stages of star and planet formation are shown here as follows: a) the gas cloud collapses, b) the protostar begins to form, c) the protoplanetary disk forms around the protostar , d) matter begins to clump together, forming protoplanets, e) the protoplanets have grown larger and the remaining gas and dust have dissipated, and f) the resulting system (in this case, the solar system)

The chemical composition of the protoplanetary disk and the resulting planets can give us important information about the mechanisms of planet formation, as well as the potential characteristics of planets. With current observing technology, we cannot always determine the composition of the planet itself. But, since this whole system comes from the same cloud of gas and dust, learning information about the chemical composition of stars can also give us information about the planets around them. We can do this relatively easily using stellar spectroscopysince different elements present in a star or planet will emit and absorb different wavelengths of light in a spectrum.


Today’s authors studied 32 known exoplanets with masses M Earth orbiting 27 different solar-type stars to find a correlation between the formation of terrestrial planets and the composition of planetary host stars. All 32 planets have masses known from radial velocity measurements and radii known from transit measurements. The authors plotted them on a mass-radius diagram and removed 10 planets which are probably mini Neptunes of the sample (see Figure 2). They then determined the abundances of Mg, Si and Fe – all major rocky elements – in the host stars using their spectra. Using pre-existing stellar composition models and these abundances, the authors estimate the iron abundance of stars and protoplanetary disks relative to Mg and Si. Using planet masses and radii and existing planetary interior models , they then estimate the iron abundance – again relative to Mg and Si – of the planets, independent of the star.

A graph showing the radii of the planets in the sample versus their masses.
Figure 2: The mass-radius diagram used to determine which exoplanets the authors retained. The mass axis is expressed in earth masses and the radius axis is expressed in earth radii. The dark blue line indicates the mass radius parameters they expect from Earth-like exoplanets. The dotted blue lines indicate the radius deviation that the authors used to distinguish between terrestrial exoplanets and mini-Neptune exoplanets. The gray line indicates the minimum planetary radius predicted by the collision model of planet formation (an explanation of this model can be found in this bite). Figure 1 in the article.


Figure 3 shows the results of plotting the resulting iron abundance of each planet against the iron abundance of its host star. Indeed, we see a positive correlation between the iron abundance of these telluric planets and their host stars. Interestingly, the results suggest that five of the planets in the sample are likely “super-Mercuries”, planets that have similar compositions to Earth, but much higher masses relative to their radii, similar to Mercury in our solar system. All five super-Mercuries orbit stars with high iron-to-silicate ratios and high iron abundances, suggesting that planetary compositions may be related to their stellar and protoplanetary compositions. Under these conditions, more collisions would likely occur during the planet formation process, supporting the theory that Mercury and Mercury-like exoplanets formed through collisional processes. Although more data is needed on super-Mercury populations, this study could potentially add another piece to the planet formation puzzle!

Two graphs of the iron-silicate ratio for the planets versus the iron-silicate ratio of their host stars.
Figure 3: For each planet, the iron abundance of its host stellar parent is plotted against its iron abundance. The color of the dots indicates the mass of the planets within the Earth masses, with lighter colors being the least massive and darker colors being the most massive. The solid black line shows the correlation between planetary iron abundance and stellar iron abundance with the super-Mercuries included in the sample, while the dotted line shows the correlation if they are excluded. The planets of the solar system are included in the plot for reference, but are not included in the calculations. Panel A shows what the distribution would be if all the iron was in the cores of exoplanets, while Panel B shows what the distribution would be if the iron was distributed between the core and the mantle of the exoplanets. Figure 3 in the article.

Astrobite edited by Katya Gozman

Featured image credit: ESO/L. Calcada

About Ali Crisp

I am a fourth year graduate student at Louisiana State University. I study Jupiter’s hot exoplanets in the Galactic Bulge. I’m from Tennessee and did my undergrad at Christian Brothers University, where I studied physics and history. In my “spare time” I enjoy cooking, hiking and photography.


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