Artist's illustration of the formation of a planet around a star. Credit: Tania Cunha (Planetário do Porto - Centro Ciência Viva & Instituto de Astrofísica e Ciências do Espaço)

Alien Stars and Their Planets Share a Surprising Link

Understanding the specific iron content of a planet is useful in assessing its potential importance for further observations - including its potential habitability. If an exoplanet is not too close to its star, the presence of a large amount of iron in its composition may indicate that it has a stable magnetic field. Some astronomers believe that such a field reduces planetary water loss, which raises the chances of the existence of complex life forms.

Astrophysicists have found a correlation between the chemical composition of rocky exoplanets and the stars around which they revolve. According to estimates based on planetary densities, for super-earths, the stoichiometric ratio of iron to silicates in the composition of the planets turned out to be related to the composition of the protoplanetary disk.

This refers to planets like Venus, Earth, and Mars. In planets similar to Mercury, however, there was much more iron. The authors of the work explain this discrepancy by another mechanism of formation.


What is the link between the chemical composition of stars and planets?

Development of stellar systems

In the process of the development of stellar systems, both the central star itself and the planets around it are formed from the same material. Therefore, scientists believe that the chemical compositions of a star and planets around it should depend on the composition of the protoplanetary disk and, accordingly, correlate with each other.

The ratio of iron to silicates

For example, in the protosolar disk, the stoichiometric ratio of iron to silicates, according to the theoretical model, is approximately 1:2. Almost the same ratio is typical for Venus, Earth, and Mars. However, Mercury does not fit into this model – for this planet, this ratio is practically the opposite.

The focus of the latest study

To test how the elemental composition of a star correlates with the elemental composition of planets around it, astrophysicists led by Vardan Adibekyan of the University of Porto studied Sun-like stars from spectral classes F, G, and K and exoplanets near them.

The temperature of these stars is from 3800 to 7400 Kelvin, and their spectra contain many lines of metals: in particular, iron, calcium, titanium, and chromium. For heavy elements in main sequence stars, the composition of the atmosphere, which can be determined from the spectrum, roughly corresponds to the composition of the entire star.

Dependence of the density of rocky exoplanets on the ratio of iron and silicates in the star around which they revolve. Credit: Vardan Adibekyan et al. / Science, 2021
Dependence of the density of rocky exoplanets on the ratio of iron and silicates in the star around which they revolve. Credit: Vardan Adibekyan et al. / Science, 2021

Research targets

In total, scientists analyzed the spectra of 21 stars and the density of 32 small exoplanets around them, not exceeding ten Earth masses in mass. Based on these data, astrophysicists have determined the ratio of magnesium, silicon, and iron in their composition. Then, scientists calculated the dependence of the density and composition of the planets on the composition of the central star.

Results

Statistical analysis showed that for earth-like rocky exoplanets there really is a correlation between the composition of the star and the density of the planets, as well as between the composition of the star and the composition of the planets.

This, however, is based on two assumptions: that there is iron only in the core of the planet or that there is iron both in the core of the planet and in its mantle. True, there was more iron in the planets than in the stars. The authors of the work believe that this discrepancy is a consequence of the process of planet formation, and it is associated with the fact that closer to the star, the proportion of iron during the condensation of matter should be higher.

Iron content in rocky exoplanets, depending on the iron content in the star around which they revolve. On the left - if iron is only in the core, on the right - if both the core and the mantle. Credit: Vardan Adibekyan et al. / Science, 2021
Iron content in rocky exoplanets, depending on the iron content in the star around which they revolve. On the left – if iron is only in the core, on the right – if both the core and the mantle. Credit: Vardan Adibekyan et al. / Science, 2021

Super-Mercuries

In addition, scientists have found that their model does not work for five planets from the entire set of 32. This is Mercury and its heavier counterparts – scientists called these planets super-Mercuries. Astrophysicists suggest that the formation mechanism of these planets was different from the mechanism of formation of Earth-like exoplanets and included multiple collisions and deprivation of the mantle of the planets.


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Sources:

Adibekyan, V., Dorn, C., & Sousa, S. G. (2021, October 15). A compositional link between rocky exoplanets and their host stars. Science.
Instituto de Astrofísica de Canarias. (n.d.). Rocky exoplanets and their host stars may have similar composition.
Physics World. (2021, October 27). Compositions of exoplanets and their stars have a surprising relationship, study reveals.
Rabie, P. (2021, October 14). To find out what an exoplanet is made of, look to its star. Inverse.

Written by Vladislav Tchakarov

Hello, my name is Vladislav and I am glad to have you here on Curiosmos. As a history student, I have a strong passion for history and science, and the opportunity to research and write in this field on a daily basis is a dream come true.

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