The planet does not fall far from the star

A compositional link between the planets and their respective host star has long been assumed in astronomy. For the first time now, a team of scientists, with the participation of researchers from the National Center of Competence in Research (NCCR) PlanetS at the University of Bern and the University of Zurich, is providing empirical evidence to support the hypothesis – and partly contradict that at the same time.

Stars and planets are formed from the same cosmic gas and dust. During the formation process, some of the matter condenses and forms rocky planets, the rest is either accumulated by the star or is part of gas planets. The hypothesis of a link between the composition of stars and their planets is therefore reasonable and is confirmed, for example, in the solar system by most rocky planets (Mercury being the exception). However, the assumptions, especially in astrophysics, do not always turn out to be true. A study conducted by the Instituto de Astrofísica e Ciências do Espaço (IA) in Portugal, which also involves researchers from the PRN PlanetS of the University of Bern and the University of Zurich, published today in the journal Science, provides the first empirical evidence for this hypothesis – and at the same time partially contradicts it.

Condensed star against rocky planet

To determine if the compositions of stars and their planets are related, the team compared very precise measurements of the two. For stars, their emitted light was measured, which bears the characteristic spectroscopic imprint of their composition. The composition of the rocky planets was determined indirectly: their density and composition were derived from their measured mass and radius. It is only recently that enough planets have been measured with such precision that meaningful investigations of this type are possible.

“But since stars and rocky planets are quite different in nature, comparing their composition is not straightforward,” as Christoph Mordasini, co-author of the study and lecturer in astrophysics at the he University of Bern and member of PRN PlanetS. Explain. “Instead, we compared the makeup of the planets with a theoretical, cooled version of their star. While most of the star’s material – mainly hydrogen and helium – remains as gas when it cools, a tiny fraction condenses, made up of rocky materials such as iron and silicate ” , explains Christoph Mordasini.

At the University of Bern, the “Bern model of the formation and evolution of planets” has been continuously developed since 2003 (see box). Christoph Mordasini says: “Information on the multiple processes involved in the formation and evolution of planets is incorporated into the model. Using this Bern model, the researchers were able to calculate the composition of this rock material from the cooled star. “We then compared that with the rocky planets,” says Christoph Mordasini.

Indications of the habitability of planets

“Our results show that our hypotheses concerning the compositions of stars and planets were not fundamentally wrong: the composition of rocky planets is in fact closely linked to the composition of their host star. However, the relationship is not as straightforward as we hoped, ”says lead study author and AI researcher Vardan Adibekyan. Scientists expected the abundance of these elements in the star to set the upper limit possible. “However, for some planets, the abundance of iron on the planet is even higher than in the star”, explains Caroline Dorn, co-author of the study and member of PRN PlanetS as well as Ambizione Fellow at the University of Zurich. , Explain. “This could be due to giant impacts on these planets which break up some of the lighter outer material, while the dense iron core remains,” according to the researcher. The results could therefore give scientists clues to the history of the planets.

“The results of this study are also very useful in constraining the supposed planetary compositions based on the density calculated from the mass and radius measurements,” explains Christoph Mordasini. “Since several compositions can correspond to a certain density, the results of our study tell us that we can reduce the potential compositions, depending on the composition of the host star,” explains Mordasini. And since the exact makeup of a planet influences, for example, the amount of radioactive material it contains or the strength of its magnetic field, it can determine whether the planet is favorable for life or not.

“Berne model of the formation and evolution of planets”

Statements can be made as to how a planet formed and how it evolved using the “Berne Model of the Formation and Evolution of Planets”. The Bern model has been continuously developed at the University of Bern since 2003. Information on the multiple processes involved in the formation and evolution of planets is incorporated into the model. These are, for example, sub-models of accretion (growth of the nucleus of a planet) or of how planets gravitationally interact and influence each other, and processes in protoplanetary discs in which planets form. . The model is also used to create population summaries, which show which planets are developing how often under certain conditions in a protoplanetary disc.

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