The planet does not fall far from the star

Date:
October 14, 2021
Source:
University of Bern
Summary:
A compositional link between planets and their respective host star
has long been assumed in astronomy. Scientists now deliver empirical
evidence to support the assumption -- and partly contradict it at
the same time.



FULL STORY ==========================================================================
A compositional link between 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 of the National Centre
of Competence in Research (NCCR) PlanetS from the University of Bern
and the University of Zu"rich, deliver empirical evidence to support
the assumption -- and partly contradict it at the same time.


========================================================================== Stars and planets are formed from the same cosmic gas and dust. In
the course of the formation process, some of the material condenses
and forms rocky planets, the rest is either accumulated by the star or
becomes part of gaseous planets. The assumption of a connection 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). Nevertheless, assumptions, especially
in astrophysics, do not always prove to be true. A study led by the
Instituto de Astrofi'sica e Cie^ncias do Espac,o (IA) in Portugal,
which also involves researchers from the NCCR PlanetS at the University
of Bern and the University of Zu"rich, published today in the journal
Science, provides the first empirical evidence for this assumption - -
and at the same time partially contradicts it.

Condensed star vs rocky planet To determine whether the compositions
of stars and their planets are related, the team compared very
precise measurements of both. For the stars, their emitted light was
measured, which bears the characteristic spectroscopic fingerprint of
their composition. The composition of the rocky planets was determined indirectly: Their density and composition were derived from their measured
mass and radius. Only recently have enough planets been measured so
precisely that meaningful investigations of this kind are possible.

"But since stars and rocky planets are quite different in nature,
the comparison of their composition is not straightforward," as
Christoph Mordasini, co-author of the study, lecturer of astrophysics
at the university of Bern and member of the NCCR PlanetS begins to
explain. "Instead, we compared the composition of the planets with a theoretical, cooled-down version of their star. While most of the star's material -- mainly hydrogen and helium - - remains as a gas when it cools,
a tiny fraction condenses, consisting of rock-forming material such as
iron and silicate," explains Christoph Mordasini.

At the University of Bern, the "Bern Model of Planet Formation
and Evolution" has been continuously developed since 2003 (see
infobox). Christoph Mordasini says: "Insights into the manifold processes involved in the formation and evolution of planets are integrated into
the model." Using this Bern model the researchers were able to calculate
the composition of this rock-forming material of the cooled-down star. "We
then compared that with the rocky planets," Christoph Mordasini says.

Indications of the habitability of planets "Our results show that our assumptions regarding star and planet compositions were not fundamentally wrong: the composition of rocky planets is indeed intimately tied to
the composition of their host star. However, the relationship is not as
simple as we expected," lead author of the study and researcher at the
IA, Vardan Adibekyan, says. What the scientists expected, was that the
star's abundance of these elements sets the upper possible limit. "Yet
for some of the planets, the iron abundance in the planet is even higher
than in the star" as Caroline Dorn, who co-authored the study and is a
member of the NCCR PlanetS as well as Ambizione Fellow at the University
of Zurich, explains.

"This could be due to giant impacts on these planets that break off some
of the outer, lighter materials, while the dense iron core remains,"
according to the researcher. The results could therefore give the
scientists clues about the history of the planets.

"The results of this study are also very useful to constrain planetary compositions that are assumed based on the calculated density from mass
and radius measurements," Christoph Mordasini explains. "Since more than
one composition can fit a certain density, the results of our study tell
us that we can narrow potential compositions down, based on the host
star's composition," Mordasini says. And since the exact composition
of a planet influences, for example, how much radioactive material it
contains or how strong its magnetic field is, it can determine whether
the planet is life-friendly or not.

"Bern Model of Planet Formation and Evolution" Statements can be made
about how a planet was formed and how it has evolved using the "Bern
Model of Planet Formation and Evolution." The Bern model has been
continuously developed at the University of Bern since 2003. Insights
into the manifold processes involved in the formation and evolution
of planets are integrated into the model. These are, for example,
submodels of accretion (growth of a planet's core) or of how planets
interact gravitationally and influence each other, and of processes in
the protoplanetary disks in which planets are formed. The model is also
used to create so-called population syntheses, which show which planets
develop how frequently under certain conditions in a protoplanetary disk.

========================================================================== Story Source: Materials provided by University_of_Bern. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Vardan Adibekyan et al. A compositional link between rocky
exoplanets and
their host stars. Science, 2021 DOI: 10.1126/science.abg8794 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211014142029.htm

--- up 6 weeks, 8 hours, 25 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)