Beryllium signal identifies stars that have swallowed rocky planets

An international team of astronomers, led by researchers from the University of São Paulo in Brazil, has unveiled a groundbreaking method for identifying stars that have ingested their orbiting planets. This novel approach hinges on detecting variations in the abundance of beryllium, a scarce chemical element. The findings, published in the journal Astronomy & Astrophysics, could offer fresh insights into the development of planetary systems.

The researchers focused on a binary star system, composed of two nearly identical solar-type stars named HD 129171 and HD 129209. While binary stars like these are expected to possess nearly identical chemical compositions since they formed from the same stellar nursery, the researchers discovered significant disparities between the two stars. HD 129171 exhibits heightened levels of refractory elements, which typically condense into solid materials like those found in rocky planets.

Study lead author Anne Rathsam, a doctoral student at the Institute of Astronomy, Geophysics and Atmospheric Sciences at USP, explained that this suggests HD 129171 has consumed planetary material throughout its lifetime. Until now, scientists suspected that some stars might incorporate planets or planetary fragments, but this study marks the first instance of verifying this through the analysis of beryllium abundance in binary stars.

Beryllium's unique property—its inability to be produced within stars throughout their evolution—makes it an invaluable marker for detecting planetary engulfment. Unlike other elements, beryllium and boron primarily arise through a process called cosmic spallation, where high-energy particles fragment heavier nuclei, resulting in lighter elements. This process provides beryllium with a more stable chemical signature, allowing astronomers to identify its presence as evidence of planetary material being ingested by a star.

The team utilized the UVES spectrograph on the European Southern Observatory's Very Large Telescope in Chile to gather data on HD 129171 and HD 129209. By breaking down starlight into different wavelengths, the instrument enabled the detection of subtle chemical signatures, revealing that HD 129171 contains a significantly higher concentration of refractory elements compared to its companion star.

The researchers estimate that the excess amount of refractory elements in HD 129171 is equivalent to more than 11 times the mass of Earth. This material could have originated from a single large planet or multiple smaller bodies. However, the authors note that for sun-like stars, internal mixing is so efficient that the final chemical signature does not allow for the differentiation between these scenarios.

The main contribution of the study lies in the chemical analysis, which identified beryllium as a reliable indicator of planetary engulfment events. The authors also discussed the various dynamical mechanisms that could cause planets to fall into their host stars, such as gravitational interactions, perturbations from companion stars, and orbital migration processes.

These mechanisms can lead to highly eccentric and unstable orbits, potentially resulting in planets being ejected from the system, colliding with one another, or being absorbed by the central star.

The study's findings suggest that stable systems similar to our solar system may be relatively uncommon. Dr. Jorge Luis Melendez Moreno, the study's adviser and an astronomer at USP, highlighted that several lines of evidence—such as computational simulations of planetary formation, exoplanet observations, and chemical studies of binary stars—support this idea.

These findings imply that systems like the solar system, characterized by giant planets in nearly circular orbits and rocky planets in stable inner regions, may be less common than previously thought. Additionally, since binary systems are prevalent in the Milky Way, with estimates suggesting that approximately half of its stars have a gravitational companion, the implications of this research could be far-reaching.