An international research team led by scientists at the University of São Paulo (USP) has developed a new technique for identifying stars that have consumed planets in their systems. The method focuses on changes in the levels of beryllium, a relatively uncommon chemical element, offering a promising new way to investigate how planetary systems evolve.
The study, published in Astronomy & Astrophysics, analyzed the binary system HD 129171 and HD 129209, two Sun-like stars that formed together from the same molecular cloud and are therefore expected to have similar chemical compositions. However, the researchers discovered notable chemical differences between the two stars.
Evidence of Planetary Material Absorption
HD 129171 is enriched in refractory elements, which tend to condense into solids and make up rocky planets. This strongly suggests that the star absorbed planetary material during its evolution, explains lead author Anne Rathsam.
Astronomers had already considered the possibility that some stars could absorb planets or fragments of them. However, this study provides the first evidence that differences in beryllium abundance between binary stars can serve as reliable indicators of such events.
Beryllium is particularly useful because stars do not produce it in their interiors as they evolve. As a result, detecting its chemical signature in starlight can provide an important clue that a star has absorbed rocky material, such as planetary debris, after its formation.
Unique Origins of Lithium, Beryllium & Boron
According to the researchers, lithium, beryllium, and boron are unusual in the chemical evolution of the Universe. Most elements formed during the Big Bang or through nuclear fusion inside stars. Beryllium and boron, however, are produced mainly through a process known as cosmic spallation.
“During cosmic spallation, high-energy particles break apart heavier atomic nuclei, including carbon, nitrogen, and oxygen, generating lighter elements,” explains astronomer Jorge Luis Melendez Moreno, a professor at IAG-USP and supervisor of the research.
Lithium is also mainly produced through cosmic spallation, although a small fraction originated during primordial nucleosynthesis and can also be formed under particular conditions inside certain types of stars.
Rathsam explains that although lithium has previously been considered a potential indicator of planetary engulfment, it is easily destroyed. Beryllium, by contrast, is more stable, allowing its chemical signature to remain detectable for a longer period.
For the study, the researchers analyzed data collected by the UVES spectrograph, an instrument mounted on the Very Large Telescope (VLT) at the European Southern Observatory (ESO) in Chile. By separating starlight into different wavelengths, UVES can detect even very small variations in the chemical composition of stars.
The observations revealed that HD 129171 contains considerably higher levels of refractory elements, including iron, magnesium, silicon, calcium, and titanium, than its companion star, HD 129209. The researchers also detected unusually high concentrations of lithium and beryllium. They estimated that HD 129171 may have absorbed over 11 Earth masses of rocky material.
Rathsam notes that this material could have originated from one massive planet or from several smaller rocky bodies. In Sun-like stars, internal mixing makes these scenarios difficult to distinguish chemically.
Mechanisms Behind Planetary Engulfment
The study identified beryllium as a marker of planetary engulfment. These include gravitational interactions between planets, disturbances caused by companion stars, and processes of orbital migration. These mechanisms can destabilize planetary orbits, causing planets to be ejected, collide, or fall into their host star.
Stable planetary systems like our Solar System may be relatively rare. Melendez points out that several independent types of evidence support this idea. Planetary models suggest Solar System-like systems may be rare, with few Sun-like stars hosting Jupiter-like planets at similar distances.
Together, simulations, exoplanet surveys, and chemical analyses suggest Solar System-like systems may be rarer than previously thought.
Melendez also highlights the prevalence of binary star systems in the Milky Way. Estimates suggest that roughly half of the galaxy’s stars have a gravitationally bound companion. Because binary stars form from the same cloud, chemical differences may indicate later events, such as planet absorption.
Planetary Instability and the Potential for Complex Life
In our planetary system, the planets generally follow stable orbits with relatively low eccentricity. If planet consumption is common, many planetary systems may undergo periods of intense gravitational instability. She notes that this possibility could have important consequences for the development of complex life.
“Life requires not only billions of years to originate and evolve, but also a planetary environment that remains sufficiently stable for long periods, without being severely disrupted by gravitational interactions,” she explains.
The findings could improve theories of planetary and stellar evolution and refine chemical tagging used to trace the Milky Way’s history.
The findings suggest the stars’ chemical differences likely came from consuming planetary material, rather than variations in their original gas cloud.
The research involved scientists from USP, the Polish and Chinese Academies of Sciences, Monash University, and Italian observatories. The study was also funded by FAPESP (São Paulo Research Foundation) through a Thematic Project led by Melendez.
Read the original article on: revistagalileu
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