USP researchers create method to detect planet-eating stars
A scientific discovery made by an international team, led by researchers from the University of São Paulo (USP), revealed an unprecedented method for detecting stars that have consumed their planets. The innovative process is based on identifying changes in the amount of beryllium, a relatively low-occurrence chemical element, promising a new approach to understanding how planetary systems develop.
The study, recently published in the journal Astronomy & Astrophysics, investigated a pair of solar-type stars, designated as HD 129171 and HD 129209. These stars, which have similar characteristics to our Sun in terms of physical, chemical and magnetic activity, form a binary system.
The initial expectation was that binary stars, born from the same molecular cloud at the same time, would have almost identical chemical compositions. However, scientists have observed notable differences between the two.
Discovery of innovative method for tracking stars
The star HD 129171 demonstrated a high level of refractory elements, those that normally condense into a solid state and form rocky planets. Anne Rathsam, doctoral student at the Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG-USP) and main author of the article, stated that this finding strongly indicates the ingestion of planetary material along the star’s evolutionary trajectory.
Although the possibility of some stars incorporating planets or their fragments had already been considered, the great contribution of this work is in proving, for the first time, that the variation in the abundance of beryllium in binary stars can serve as a faithful indicator of this phenomenon.
The essential role of beryllium in identifying cosmic events
Beryllium is notable for not being produced in the stellar core during the life of a star. Thus, its detection in the light emitted by a star acts as a warning sign, pointing out that the star absorbed rocky material, such as remnants of planets, long after its initial formation.
The researchers clarify that lithium, beryllium and boron represent a peculiarity in the chemical composition of the universe. While other chemical elements arise from primordial or stellar nucleosynthesis, beryllium and boron are created primarily by a process called ‘cosmic spallation’. In it, high-energy particles disintegrate denser nuclei, such as carbon, nitrogen and oxygen, generating lighter elements.
Lithium, although also mostly originated by spallation – with a minimal portion from primordial nucleosynthesis and rare production in specific types of stars – was previously used as a possible marker of planetary engulfment. Rathsam, however, highlights that beryllium is more durable, which allows its chemical signature to persist for a longer period of time.
Detailed observations reveal ingestion of planetary matter
To carry out the research, the team used data from the UVES spectrograph, an instrument installed on the Very Large Telescope (VLT) of the European Southern Observatory (ESO), located in Chile. This high-precision equipment is capable of decomposing starlight into its various wavelengths, allowing the identification of extremely subtle chemical signatures.
The results of the observations showed that HD 129171 has a significantly greater amount of refractory elements, such as iron, magnesium, silicon, calcium and titanium, when compared to its companion HD 129209. Additionally, this star has excesses of both lithium and beryllium. According to scientists, the observed pattern is consistent with the absorption of rocky material equivalent to more than 11 times the mass of Earth.
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Rathsam explained that the origin of this material could be either from a single large planet or from the sum of several smaller bodies. However, in stars similar to the Sun, internal mixing is so efficient that the final chemical signature does not allow us to differentiate between these two scenarios.
Violent dynamics of stellar systems and the rarity of stability
Although chemical analysis is the main original contribution of the study, with the election of beryllium as a marker of planetary engulfment, the authors also explored the dynamic mechanisms that can lead planets to be absorbed by their host stars. Such mechanisms include gravitational interactions between planets, perturbations caused by companion stars and orbital migration processes. These factors can result in extremely eccentric and unstable orbits, leading to the ejection of planets, collisions between them or their eventual absorption by the central star.
A crucial conclusion from the research suggests the possible scarcity of stable systems, like our Solar System. Jorge Luis Melendez Moreno, professor at IAG-USP and advisor of the study, notes that several independent pieces of evidence corroborate this hypothesis. Computer simulations of planetary formation indicate that configurations similar to the Solar System — with gas giants in nearly circular outer orbits and rocky planets in stable inner orbits — are not a frequent outcome. Furthermore, observational surveys of Sun-like stars have revealed few Jupiter analogues in orbits comparable to our gas giant.
Melendez points out that, when analyzing data from dynamical simulations, observations of exoplanets and chemical studies of binary stars, a consistent scenario emerges, suggesting that systems like the Solar may be less common than previously imagined. This implies that orbital stability, crucial for the persistence of habitable environments for billions of years, may be a rare exception, increasing understanding of the conditions necessary for the evolution of complex life in the universe.
Implications for star formation and the search for complex life
Melendez adds that binary systems are widely found in the Milky Way, with estimates indicating that about half of the stars in the galaxy have a gravitational companion. As the two stars of a binary system form simultaneously and from the same molecular cloud, the chemical differences observed between them serve as a strong indication that subsequent processes, such as the ingestion of planets, have altered their original composition.
Rathsam points out that while the planets in our system have relatively stable, low-eccentricity orbits, the frequency of planetary engulfment suggests that many star systems go through turbulent dynamic phases. Such instability, she emphasizes, has direct implications for the existence of complex life. For life to not only emerge and evolve over billions of years, but also to thrive, a planet must maintain a sufficiently stable orbit, protected from significant gravitational perturbations.
In addition to shedding new light on the evolution of planetary systems, the study also impacts theories of star formation and the technique known as “chemical tagging”, used to reconstruct the history of the Milky Way based on the chemical composition of stars.
If the chemical variations observed in binary stars originated from heterogeneities in the primordial cloud that generated them, a review of current models of star formation would be necessary. The results achieved by the team, however, reinforce the hypothesis of planetary ingestion.
The research included the participation of scientists from USP, the Polish Academy of Sciences, the Chinese Academy of Sciences, Monash University, in Australia, and Italian astronomical observatories, with financial support from Fapesp through a Thematic Project coordinated by Melendez.
