New Study Suggests One in Ten Stars Eat a Jupiter-Sized Planet

In a fascinating study titled "Giant planet engulfment by evolved giant stars: light curves, asteroseismology, and survivability," researchers have estimated that one in ten evolved stars in the Milky Way galaxy have engulfed Jupiter-sized planets. The study, conducted by Christopher O'Connor, a Ph.D. student at Cornell University's Department of Astronomy, sheds light on the intricate process of stellar engulfment and its consequences.

When stars like our sun reach the later stages of their lives, they transform into red giants. These celestial objects undergo nuclear fusion, converting mass into energy, causing them to shed mass and expand. Unfortunately for any planets in close proximity, this expansion spells doom as they are eventually consumed and completely destroyed.

The research, which focuses on two types of evolved stars, namely the red giant branch (RGB) stars and asymptotic giant branch (AGB) stars, explores the frequency of planetary engulfment events and the subsequent response of these stars. Both RGB and AGB stars have left the main sequence and are characterized by their expanding size and mass loss.

Utilizing the Modules for Experiments in Stellar Astrophysics (MESA) software instrument, the researchers tracked the stellar response to planetary engulfment while simultaneously evolving the planetary orbit. The study found that approximately 10% of sun-like stars, those with 1 to 2 solar masses, will engulf a planet ranging between 1 to 10 Jupiter masses. The process of in-spiral, where the planet spirals inward toward the star, takes between 10 and 100 years or 100 and 1000 orbits.

During the initial phase of engulfment, stellar expansion and orbital decay draw the star and planet closer together. Tidal friction caused by the star's convective envelope plays a significant role in this phase, leading to the planet's orbital decay. As the star and planet come into contact, the dynamics shift, and drag forces become dominant. Referred to as the "grazing" phase, this stage involves complex hydrodynamical interactions between the star and the planet.

The study primarily focuses on the later "inspiral" phase when the planet is fully immersed in the star's envelope. At this point, the planet deposits heat into the star, contributing to its response. The mass of the engulfed planet influences the amount of heat transferred. These engulfment events cause the star's envelope to expand and contract, resulting in variations in brightness.

The research aligns with previous findings, indicating that planet engulfment leads to bursts of optical and infrared luminosity. The magnitude and duration of these bursts depend on the mass of the planet and the star. The study reveals that for both types of evolved stars, engulfing planets on the lower end of the range (up to three Jupiter masses), the structural changes in the star are mild to moderate. The brightness of the star can increase by up to one magnitude within a few years, with brighter stars potentially experiencing a double peak.

In the case of stars in the advanced stages of the AGB phase, the engulfed planet can trigger supersonic expansion of the star's outer layers, resembling Luminous Red Novae (LRN) with their bright, red, dusty eruptions.

It is important to note that this study's findings have limited applicability to our solar system. While our sun will eventually become a red giant, Jupiter is unlikely to be engulfed unless unforeseen events occur. Instead, the inner rocky planets may face such a fate.

Although the study is based on simulations rather than direct observations, the insights gained from these simulations provide valuable information for astronomers in identifying real planetary engulfment events. These transient events occur relatively quickly compared to other astrophysical phenomena, making them challenging to observe directly. However, with the upcoming launch of the Vera Rubin Observatory, equipped with its focus on transients and time-domain astronomy, astronomers will have an enhanced capability to detect and study evolved stars engulfing Jupiter-mass planets.

This figure from the paper shows heat deposited in stars in the later inspiral phase. The RGBs and AGBs in the legend are modelled host stars with different masses. The x-axis shows planetary mass, and the y-axis shows the amount of heat deposited. Clearly, the more massive the planet, the more heat is deposited. Credit: O’Connor et al. 2023

The findings of this study contribute to our understanding of the complex interactions between stars and planets in the later stages of stellar evolution. By simulating and analyzing the process of planetary engulfment, scientists gain insights into the mechanisms involved, the changes in stellar structure and brightness, and the potential for significant disruptions in the star's outer layers.

Future research in this field will likely explore additional factors that can influence the engulfment process, such as the rotation of the star and the role of optical and X-ray transients triggered by shocks during the grazing phase. Understanding these phenomena in more detail will provide a comprehensive picture of the dynamics of planetary engulfment and its impact on evolved stars.

This figure from the study shows the changes in radius and magnitude for one of the host stars modelled in the study. The top panel shows how a star can expand and contract multiple times during engulfment. The bottom panel shows how the star’s magnitude changes. Credit: O’Connor et al. 2023

While our own solar system may not be directly affected by these events, the study highlights the fate that awaits planets in close proximity to evolving stars. It underscores the destructive power that stars can unleash upon their planetary companions, providing a stark reminder of the violent and ever-changing nature of the cosmos.

As our knowledge of planetary engulfment grows, so too does our ability to uncover the mysteries of the universe. With further advancements in observational technology and refined simulations, scientists are poised to uncover more insights into the life cycles of stars and the fate of planets.

Journal Information: Christopher E. O'Connor et al, Giant planet engulfment by evolved giant stars: light curves, asteroseismology, and survivability, arXiv (2023). DOI: 10.48550/arxiv.2304.09882